Atoms- Smallest Unit of Matter: Videos & Practice Problems

Atoms, the smallest units of matter, consist of protons, neutrons, and electrons. The atomic number, defined as the total number of protons, determines the element's identity. The mass number combines protons and neutrons, while atomic mass is the average mass of an element's isotopes. Electrons occupy energy shells, with valence electrons in the outermost shell influencing reactivity. The octet rule states that atoms are more stable when their valence shells are full, typically holding 2 electrons in the first shell and 8 in the second, guiding chemical bonding and interactions.

Atoms- Smallest Unit of Matter

In this video, we're going to begin our lesson on atoms, which are the smallest unit of matter. Now, the term matter is used by scientists to refer to anything that takes up space and has mass. So this includes all living organisms like me and you, but also nonliving things too like rocks, oceans, and the device that you're watching this video through. Pretty much anything that takes up space and has mass will be matter. So matter is really, really, really broad, and it's going to be at the top here of our lesson for that reason. It includes living and nonliving things. Now all matter, regardless of if it's living or nonliving, is going to be made up of at least one chemical element and that's exactly what we're saying right here.

It's gonna be made up of at least one chemical element. Now the term chemical element is defined as a pure substance that's made up of only one type of atom. Over here in our image, you can see that all matter is made up of at least one chemical element and chemical elements are made up of atoms. And so the atom can therefore be defined as the smallest unit of an element. Now because atoms make up elements and chemical elements make up matter, we can also define the atom as the smallest unit of matter and that's exactly what we're saying right here and up above in our title.

Once again, atoms are going to make up both living and nonliving matter. So let's take a look down below at our example image to get a better idea of some of these concepts. Notice on the far left over here we're showing you an image of a diamond and of a honey bee in this plant. Now, the diamond and the honey bee, because they both take up space and have mass, they're both considered types of matter. Now the diamond of course is going to be nonliving matter whereas the honey bee and the plant are going to be living matter.

Now if we zoom into the diamond here, which you can see is the diamond structure that we're showing you over here, which is made up of all of these Cs that we see here. And these Cs represent a type of element, and so it's made up of just one type of element, which is the element carbon, which is abbreviated with just a C. Now if we zoom into one of these chemical element symbols, one of those Cs, what you'll see is that it is a carbon atom. And so here on the far right, we're showing you a representation of a carbon atom.

Now, in our next video, we're going to talk more about the structures and properties of atoms. But for now, you should notice that nonliving matter like diamond is gonna be made up of at least one chemical element and the smallest part of a chemical element is going to be the atom itself. Now, similarly down below with the honey bee and the plant, when we zoom into one of its chemical structures, you can see that it's going to have a sugar like, for example, glucose is a sugar that can be found in these living organisms.

What you'll notice is that this glucose structure over here actually has multiple types of elements. In fact, it has three types of elements. It has the same carbon element as the diamond, but then it also has the element oxygen and the element hydrogen, and you can see those throughout this glucose structure here. Now if we zoom into just one of the chemical elements of hydrogen here, what you'll see is that the smallest unit of this element is a hydrogen atom. And so this is the representation of the hydrogen atom. Once again, we'll talk more about the components of the atom and the properties of the atom moving forward in our course.

But for now, this here concludes our introduction to how atoms are the smallest unit of matter. And once again, the biggest takeaway is that all matter is going to be made up of chemical elements, and the smallest unit of a chemical element is the atom. So I'll see you guys in our next video.

Atomic Structure

In this video, we're going to talk about atomic structure or the structure of atoms. Now, atoms are made up of three subatomic particles, each with their own characteristic charge, mass, and location within the atom. Notice over here, we have this table that gives you information on the three subatomic particles that make up an atom. And so those three subatomic particles are protons, neutrons, and electrons. When it comes to the electric charge of a proton, it's going to be plus one, so it has a positive charge. You can think that the "p" here positively charged. When it comes to the electric charge of the neutron, the neutron is going to be, as its name implies, neutral, meaning that it has a neutral electric charge of zero, and so its electric charge is going to be zero. When it comes to the electron and its electric charge, it's going to have a negative one electric charge. The way that you can think about it is that electron kinda sounds like getting electrocuted, and getting electrocuted is definitely not something positive, it's something negative, and that's how you can remember that it's going to have a negative one charge.

When it comes to the mass of subatomic particles, the subatomic particles are so incredibly small that it doesn't really make a lot of sense to measure their mass in pounds or ounces or kilograms or even grams. They are so small that scientists have come up with a new unit to measure their mass, and that unit is called the atomic mass unit or AMU for short. Protons have an atomic mass unit of one. Neutrons also have an atomic mass unit of one. However, electrons are going to be different. Electrons have a mass that is so incredibly small that it's practically negligible, meaning that we can pretty much ignore its mass and just round it off to say that it is zero. We can go ahead and say that the electron has an atomic mass unit of zero. In terms of the location of these subatomic particles within the atom, you can see that the proton is found within the nucleus of the atom. The neutron is also found within the nucleus of the atom. However, the electrons, once again, are going to be different. Instead of being found within the nucleus, the electrons are going to be found orbiting the nucleus or revolving around the nucleus.

If we take a look at our image over here, you can see that we're showing you the image of a carbon atom right here. Notice that the nucleus is indicated with the purplish background. Notice that within the nucleus, we have these positively charged protons that look like this, and then we also have these neutral, gray, circles that represent the neutrons, and both the protons and the neutrons are found packed within the nucleus of the atom, just like what we see here. However, notice that the electrons, which are these little blue circles, are not found within the nucleus. Instead, the electrons are going to be found orbiting the nucleus in these electron shells that we see here. We'll talk more about these electron shells as we move forward in our course. But for now, what you guys should know are the subatomic particles, our protons, neutrons, and electrons, you should know their characteristic electric charge, their characteristic atomic mass unit, and the location of each subatomic particle. This here concludes our introduction to atomic structure and as we move forward, we'll be able to get some practice. So I'll see you all in our next video.

Atoms- Smallest Unit of Matter Example 1

Alright. So here we have an example problem that wants us to fill in the sentence here using one of these five potential answer options down below. It says that negatively charged particles of atoms with almost no mass are called either electrons, protons, neutrons, ions, or polymers. Now so far at this point in our course, we have not yet introduced ions or polymers. However, later in our course, we definitely will talk about these two terms, but because we have not yet introduced these two terms, we should have been able to figure out that these two are not going to be the correct answer for this problem. So now we're between either electrons, protons, or neutrons, which are the three subatomic particles of a typical atom. Recall that the "p" in protons can remind us that this subatomic particle is positively charged. So you can think the "p" in protons is for the "p" in positively charged. And so because protons are positively charged, they are not going to be negatively charged, and so we can eliminate answer option b. And also recall that neutrons are, as their name implies, neutral, and neutral means that it has a net charge of 0 and so it will not be negatively charged, it will have a charge of 0. So once again, we can eliminate neutrons. And of course, this only leaves the electrons, which is the correct answer for this example problem, and so electrons are negatively charged and recall that electrons kinda sounds like getting electrocuted and getting electrocuted is something negative. And so that can remind us that electrons are negatively charged. And also, electrons have almost no mass, and so we can pretty much say that their mass is about equal to 0. And so, we can go ahead and, indicate that a here is the correct answer to this example problem, and that concludes this example. So I'll see you guys in our next video.

A proton ___________:

• A. Has one positive charge.
• B. Has one AMU.
• C. Is found in the nucleus of the atom.
• D. Only a and b are true.
• E. a, b, and c are true.

Answer: (E)

Elements of Life

In this video, we're going to begin our lesson on the elements of life. It's interesting to note that of all the known elements in the universe, only a small subset of these is found in living organisms. The periodic table of elements, which you've probably seen before in some of your previous courses, actually arranges all of the known elements based on their chemical properties. If we take a look at our image down below, you'll be able to see all of the known elements in the periodic table. Once again, living organisms do not utilize all of these elements; they only utilize a small subset. In fact, 97% of the mass of most living organisms is composed of just 6 elements, and those 6 elements are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. If you take the chemical symbol of those 6 elements, which is really just the first letter of those 6 elements, you can spell the word CHNOPS. This mnemonic is a helpful way to remember the 6 most abundant elements that make up most living organisms. We also refer to these 6 elements as the bulk elements. You'll notice that here in blue, we're labeling the blue boxes within the periodic table as the bulk elements. The bulk elements are very abundant within living organisms. Again, those elements include hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur. If you use these elements, you can spell the word CHNOPS, which can be very helpful to remember the bulk elements.

Although these six elements make up the vast majority of the mass of most living things, there are some trace elements as well. Trace elements are those that are required for life but are needed in very small amounts. Whereas the bulk elements are found in large amounts, the trace elements are found in very small amounts. Notice that in our image down below, the trace elements are in yellow boxes. Once again, these trace elements are necessary for life, but they are only required in very small amounts. You'll see all of these yellow-highlighted boxes here are part of the trace elements. There's no need for you to memorize all of these trace elements; instead, you should just be aware of the fact that there are trace elements required for life in small amounts and you should be more aware of the fact that the bulk elements are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and CHNOPS can help you remember that. This concludes our brief introduction to the elements of life, and we'll be able to get some practice applying these concepts as we move forward. So, I'll see you all in our next video.

Atomic Properties

Alright. So here in this video, we're going to talk about some atomic properties. And so each atom of an element has unique properties, and we're going to talk about 3 specific properties that you all should be familiar with. And we're going to talk about them in these three lines of text here. Now the very first property that you all should be familiar with is the atomic number. Now, the atomic number is defined pretty simply. It's a pretty straightforward easy idea. All it is is the total number of protons in the nucleus of an atom. And so all you need to do to get the atomic number is count up the total number of protons and that's it. Now, the atomic number or the total number of protons is actually what defines each element. And so if we change the total number of protons in the nucleus of an atom, then we change the element that it falls under. However, if we change the other subatomic particles like neutrons or electrons, then we do not change the element. And so only changing the number of protons or only changing the atomic number is going to change the element, and that's why the atomic number or the number of protons defines each element.

So let's take a look at our example down below to clear some of this up. So here we're taking a look at the atomic properties of a carbon atom, more specifically this carbon atom that we're showing you right over here. And so if we want to determine the atomic number, which is once again just the number of protons in the nucleus, all we need to do is count up the total number of protons in this nucleus over here. And so when we do that, what we'll see is that there are a total of 6 protons in the nucleus, which means that the atomic number of this atom is 6. And so once again, the protons are here in red, and we're just counting up the number of red circles here and there are 6 of them, which is why the atomic number is 6. So that's pretty straightforward. That's called the atomic number. Now once again, if we were to add a 7th proton in here, then we would be changing the element and it would no longer be carbon. Instead, it would be nitrogen. And so, the number of protons in the nucleus is going to define the element.

Now if we were to add another neutron here, pretend this were a gray circle, if we were to add another neutron then it would still be a carbon atom. If we were to add another electron, here in the revolving around the nucleus, then it would also still be a carbon atom. And so, the number of electrons and neutrons do not affect the type of element that it is. And once again, it's only the number of protons that determines each element. And so that really, that's it for the atomic number. Moving on now, what we have next is the mass number, and the mass number is also a pretty straightforward idea. It's really just the mass of the nucleus of a single atom. And so if we want to take the mass of the nucleus, then we need to consider the subatomic particles that are inside of the nucleus, which we know are both protons and neutrons as well. And so the mass number is going to be the total number of protons and neutrons found in the nucleus. So once again, let's take a look at our example down below. And so of course, if we want to get the mass number, we need to get the mass of the nucleus and consider the number of protons and neutrons in the nucleus. We already know the number of protons, and so if we count the number of neutrons, the number of gray circles here in the nucleus, what you'll count is that there are a total of 6. 1, 2, 3, 4, 5, and 6. So we can put a 6 here as well. And so if we want the mass number, all we need to do is total up the 2. We have 6 protons plus 6, neutrons will give us the mass number 6 plus 6 is, of course, equal to 12. And so this is going to be the mass number that we just defined up above here. Now last but not least, what we have here is the atomic mass, which is also sometimes referred to as the atomic weight. Now the atomic mass or the atomic weight sounds kind of similar to this mass number idea, and really they are very, very similar. However, there's one big difference, and this is the idea that the atomic mass or the atomic weight, instead of being the mass of the nucleus of 1 atom, it's actually going to be an average total mass of all of the atoms of an element. And so, it is going to be an average whereas the mass number is not an average, it's the mass of just one atom. Now if we take a look at our image over here on the right hand side, what you'll notice is that a lot of periodic tables in your textbooks, are gonna have a periodic table view that looks somewhat like this for the elements. And so when you see this view here, you'll notice that there are some specific labels here. Now the very first one that you'll see here is this, number up here which is 12.011. Now, this number here is what we refer to as the atomic mass or the atomic weight that we just talked about, which is once again an average total mass of all all of the atoms of an element, which is why it looks like a strange number here with the point 011. And so once again, we'll be able to understand this idea here of atomic mass or atomic weight much better later in our course once we start talking about isotopes. But for now what you should note is that the atomic mass is going to be very very similar to the mass number of 12. Notice that they're very, very close, but they're going to be slightly different, because once again, the atomic mass is an average. And we'll be able to understand that idea better in a different video once we talk about isotopes. Now, notice that what we also have in here is the atomic number, and the atomic number tells us the total number of protons in the nucleus, which we know is 6 here for the carbon atom. You'll also note that the chemical symbol is always going to be shown here. For carbon, the chemical symbol is C, and the element's name is usually provided as well, which is carbon. And so this here concludes our introduction to these atomic properties of atomic number, mass number, and atomic mass or atomic weight. And we'll be able to get some practice applying these concepts moving forward in our course, so I'll see you all in our next video.

Atoms- Smallest Unit of Matter Example 2

Alright. So here we have an example problem that wants us to complete the sentence here using one of these 5 potential answer options down below. And it says that the atomic number of an element is equal to the number of neutrons only, neutrons plus electrons, protons plus electrons, protons only, or protons plus neutrons. Now, of course, we know from our last lesson video that the atomic number is going to be exactly equal to the number of protons only that are found in the nucleus. And so this here is our atomic number, and it is the correct answer to this example problem. Now, of course, protons plus neutrons might sound familiar, but that's because this is referring to the mass number or the mass of the nucleus of an atom. But it's not the same thing as the atomic number, so that's why it's not correct. The protons plus electrons are going to dictate the net charge of the atom because remember protons are positively charged and electrons are negatively charged. And so the balance of protons and electrons, which are oppositely charged, is going to dictate the overall net charge of the atom. But once again, option b here is not going to be the atomic number. And then neutrons only and neutrons plus electrons, they, once again, are not going to be the atomic number. So that concludes this example problem, and I'll see you in our next video.

The average oxygen atom has a mass number of 16 and an atomic number of 8. This means that the number of neutrons in this oxygen atom is:

• A. 24.
• B. 8.
• C. 16.
• D. 4.
• E. 2.

Answer: (B)

Electron Orbitals & Energy Shells

In this video, we're going to continue to talk about atoms by focusing on their electron orbitals and energy shells. Now, electron orbitals are really defined as three-dimensional regions around the nucleus of an atom where electrons can be found. Although electron orbitals are three-dimensional regions, they can still be envisioned in two dimensions as energy shells. In our course, we're mainly going to focus on the energy shell aspect. You'll learn more about the three-dimensional shapes of electron orbitals in a chemistry class. Let's take a look at our example image here of the carbon atom on the far left to get a better understanding of this idea of energy shells. You'll notice that the chemical symbol right in the middle represents the nucleus of this carbon atom, but revolving around the nucleus, what we have are these black circles, two of them for that matter, that represent energy shells. Once again, energy shells are just the two-dimensional representation of the electron orbitals, which are really three-dimensional regions. The energy shells contain electrons, and these little blue circles you see around here are the electrons orbiting around the nucleus of the atom in these energy shells. Any typical atom could have multiple energy shells as you can see from our image. Focusing on the carbon atom here, notice that it has two energy shells: one that is closer to the nucleus and a second energy shell that is further away from the nucleus. The energy shells that are closer to the nucleus are lower in energy than the energy shells that are more distant from the nucleus. The distant shells, being higher in energy, are more reactive, which is why scientists tend to focus on them when looking at chemical bonds. This leads us to the idea of valence electrons, defined as the electrons found in the outermost energy shell or the valence shell. For carbon, the valence shell is the one furthest from the nucleus. Electrons in the valence shell are the ones that are higher in energy and more reactive. Other electrons that are closer to the nucleus are not nearly as reactive and are not the ones to form chemical bonds with other atoms. Below the atom, you'll find a different way to represent atoms: the chemical symbol is shown with the mass number to the top left, representing the total number of protons and neutrons, and the atomic number to the bottom left, representing the total number of protons in the nucleus. The carbon atom has 6 protons in its nucleus and also has 6 electrons, resulting in a neutral net charge. All atoms shown have a neutral net charge. The first energy shell always holds a maximum of 2 electrons, and the second energy shell holds a maximum of 8 electrons. Our biology course requires knowing that the first shell holds up to 2 electrons, and the second shell holds up to 8 electrons.

Moving on to the hydrogen atom, it has an atomic number of 1, meaning it has 1 proton, and its mass number is also 1, indicating it has 1 proton and no neutrons. Since it has only one electron, it only needs one energy shell. For the nitrogen atom, it has 7 protons in its nucleus, and its energy shell configuration is such that the first shell is filled with 2 electrons, with remaining electrons in the second shell. The oxygen atom, with an atomic number of 8 and a mass number of 16, fills its first shell with 2 electrons, with others moving to the second shell, which can hold up to 8 electrons. For the phosphorus atom, with an atomic number of 15 and a mass number of 31, its first shell is filled with 2 electrons, its second shell with 8, and additional electrons move to the third shell. Similarly, the sulfur atom, with an atomic number of 16 and a mass number of 32, follows the same shell-filling pattern. This concludes our lesson on electron orbitals and energy shells and sets the stage for applying these concepts in our upcoming videos. I'll see you in our next one.

How many valence electrons does an atom with five total electrons have?

• A. 5.
• B. 7.
• C. 3.
• D. 2.
• E. 1.

Answer: (C)

Which of the following is true about electron energy shells?

• A. They represent regions around the nucleus in which the electrons orbit.
• B. The shells closest to the nucleus contain electrons with higher energy.
• C. They contain electrons of the same energy.
• D. a and b only.
• E. a and c only.

Answer: (A)

Octet Rule

In this video, we're going to introduce the octet rule. The octet rule is really just a rule of thumb that applies to most of the simple atoms that we're going to discuss in our biology course. The octet rule states that atoms are more stable and less reactive when their valence electron shells or their outermost electron shells are fully occupied. Recall from some of our previous lesson videos that the first energy shell of an atom holds up to 2 electrons, but the second energy shell of an atom holds up to 8 electrons. This 8 here is where the octet rule gets its name since "oct" is a root that means 8. The main point of the octet rule is that atoms are going to be less reactive when their outer valence shells are fully occupied or when they are full.

If we take a look at our image down below, we have this interesting analogy for the octet rule. You'll notice we have a bunch of blue circles all around, and these blue circles represent electrons. Notice that over here on the left, we have one atom, and over here on the right, we have a second atom. At the center of the atom, what we have is this turkey, and the turkey represents the nucleus of those atoms. It's almost as if these electrons are sitting at a dinner table, surrounding the nucleus of the atom.

Notice here in the middle, what we have is a single electron here that's asking, "Hey, want to react?" This atom over here on the left only has a total of 7 electrons. Recall that the second energy shell holds up to 8 electrons, so this atom does not have a full valence shell. It has one open slot, or you can think of this as one open seat at the dinner table. Notice that the electron is saying, "Yeah, there's a spot for you," and that spot again is this open spot that you see here, so because this atom does not have a full valence shell, it can react. Notice that we have a green check mark here to indicate that this electron would be able to react with this atom.

Notice over here on the right-hand side, this atom has a full valence shell with 8 electrons in its outermost shell. It does not have any spots open, and it says, "Nope, we are full," so notice we have this red X here to indicate that this electron would not be able to react with this atom. Once again, the main takeaway of the octet rule is that atoms are going to be less reactive when their valence shells are full, and this one has a full valence shell and does not react.

This here concludes our brief introduction to the octet rule, and we'll be able to get some practice applying these concepts as we move forward. So, I'll see you all in our next video.

Atoms- Smallest Unit of Matter Example 3

Alright. So here we have an example problem that says according to the octet rule, electron distribution in each shell of a neutral nitrogen atom with an atomic number of 7 is which one of these 4 potential answer options down below, where the first number of each answer option suggests the total number of electrons in the first energy shell, and the second number of each answer option represents the total number of electrons in the second energy shell. And so really what we need to do is draw ourselves a little sketch of a neutral nitrogen atom, which once again has an atomic number 7. So let's go ahead and say that this green circle here represents the nucleus of our nitrogen atom, which once again we know has an atomic number of 7, so we know that it has 7 positively charged protons in its nucleus. But then it tells us that the nitrogen atom is neutral, which means that it's going to have an overall net charge of 0, which means that the positively charged protons are going to be balanced out perfectly with the number of negatively charged electrons. So, if there are 7 positively charged protons, there must be 7 negatively charged electrons in order to make this nitrogen atom neutral. So we have to draw a total of 7 electrons. So considering that, we can go ahead and draw our first energy shell, and we know that the first energy shell holds a maximum of 2 electrons. So we can go ahead and fill this first energy shell here with 2 electrons as we see here, and then the other electrons need to go into the next shell. So, we can go ahead and draw our second energy shell, and, of course, we can put the remaining electrons here. So we know that there must be a total of 7 electrons. We've already got 2, so we got to put 5 more in the second shell. So we can go ahead and put 1, 2, 3, 4, and 5 electrons in this outer shell. So once again, there's a total of 7 electrons here that balance out the 7 protons, which makes this nitrogen atom neutral. And so now all we need to do is take a look at the answer options. And once again, we can see that the first energy shell here has a total of 2 electrons, not one electron as option A and option D indicate. So we can go ahead and eliminate answer option A and answer option D just based on that. However, when we look at answer option B and answer option C, notice that they both suggest the first energy shell has 2 electrons, which once again is correct. We can see the first energy shell does have 2 electrons. So then we need to look at the second number, which is really how these two answers differ, and you can see that the second energy shell, when you count them up, there's a total of 5 electrons, not 4 electrons. So we can go ahead and eliminate answer option B here. We can eliminate answer option B and go ahead and indicate that this answers option C here that is the correct answer for this example problem. And the reason for that is once again the 2 here indicates the number of electrons in the first energy shell, which you can see there's one here, and the other is right here. And then the second number of 5 indicates the number of electrons in the second energy shell. And once again, there are 5 when you count those up. And so C here is the correct answer to this example, and I'll see you guys in our next practice video.

A neon atom (Ne) is unreactive for which of the following reasons?

• A. It has 7 valence electrons.
• B. It has 8 valence electrons.
• C. Its valence shell is full of electrons.
• D. It has 20 valence electrons.
• E. b and c only.

Answer: (E)

How many electrons does an Oxygen atom need to fulfill the octet rule by filling its valence shell?

• A. 8.
• B. 4.
• C. 1.
• D. 2.
• E. 6.

Answer: (D)

Here’s what students ask on this topic:

What are the three subatomic particles that make up an atom and their characteristics?

Atoms are composed of three subatomic particles: protons, neutrons, and electrons. Protons have a positive charge (+1) and are located in the nucleus of the atom. They have a mass of approximately 1 atomic mass unit (AMU). Neutrons are neutral particles with no charge and are also found in the nucleus. They have a mass similar to protons, about 1 AMU. Electrons are negatively charged particles (-1) that orbit the nucleus in energy shells. Their mass is so small that it is considered negligible, approximately 0 AMU. Understanding these particles is crucial for studying atomic structure and chemical behavior.

How is the atomic number of an element determined?

The atomic number of an element is determined by the total number of protons in the nucleus of an atom. This number is unique to each element and defines its identity. For example, carbon has an atomic number of 6, meaning it has 6 protons in its nucleus. Changing the number of protons changes the element itself. The atomic number is crucial for organizing elements in the periodic table and understanding their chemical properties.

What is the octet rule and why is it important in chemistry?

The octet rule is a principle that states atoms are more stable and less reactive when their outermost electron shell, or valence shell, is fully occupied. For most atoms, this means having 8 electrons in their valence shell. The first energy shell holds up to 2 electrons, while the second holds up to 8. Atoms will often gain, lose, or share electrons to achieve a full valence shell, leading to the formation of chemical bonds. This rule helps explain the reactivity and bonding behavior of elements in chemical reactions.

What is the difference between atomic mass and mass number?

The mass number of an atom is the total number of protons and neutrons in its nucleus. For example, a carbon atom with 6 protons and 6 neutrons has a mass number of 12. Atomic mass, or atomic weight, is the average mass of all the isotopes of an element, taking into account their relative abundance. It is usually a decimal number, such as carbon's atomic mass of 12.011. While mass number is specific to a single atom, atomic mass represents an average for all atoms of an element.

What are valence electrons and why are they important?

Valence electrons are the electrons located in the outermost energy shell of an atom, known as the valence shell. These electrons are crucial because they determine an atom's chemical properties and its ability to form bonds with other atoms. Atoms with a full valence shell are generally more stable and less reactive, while those with incomplete valence shells tend to react to achieve stability. Understanding valence electrons is essential for predicting how elements will interact in chemical reactions.