Q: A student is given a sample of magnesium ($$Mg) $$and asked to determine the number of atoms in a $$12.15 \ g$$ sample. (Molar mass of $$Mg = 24.3 \ g/mol) $$Which calculation should they perform?

$$\frac{12.15}{24.3} \times 6.022 \times 10$$^{23}$$

Question 1

Which of the following best describes a mole in chemistry?

Accepted Answer

A unit that measures the number of atoms or molecules, specifically $$6.022 	imes 10$$^{23}$$ particles.

Question 2

If you have 18 grams of water ($$H_2O$$), how many moles of water do you have? (Molar mass of $$H_2O = 18 \ g/mol)$$

Accepted Answer

$$1$$ mole

Question 3

Which of the following elements has 6 valence electrons?

Accepted Answer

Oxygen

Question 4

What is the electron configuration for potassium ($$K$$, atomic number 19)?

Accepted Answer

$$1s^2$$ $$2s^2$$ $$2p^6$$ $$3s^2$$ $$3p^6$$ $$4s^1$$

Question 5

Which group on the periodic table contains elements with a full outer shell of electrons?

Accepted Answer

Noble gases

Question 6

A sample contains $$3.01 	imes 10$$^{23}$$ molecules of CO_2$. How many moles of CO_2$ are present?

Accepted Answer

$$0.5$$ moles

Question 7

Which of the following statements about transition metals is TRUE?

Accepted Answer

Their valence electrons can include electrons from both the outermost s and d orbitals.

Question 8

Which analogy best describes the relationship between electron shells, subshells, and orbitals?

Accepted Answer

A hotel with floors (shells), hallways (subshells), and rooms (orbitals)

Question 9

How many valence electrons does chlorine have?

Accepted Answer

7

Question 10

Which of the following is the correct order of filling for the first four electron subshells according to the Aufbau principle?

Accepted Answer

$$1s \rightarrow 2s \rightarrow 2p \rightarrow 3s$$

Question 11

A student is given a sample of magnesium ($$Mg) $$and asked to determine the number of atoms in a $$12.15 \ g$$ sample. (Molar mass of $$Mg = 24.3 \ g/mol) $$Which calculation should they perform?

Accepted Answer

$$\frac{12.15}{24.3} 	imes 6.022 	imes 10$$^{23}$$

Question 12

Which of the following elements is a non-metal?

Accepted Answer

Oxygen

Question 13

What is the main reason electrons fill the lowest available energy levels first?

Accepted Answer

To minimize the energy of the atom and increase stability

Question 14

Which of the following is the correct electron configuration for carbon ($$C$$, atomic number 6)?

Accepted Answer

$$1s^2$$ $$2s^2$$ $$2p^2$$

Question 15

Which of the following best explains why transition metals often have variable oxidation states?

Accepted Answer

They can lose electrons from both their outermost s and d orbitals.

Question 16

A chemist needs to prepare $$0.25$$ moles of sodium chloride ($$NaCl). $$What mass of $$NaCl$$ should they weigh out? (Molar mass of $$NaCl = 58.44 \ g/mol)$$

Accepted Answer

$$14.61 \ g$$

Question 17

Which of the following statements about the periodic table is correct?

Accepted Answer

Elements in the same group have the same number of valence electrons.

Question 18

Which element has the electron configuration $$1s^2$$ $$2s^2$$ $$2p^6$$ $$3s^2$$ $$3p^5$$?

Accepted Answer

Chlorine

Question 19

Why is it more complicated to determine the number of valence electrons for transition metals compared to main group elements?

Accepted Answer

Because their valence electrons can include both s and d electrons, and their electron configurations do not always follow a simple pattern.

Question 20

Which of the following best describes the trend in reactivity for alkali metals as you move down the group?

Accepted Answer

Reactivity increases because the outer electron is further from the nucleus and more easily lost.

Valence Electrons & the Periodic Table: Foundations for Understanding Chemical Properties

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