Atoms, Elements, and Atomic Structure: Foundations for General Biology

Atoms and Matter

Definition and Composition of Matter

Matter is anything that occupies space and has mass, including organisms, rocks, oceans, and air. All matter consists of at least one chemical element.

• Chemical Elements: Pure substances made of only one type of atom.

• Atoms: The smallest unit of an element, and thus the smallest unit of matter.

• Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Example: Diamonds and honey bees are composed of atoms, which are the fundamental units of matter.

Atomic Structure

Subatomic Particles

Atoms are made of subatomic particles, each with a characteristic charge, mass, and location within the atom.

Subatomic Particle	Electric Charge	Atomic Mass Unit (AMU)	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	~0	Orbiting Nucleus

Example: Negatively charged particles with almost no mass are called electrons.

Elements of Life

Essential Elements

Of all known elements, only a small subset is found in living organisms. The periodic table arranges all known elements based on their chemical properties.

• CHNOPS: 97% of the mass of most life is composed of Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S).

• Trace elements are required for life in small amounts.

Element	Symbol	Role in Life
Carbon	C	Structural backbone of organic molecules
Hydrogen	H	Component of water and organic molecules
Nitrogen	N	Component of proteins and nucleic acids
Oxygen	O	Component of water and organic molecules; required for respiration
Phosphorus	P	Component of nucleic acids and ATP
Sulfur	S	Component of some amino acids and vitamins

Atomic Properties

Atomic Number and Mass Number

Each atom of an element has unique properties:

• Atomic Number (Z): Number of protons in the nucleus of an atom.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

• Atomic Mass: The average mass of all atoms of an element.

Formula:

$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$

Example: The atomic number of carbon is 6, meaning it has 6 protons.

Electron Orbitals & Energy Shells

Electron Arrangement

Electrons are arranged in energy shells around the nucleus. Shells closer to the nucleus are lower in energy, while shells farther away are higher in energy.

• Valence Electrons: Electrons found in the outermost energy shell; they determine chemical reactivity.

• First shell holds up to 2 electrons; second shell holds up to 8 electrons.

Element	Atomic Number	Valence Electrons
Carbon	6	4
Hydrogen	1	1
Nitrogen	7	5
Oxygen	8	6
Phosphorus	15	5
Sulfur	16	6

Example: Oxygen has 6 valence electrons.

Octet Rule

Stability of Atoms

Atoms are more stable (less reactive) when their valence shells are fully occupied. This is known as the octet rule.

• First energy shell holds up to 2 electrons; second energy shell holds up to 8 electrons.

• Atoms are reactive when their outer valence shells are not full.

Example: Neon is unreactive because its valence shell is full with 8 electrons.

Isotopes

Definition and Properties

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

• Atomic Number: Number of protons (same for all isotopes of an element).

• Mass Number: Varies due to different numbers of neutrons.

• Atomic Mass: The average mass of all isotopes of an element.

Isotope	Protons	Neutrons	Electrons	Abundance
Carbon-12	6	6	6	~99%
Carbon-13	6	7	6	~1%
Carbon-14	6	8	6	Trace

Example: Carbon-13 and Carbon-14 are isotopes of carbon with different numbers of neutrons.

Radioactive Isotopes

Properties and Applications

Radioactive isotopes are unstable and break down, emitting energy in the form of rays or particles. The time it takes for half of a radioactive sample to break down is called its half-life.

• Half-life: Time required for half of the radioactive atoms in a sample to decay.

• Radioactive isotopes are used in medicine, biological research, and dating fossils.

Example: Carbon-14 is used in radiometric dating of fossils. Its half-life is 5,730 years.

Formula:

$\text{Number of years for decay} = \text{Half-life} \times \text{Number of half-lives}$

Summary Table: Key Atomic Concepts

Concept	Definition	Example/Application
Atom	Smallest unit of matter	Hydrogen atom
Element	Pure substance made of one type of atom	Oxygen (O)
Isotope	Atoms of same element with different numbers of neutrons	Carbon-12, Carbon-14
Valence Electron	Electron in outermost shell	Oxygen has 6 valence electrons
Octet Rule	Atoms are stable with 8 electrons in valence shell	Neon is unreactive
Radioactive Isotope	Unstable isotope that emits energy	Carbon-14 used in dating fossils

Additional info: These notes expand on the original content by providing full definitions, examples, and context for each concept, ensuring a self-contained study guide suitable for General Biology students.