Atoms, Elements, and the Chemical Basis of Life: General Biology Study Notes

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Atoms, Elements, and Matter

Definition and Structure 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.
Atom: The smallest unit of an element, retaining its chemical properties.
Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Example: Diamond and honey bee structures are composed of atoms, such as carbon and hydrogen.

Subatomic Particles

Atoms are made of three main subatomic particles, each with distinct properties:

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 electrons.

Elements of Life

Major and Trace Elements

Of all known elements, only a small subset is found in living organisms. The Periodic Table of Elements arranges all elements based on their chemical properties.

Approximately 97% of the mass of most life is composed of Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S) (CHNOPS).
Trace elements are required in smaller amounts for life.

Major Elements	Trace Elements
C, H, N, O, P, S	Fe, Zn, Cu, I, etc.

Example: The periodic table highlights elements essential for life.

Atomic Properties

Atomic Number and Mass Number

Each atom of an element has unique properties:

Atomic Number (Z): Number of protons in the nucleus.
Mass Number (A): Total number of protons and neutrons.
Atomic Mass: Average mass of all atoms of an element.

Formula:

Example: Carbon has atomic number 6 and mass number 12.

Electron Orbitals and Energy Shells

Electron Arrangement

Electrons are arranged in energy shells around the nucleus:

Shells closer to the nucleus are lower in energy; shells farther are higher in energy.
Valence Electrons: Electrons found in the outermost shell, important for chemical bonding.
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: Energy shells for C, H, N, O, P, S show their 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 and Atomic Mass

Isotopes

Atoms of the same element can have different numbers of neutrons, resulting in different isotopes.

Isotope: Atoms with the same number of protons but different numbers of neutrons.
Atomic Mass: Weighted average of all isotopes.

Isotope	Protons	Neutrons	Electrons
Carbon-12	6	6	6
Carbon-13	6	7	6
Carbon-14	6	8	6

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

Radioactive Isotopes

Radioactivity and Half-Life

Some isotopes are unstable and break down, emitting energy in the form of rays or particles. These are called radioactive isotopes.

Half-Life: Time required for half of a radioactive sample to decay.
Radioactive isotopes are used in medicine, cancer treatment, and dating fossils.

Formula:

where is the remaining amount, is the initial amount, is time, and is half-life.

Example: Carbon-14 dating is used to determine the age of fossils.

Additional info: These notes expand on the original content by providing definitions, formulas, and context for key concepts in atomic structure and the chemical basis of life, suitable for General Biology students.