Atoms & Elements

Introduction to Matter and Chemical Elements

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 are pure substances made of only one type of atom, and atoms are the smallest units of matter.

• Matter: Anything with mass and volume.

• Chemical Element: Pure substance of one type of atom.

• Atom: Smallest unit of an element.

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

Atomic Structure

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

• Protons: Positively charged, found in the nucleus.

• Neutrons: Neutral charge, found in the nucleus.

• Electrons: Negatively charged, orbit the nucleus.

Example: Carbon atom structure showing protons, neutrons, and electrons.

Elements of Life

Major Elements in Living Organisms

Of all known elements, only a small subset is found in living organisms. The Periodic Table arranges elements by chemical properties. About 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: Elements required in small amounts for life.

• Major Elements: Elements required in large amounts 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: Weighted average mass of all atoms of an element.

Formulas:

• Atomic Number:

• Mass Number:

Example: Carbon atom: ,

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.

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

Example: Energy shells for C, H, N, O, P, S showing electron configuration.

Octet Rule

Stability of Atoms

Atoms are more stable (less reactive) when their valence shells are fully occupied. The octet rule states that atoms tend to have eight electrons in their valence shell.

• First energy shell: up to 2 electrons.

• Second energy shell: up to 8 electrons.

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

Example: Neon atom is unreactive because its valence shell is full.

Isotopes

Definition and Properties

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. This leads to different mass numbers.

• Isotope: Same atomic number, different mass number.

• Atomic Mass: Weighted average of all isotopes.

Radioactive Isotopes

Radioactivity and Half-Life

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

• Radioactive Isotopes: Used in medicine, dating fossils, and cancer treatment.

• Half-Life Formula:

Example: Carbon-14 has a half-life of 5,730 years, used in radiocarbon dating.

Practice Questions

Sample Questions for Review

• What is the atomic number of an element equal to?

• How many neutrons are in an oxygen atom with mass number 16 and atomic number 8?

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

• What makes an atom unreactive?

• How are isotopes of carbon different?

• What is the half-life of Carbon-14?

Additional info: These notes expand on the original content by providing definitions, examples, and formulas for key concepts in atomic structure and elements, suitable for General Chemistry students.