Atoms & Elements

Definition of 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: Pure substances made of only one type of atom.

• Atom: The smallest unit of an element, retaining the element's properties.

Example: Atoms are the smallest units of matter, as illustrated by the atomic composition of diamonds, honey bees, and glucose.

Atomic Structure

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

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

Elements of Life

Essential Elements and the Periodic Table

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, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur.

• Trace elements are required for life in small amounts.

Example: The periodic table highlights essential and trace elements 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.

Example: Atomic properties of a carbon atom: , .

Electron Orbitals & Energy Shells

Electron Arrangement

Electrons are arranged in 3D regions around a nucleus called energy shells or orbitals.

• Shells closer to the nucleus are lower in energy than distant shells.

• 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 show the distribution of electrons in each shell.

Octet Rule

Stability of Atoms

Atoms are more stable (less reactive) when their valence shells are fully occupied.

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

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

Example: The octet rule explains the electron distribution in nitrogen and neon atoms.

Isotopes & Atomic Mass

Isotopes

Atoms of an element have the same number of protons but may have different numbers of neutrons.

• Isotopes: Atoms of the same element with different mass numbers.

• Atomic Mass: Weighted average of all isotopes.

Example: Atomic mass of carbon is calculated from its 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: Time required for half of a radioactive sample to decay.

• Radioactive isotopes are used in medicine, dating fossils, and cancer treatment.

Example: Carbon-14 is used in radiometric dating of fossils.

Equation:

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 is true about electron energy shells?

• How are isotopes of carbon and nitrogen different?

• How is radioactive carbon-14 used in science?

Additional info: These notes cover foundational topics in General Chemistry, including atomic structure, elements, isotopes, electron configuration, and radioactivity, suitable for introductory college-level study.