2.1 - Elements and Atoms: The Building Blocks of Matter

Matter

Anything that occupies space and has mass

Element

A pure substance that cannot be broken down by ordinary chemical means

Compound

Substance composed of two or more atoms of different elements

Atom

The smallest unit with unique properties of an element; made of protons, neutrons, and electrons

Molecule

A stable grouping of two or more atoms held together by chemical bonds

Proton

Heavy subatomic particle with positive charge found in the nucleus

Neutron

Heavy subatomic particle with no charge found in the nucleus

Electron

Subatomic particle with negative charge and negligible mass found orbiting the nucleus

Atomic Number

Number of protons in an atom

Mass Number

Sum of protons and neutrons in the nucleus

Isotope

Atoms of the same element with the same number of protons but different numbers of neutrons

Radioactive Isotope

Unstable, heavy isotope that gives off subatomic particles as it decays

Electron Shell

Area that’s a given distance from an atom’s nucleus where electrons are group at a distinct energy level

Valence Shell

An atom’s outermost electron shell; determines reactivity

Octet Rule

All atoms are most stable with 8 electrons in their valence shell

Atomic Weight

The weighted average mass of all naturally occurring isotopes of an element