Key Terms
Matter
Anything that occupies space and has mass. Mass: the amount of matter in an object.
Element
A pure substance that cannot be broken down by ordinary chemical means. 92 occur in nature.
Compound
Two or more elements joined by chemical bonds. Always in fixed ratios.
Atom
The smallest unit of an element that retains that element's properties.
Atomic number
Number of protons in the nucleus. This defines the element.
Mass number
Protons + neutrons. Electrons excluded because their mass is negligible.
Neutral atom
Protons = electrons. Overall charge is zero.
Radioactive isotope
Nucleus decays, releases subatomic particles and electromagnetic energy. Half-life: time for half of a sample to decay.
Valence shell
The outermost shell. This is what drives chemical behavior.
Octet rule
Atoms give up, gain, or share electrons until their valence shell holds 8 (or 2 for hydrogen/helium). Filled valence she
Ion
Atom that has gained or lost an electron; now has a charge. Cation: positive ion (lost electrons).
Anion
Negative ion (gained electrons). Example: Cl- (chloride accepts 1 electron)
Ionic bond
Attraction between opposite-charged ions. Salt (NaCl) is the classic example.
Nonpolar covalent
Electrons shared equally. No charge difference across the molecule.
Polar covalent
Electrons shared unequally. One end is slightly negative, the other slightly positive (partial charges, indicated with d