exam chem definitins

Isotope

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

Carbon isotopes:

• Carbon-12 → 6 protons, 6 neutrons

• Carbon-14 → 6 protons, 8 neutrons

Element

A pure substance that cannot be broken down any further

Atom

An atom is the smallest particle of an element that retains its chemical properties.

Smallest unit of a element

Compound

A compound is a substance made of two or more different elements chemically bonded together.

• H₂O (water) = hydrogen + oxygen

• CO₂ (carbon dioxide) = carbon + oxygen

• NaCl (table salt) = sodium + chlorine

Key idea:

• Different elements must be joined by a chemical bond

Molecule

• Same atoms → molecule of an element

  • O₂ (oxygen), H₂ (hydrogen)

• Different atoms → molecule of a compound

  • H₂O (water), CO₂ (carbon dioxide)

Atomic number

The atomic number is the number of protons in the nucleus of an atom.

Key idea:

• It determines what element it is

Atomic mass

Atomic mass is the total number of protons and neutrons in the nucleus of an atom.

The atomic mass (also called mass number in school science) is the total number of protons + neutrons in an atom’s nucleus.

Key idea:

• Electrons are not included (they’re too small)

Example:

Carbon-12:

• Protons = 6

• Neutrons = 6

• Atomic mass = 12

Physical property

A physical property is a characteristic of a substance that can be observed or measured without changing what the substance is.

Chemical property

A chemical property is a characteristic that describes how a substance can change into a new substance.