Chemistry Definition Atoms

Period

A horizontal row in the periodic table

Compound

A pure substance made up of only one type of molecule, and in which the molecules are made up of different types of atoms

Element

A pure substance which is made up of only one type of atom

Mixture

A substance (not pure) which is made up of different types of molecules

Subatomic

A term that means 'smaller than an atom'

Group

A vertical column in the periodic table

Atom

An atom is the smallest part of an element that can exist. Atoms are electrically neutral, with a positively charged, dense nucleus (filled with protons and neutrons) and surrounded by one or more electrons

(Niels) Bohr

The man who discovered electron energy levels

Relative atomic mass

The weighted average of the isotopic masses of an element, on a scale where an atom of carbon-12 has a mass of exactly 12 atomic mass units

Electron

a negatively charged subatomic particle that is found in the space outside the nucleus of an atom

Neutron

a neutral subatomic particle that is found in the nucleus of an atom

Proton

a positively charged subatomic particle that is found in the nucleus of an atom

Isotopes

atoms of an element that have the same number of protons but different numbers of neutrons

Electron configuration

the arrangement of electrons in the levels (or shells) of an atom

Nucleus

the dense, positively charged mass located in the centre of an atom

Atomic number

the number of protons in an atom of an element

Electron levels (or shells)

the possible energies that electrons in an atom can have

Mass number

the sum of the number of protons and neutrons in the nucleus of an atom of an element