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Atom – Smallest particle of an element.

Compound – A substance made of two or more chemically combined elements. Water is a compound because it consists of the elements hydrogen and oxygen.

Element – A substance that cannot be broken down.

Ion – An atom that has an electric charge as an outcome of losing or gaining one or more electrons.

Isotope – Atoms of the same element that has different numbers of neutrons.

Matter – Anything that occupies space and that has mass. This computer is matter because it takes up space.

Family – A vertical column on the Periodic Table.

Period – A horizontal row on the Periodic Table.

Mass Number – The total number of protons and neutrons. Since there are 19 protons and 20 neutrons in the element potassium, the mass number is 39.

Phase Change – When a substance in a solid, liquid or gas form changes from one phase to another. When ice turns to water, that’s a phase change because it went from a solid to a liquid.

Electron – A negatively charged particle that is located outside the nucleus in an atom.

Proton – A positively charged particle that is located in the nucleus.

Neutron – A particle with no charge that is located in the nucleus.