An element is a substance which contains only one kind of atom:  lead (Pb), silver (Ag), hydrogen (H₂), oxygen (O₂).

An atom is the smallest particle of an element which exhibits the physical and chemical characteristics of that element.

A compound is a substance which contains two or more kinds of atoms:  carbon dioxide (CO₂), calcium oxide (CaO), sodium hydroxide (NaOH).

A molecule is the smallest particle of a substance which exhibits the physical and chemical characteristics of that substance.

    Monoatomic molecules of elements - Cu, Ag, Na, etc.

    Multiatomic molecules of elements - H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂, P₄, S₈

    Molecules of compounds - Na₂O, KOH, CaSO₄, HBr, H₂CO₃

   Examples:  double the answer and p (contain an unlimited number of decimal places)

Special case - when some zeros are significant and some are not, use a line to show which ones are.

   Examples:  3400 (3 sig fig)    2000 (3 sig fig)

Composition and Uses of Some Alloys

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