Element, Atom, Compound, Molecule
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)