Ar values of elements

The table shows some Ar values:

Element Relative atomic mass

Hydrogen (H) 1

Carbon (C) 12

Oxygen (O) 16

Magnesium (Mg) 24

Chlorine (Cl) 35

For example:

Hydrogen gas is made of molecules. Each molecules contain Each water molecule contains 2 hydrogen atoms and 1 oxygen

2 hydrogen atoms, so its mass is 2 (2x1=2) atom, so its mass is 18 (2x1+16=18)

Relative formula mass

Relative atomic masses can be used to find the relative formula mass of a compound.

To find the relative formula mass (Mr) of a compound, you add together the relative atomic mass values (Ar values) for all the atoms in its formula.

The masses of atoms,molecules, and ions

A standard carbon atom.
A single atom weighs almost nothing, therefore you can't use scales to weigh it so the scientist have to choose an atom of carbon-12 to be the standard atom. They fixed its mass as exactly 12 atomic mass units, then they compared all the other atoms with this standard atom by using a machine called a mass spectrometer.

RELATIVE ATOMIC MASS
The relative atomic mass, Ar, for an element is the average mass of its naturally-occuring isotopes, relative to the mass of a carbon-12 atom.

R.A.M = mass of WESYWTM/mass of H