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Protons and neutrons have the same mass, so the relative mass of a proton and neutron are both one.  The mass of an electron is very small compared with a proton or neutron.  The mass of an atom is made up almost entirely of protons and neutrons.  The total number of protons and neutrons in an atom is called the mass number.

Atoms of the same element all have the same atomic number.  The number of protons and electrons in an atom are always the same (not an ion), but the number of neutrons can be different.  Atoms of the same element with different numbers of neutrons are called isotopes.  The number of neutrons in an atom is equal to the mass number minus the atomic number.  In the periodic table, the number above the chemical symbol is the mass number.  The number below it is the atomic number.  Hydrogen has three isotopes - hydrogen, deuterium and tritium.  They all have different properties.  Hydrogen has no neutrons, deuterium has one and tritium has two.


Atoms are too small to weigh, so we use relative atomic masses (Ar) in calculations.  Relative atomic masses are sometimes shown in the periodic table.  The relative atomic mass of an element in grams is called one mole of atoms of the element.

We use an atom of carbon as a standard atom and compare the masses of all other elements with this.  The relative atomic mass of an element (Ar) is an average value that depends on the isotopes the element contains.  When rounded to a whole number, the relative atomic mass is often the same as the mass number as the main isotope of the element.

The relative formula mass (Mr) of a substance is found by adding up the relative atomic masses of the atoms in its formula.

For example, calculate the relative formula mass of calcium chloride, CaCl2.

Answer: relative atomic mass of calcium is 40, Ar of Cl is 35.5, so Mr = 40 + 2(35.5) = 111

The relative formula mass of a substance in grams is one mole of the substance.  Using moles of substances is useful when we need to work out how much of a substance reacts or how much product we will get.

What is the mass of one mole of sodium hydroxide, NaOH?

Ar of Na is 23, Ar of O is 16, Ar of H is 1.  23 + 16 + 1 = 40g.


We can calculate the percentage of any of the elements in a compound from the formula of the compound.  Divide the relative atomic mass of the element by the relative formula mass of the compund and multiply the answer by 100% to convert into a percentage.  This is useful when deciding if a compound is suitable for a particular purpose.

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