Relative atomic mass

All of the elements on the periodic table make up everything in our world! The periodic table consisting of most of the known elements was developed by a Russian man called Dimitri Mendelev around He managed to put all the elements in order of weight ! without sophisticated scientific equipment.

Mendeleevs First Periodic Table

Relative atomic Mass (Ar) numbers All atoms were measured against Carbon. The carbon atom was assigned an atomic mass of 12 C 6 12 This tell us that each carbon atom has a mass of 12 It also tells us the number of protons and neutrons We say the relative atomic mass (Ar) of Carbon is 12

H 1 1 hydrogen has a relative mass of 1 compared to carbon Relative atomic Mass (Ar) numbers We say the relative atomic mass (Ar) of hydrogen is 1

O 8 16 oxygen has a relative mass of 16 compared to carbon Relative atomic Mass (Ar) numbers We say the relative atomic mass (Ar) of oxygen is 16 All of the relative atomic masses for all the atoms are found on the periodic table

Atomic Mass (Ar) and Molecular Mass (Mr) We can find the molecular mass (Mr) of a compound by adding the atomic mass (Ar) numbers of the atoms in a compound. Be aware that the terms atomic mass and molecular mass are also known as molar masses

Ar and Mr are very similiar Ar = the relative atomic mass and is used for single atoms eg Ca, N, O etc in gmol -1 Eg The Ar of Ca = 40 gmol -1 Mr = the relative molecular mass of a substance and is the sum of the Ar values Eg 1 mole of H 2 = = 2 gmol -1 Eg What is the Mr of H 2 O? (use your PT to find out) (2 x H = 2 ) + (1 x O = 16) = 18gmol -1 What is the Mr of H 2 SO 4 ? (2 x H = 2) + (1 x S = 32) + (4 x O = 64) = 98 gmol -1