Chapter 7

Relative Masses of Atoms and Molecules

contents

You will learn about:• Relative atomic mass• Relative molecular mass• The formula of a compound

Objectives At the end of the lesson, students should be able to:

– define relative atomic mass, Ar – define relative molecular mass, Mr – calculate relative atomic mass, relative

molecular mass and relative formula mass

Introduction

Atoms are soooo small and their masses are so small! Is there a way to measure

them conveniently???

Since atoms are so small, let’s compare them to each other on the atomic scale.

We can compare every element to H or 1/12 of C-

12

relative atomic mass

NH H

H H HH H H H

H H H H H

The lightest atom is the hydrogen atom. As one nitrogen atom is 14 times heavier than a hydrogen atom, nitrogen is said to have a relative atomic mass of 14.

relative atomic massRelative masses is about comparing the masses of atoms and molecules.

relative atomic mass of an element is the average mass of one atom of the element

when compared with 1/12 of the mass of an atom of carbon-12

relative atomic mass of an element

=average mass of one atom of the element

mass of 1/12 of an atom of carbon-12

symbol is Ar

has no units

Isotope carbon-12 was chosen (instead of hydrogen) for comparing the masses of atoms as it is in abundance and has isotopes.

relative atomic mass of an elementRelative atomic masses of common elements:

ElementsElements Relative Atomic Relative Atomic MassMass SymbolSymbol

hydrogen 1 Hhelium 4 Hecarbon 12 C

nitrogen 14 Noxygen 16 Osodium 23 Na

magnesium 24 Mgaluminium 27 Al

silicon 28 Siphosphorus 31 P

sulphur 32 S

relative atomic mass

relative atomic mass of an elementRelative atomic masses of common elements:

ElementsElements Relative Atomic MassRelative Atomic Mass SymbolSymbol

chlorine 35.5 Clpotassium 39 K

calcium 40 Cairon 56 Fe

copper 64 Cuzinc 65 Zn

bromine 80 Briodine 127 Ilead 207 Pb

relative atomic mass

Question • What is the difference between nucleon number and

relative atomic mass, Ar?Nucleon number

represents the total number of proton and neutron in an atom but Ar takes into account the

presence of isotopes.

Question Why are some Ar values not whole numbers?Example: Ar of Cl is 35.5.

Because most naturally occurring elements contain a mixture of isotopes.Eg. 75% of chlorine-35

25% of chlorine-37To calculate Ar of Cl:Ar of Cl = (75/100 x 35) + (25/100 x 37)

= 26.25 + 9.25 = 35.5

The relative atomic mass of chlorine is 35.5.

Question

1. The mass of one atom of an element X is 1 1/3 that of carbon. What is its relative atomic mass?

2. What is the relative atomic mass of element Y, given that it is four times the mass of a carbon-12 atom?

1. Relative atomic mass of X = (4/3) x 12 = 162. Relative atomic mass of Y = 4 x 12 = 48

relative molecular massrelative molecular mass of a molecule is the average mass of one molecule of a

substance when when compared with 1/12 of the mass of an atom of carbon-12

relative molecular mass of a molecule

=average mass of one molecule of a substance

mass of 1/12 of an atom of carbon-12

symbol is Mr

has no units is the sum of the relative atomic masses of all

the atoms in the molecule for ionic compounds, relative formula mass is

used

relative molecular massrelative molecular mass of a molecule

to find relative molecular mass of carbon dioxide (CO2),

relative atomic mass of carbon 2 x relative atomic mass of oxygen

relative molecular mass of CO2

= 12 = 2 x 16 = 32 (total) = 44

relative molecular massrelative molecular mass of a molecule

to find relative molecular mass of sugar (C12H22O11),

12 x relative atomic mass of carbon22 x relative atomic mass of hydrogen

11 x relative atomic mass of oxygenrelative molecular mass of C12H22O11

= 12 x 12 = 144 = 22 x 1 = 22 = 11 x 16 = 176 (total) = 342

Relative molecular mass of some molecules

Substance Formula Atoms in formula

Calculating Mr

Nitrogen N2 2N 14 x 2 = 28

Water H2O 1O; 2H 1 x 2 + 16 = 18

Carbon dioxide

CO2 1C; 2O 12 + 16 x 2 = 44

Hydrogen chloride

HCl 1H; 1Cl 1 + 35.5 = 36.5

Sulfur dioxide

SO2 1S; 2O 32 + 16 x 2 = 64

relative formula massrelative formula mass of ions/ionic compounds Ionic compounds do not contain molecules Sum of relative atomic masses of atoms called relative

formula mass Also given the symbol Mr

= 12 = 3 x 16 = 48 (total) = 60

relative atomic mass of carbon 3 x relative atomic mass of oxygen

relative formula mass of CO32-

relative formula massrelative formula mass of ions/ionic compounds Relative formula mass of ammonium ion, NH4

+

relative atomic mass of nitrogen 4 x relative atomic mass of hydrogen

relative formula mass of NH4+

= 14 = 4 x 1 = 4 (total) = 18

Relative formula mass of ionic compounds

Name Formula Atoms present Calculating Mr

Magnesium sulphate

MgSO4 1Mg; 1S;4O 24 + 32 + 4 x 16 = 120

Iron (III) chloride FeCl3 1Fe;3Cl 56 + 3 x 35.5 = 162.5

Iron (II) sulphate crystals

FeSO4.7H2O 1Fe;1S;4O;7H2

O56 + 32 + 16 x 4 + 7(1 x 2 + 16) = 278

Calcium carbonate CaCO3 1Ca;1C;3O 40 + 12 + 16 x 3 = 100

Sodium phosphate Na3PO4 3Na;1P;4O 23 x 3 + 31 + 16 x 4 = 164

Question 1. Use the Periodic Table to answer the following questions.

a) Four nitrogen atoms have the same mass as one atom of the element X. Identify X.

b) How many calcium atoms have about the same mass as one mercury atom?

a) 4 x 14 = 56The Ar of X is 56 and X is iron, Fe.

b) Ar of mercury = 201

Ar of calcium = 40

No. of calcium atoms that have about the same mass as a Hg atom = 5