REVISION
All matter is composed of tiny invisible particles called atoms .
The many different kinds of atoms are referred to as different elements . Different kinds of matter are different because they contain different combinations of elements.
Physical change involves a change in the state or condition of matter but not a change in its chemical identity:
Example: Salt, NaCl is chemically unchanged - i.e. it remains NaCl - when the solid is melted to form a liquid or if the solid is dissolved in water.
Chemical change , i.e. a chemical reaction involves the conversion of one type of matter into another:
Example: Na + Cl

NaCl
The total mass remains constant during a chemical reaction
( Law of Conservation of Mass ).
A pure compound always contains constant proportions of the elements by mass ( Law of Constant Composition )

The Structure of Chemical Compounds
Atoms in chemical compounds, i.e. in compound matter, are not just bundled together like a collection of billiard balls, but are held together by forces called chemical bonding .
Two types of chemical compound can be distinguished:
Molecular and Non-molecular . Molecular substances are defined as substances that contain molecules .
A molecule is a distinct finite number of atoms held together (bonded) by chemical forces and capable of behaving as a single unit .
A molecule has distinct physical and chemical properties that are characteristic of the molecular unit as a whole and which are different from those of the constituent atoms . If the molecule is broken down into its constituent atoms its characteristic properties are lost. The molecule is therefore the smallest unit of a compound that retains the chemical properties of the substance as a whole .

Examples of molecular substances :
H
2
Hydrogen molecule
O
2
Oxygen molecule
CO
2
Carbon dioxide molecule
SO
3
Sulphur trioxide molecule
N
2
O
4
Dinitrogen Tetroxide molecule
C
76
H
52
O
46
Tannic acid molecule
Molecular formula: The composition of a molecule is given in the molecular formula by the symbol for each of the constituent elements followed by the number of atoms of that element involved as a subscript. For instance, the formula of tannic acid above specifies that the molecule contains 76 carbon atoms, 52 hydrogen atoms and 46 oxygen atoms.
By convention, in molecules containing carbon and hydrogen together with other elements, the formula is written with carbon and hydrogen first followed by the other elements in alphabetical order. Examples: tannic acid (see above) or nitrobenzene, C
6
H
5
N O
2
.

There are three kinds of non-molecular substances :
1.
Mono-atomic substances: e.g. He, Ne, Kr, Ar and Xe
(known as ‘Noble Gas’ elements)
.
2.
Giant molecules: e.g. silicon carbide i.e. SiC or (SiC) n
.
In this compound there are equal numbers of silicon and carbon atoms. Each silicon atom is surrounded by and bonded to four carbon atoms, while each carbon is
C similarly attached to four silicon atoms. The size of the particle of solid silicon carbide
Si is the only limitation on the
C
Si Si
C number of atoms in the network
– which is so large as, for all
C intents and purposes, to be
Si effectively infinite .
3.
Ionic solids : In ionic solids the component atoms (or group of atoms) bear positive or negative electrical charges and are called IONS . The ions are held together by electrostatic attraction between the opposite charges.

Since an ionic compound must be neutral overall there must be equal numbers of charges on the positive ions
( cations ) as there are on the negative ions ( anions ) and this determines the formula for that compound.
Example: Sodium chloride (NaCl, 'table salt') is composed of sodium cations ( Na + ) and chloride anions ( Cl ). For the charges to balance there must be equal numbers of cations and anions so that the formula must be NaCl. However there is no distinct, finite NaCl molecule – in the solid no single Na + cation can be considered to be associated with just one Cl anion - each cation is surrounded by six identical anions and each anion by six identical cations.

Atomic Mass: The atoms of the ca. 118 known elements each have a characteristic mass – the atomic mass (aka atomic weight). The masses of atoms are conveniently expressed in a tomic m ass u nits (abbreviated ‘amu’).
The amu is defined as exactly
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of the mass of the most common form of the carbon atom, i.e.
12
,
6
C.
On this scale hydrogen, H, has a mass of 1 amu and the heaviest presently known element has a mass of around
290 amu .
Listings of atomic masses are found in the inside cover of most First Year chemistry textbooks.
The given atomic masses of the naturally-ocurring elements are actually average figures. This is because naturally occurring elements generally exist as a mixture of so-called isotopes.
Isotopes are atoms with identical chemical properties but different masses which are present in nature in different relative ratios depending on the element involved.

Example: Magnesium, Mg.
Naturally-occurring magnesium contains three different isotopes:
Atomic mass 23.98 amu 78.70%
24.98
10.13%
25.98
11.17%
Consider 10000 magnesium atoms:
7870 will have a mass of 23.98 amu = 188722.6 amu
1013 will have a mass of 24.98 amu = 25304.7 amu
1117 will have a mass of 25.98 amu = 29019.6 amu
10000 atoms have a total mass of 243046.9 amu
Therefore the weighted average mass of a single magnesium atom will be 24.304 amu.

Molecular Mass (aka Molecular Weight) : The mass of a molecule is simply the sum of the atomic masses of the constituent atoms:
H
2
: 2 x 1.008 = 2.016 amu
CO
2
: 12.011 + (2 x 15.99) = 43.99 amu
C
6
H
12
O
6
: (6 x 12.011) + (12 x 1.008) + (6 x 15.999) =
180.00 amu
For non-molecular substances a formula mass ( aka formula weight ) is determined from the empirical formula
(i.e. the smallest whole-number ratio of atoms involved):
Silicon carbide, SiC: 28.086 + 12.011 = 40.10 amu
Calcium Chloride, CaCl
2
: 40.08 + (2 x 35.453) = 110.99 amu.
The Mole Concept:
So far, atomic mass units (amu) have been used for atomic and molecular masses, i.e. the masses of 1 atom, 1 molecule or 1 formula unit. These mass values, however, have practical limitations, because chemical reactions are not normally carried out on individual atoms or molecules.

In practice, chemical reactions are carried out on many billions of atoms or molecules at a time. For this reason a more practically useful unit, the mole , abbreviated as mol and defined as 6.02 x 10 23 (known as Avogadro's
Number , symbol N ) of atoms, molecules or formula units, was introduced.
Why did Avogadro choose the number 6.02 x 10 23 ?
Because 6.02 x 10 23 is the number of atomic mass units
(amu) in 1g. Therefore the mass in grams of a mole of atoms, molecules or formula units is numerically the same as the corresponding atomic, molecular or formula unit masses in amu.
Example:
Mass of one Sodium (Na) atom = 22.99 atomic mass units
Mass of one mole of Na atoms = 22.99 x 6.02 x 10
23
amu
Converting the mass of a mole of Na atoms in amu to mass in grams using the conversion factor 6.02 x 10
23
amu = 1g:
22.99 x 6.02 x 10
6.02 x 10
23
amu
23
amu g
-1
= 22.99 grams

Example:
HCl, hydrogen chloride:
Molecular mass = 1.008 + 35.453 = 36.45 amu
Mass of 1 mol of HCl molecules = 36.45 g
When using moles in chemical calculations it is essential to be clear about the precise chemical species that you are dealing with.
Example:
Cl, chlorine atom:
Mass of one chlorine atom = 35.45 amu
Mass of one mol of Cl atoms = 35.45 g
Cl
2
, chlorine molecule:
Mass of one chlorine molecule = 70.9 amu
Mass of one mol of Cl
2 molecules = 70.9 g

REVISION
1.
One mole of any chemical species (atom, molecule, formula unit) contains Avogadro's Number , i.e. 6.02 x
10 23 , of that species.
2.
The mass in atomic mass units (amu) of any individual chemical species (i.e. atom, molecule or formula unit) is numerically equal to the mass in grams of one mole of that chemical species (i.e. atom, molecule or formula unit) .
3.
In working with moles in chemical calculations it is essential to specify precisely the chemical species that you are dealing with.
4.
The mass of a mole of atoms in grams is called the gram atomic weight , the mass of a mole of molecules in grams is called the gram molecular weight and the mass of a mole of formula units is called the gram formula weight . These are commonly - though less precisely - referred to simply as atomic weight, molecular weight and formula weight .