Chemistry is the study of the combining of atoms to form compounds and the study of the
properties of the matter made from compounds.
Compounds and Molecules
Atoms combine chemically to form compounds.
Two types of compounds – molecular and non-molecular.
Molecular compounds contain molecules.
Non-molecular compounds are ionic compounds, giant molecules, metals or monoatomic.
Molecules and molecular compounds
A molecule contains a number of atoms held together by chemical bonding.
A molecule contains a definite number of atoms in definite proportions.
NO – nitric oxide
NO2 – nitrogen dioxide
N2O – nitrous oxide
N2O5 – dinitrogen pentaoxide
are all different molecules having different chemical properties.
A molecule has a molecular formula which shows the number of atoms in the molecule.
H2O, C6H6, NaHSO4, CH3CO2H …
A molecule has mass, volume and shape
A molecule has mass which is the sum of the mass (A) of the atoms in the molecule.
N2O is linear NNO;
NO2 is bent (V shaped);
N2O5
O
O
O
N
N
O
O
Molecules containing only one type of atom are called elements.
e.g. oxygen gas O2; hydrogen gas H2; sulphur S8 …
Molecules - Chemical Bonding
Molecules are formed when atoms are bonded together.
Two atoms are bonded together if they share electrons.
H – Cl : the line represents electrons shared between the atomic nuclei. The negatively
charged electrons are attracted to both, positively charged, nuclei.
This ‘holds’ the nuclei together – a chemical bond.
Chemical Bond
= H
1H-
1
electron
around
nucleus of
1 proton
Cl
35Cl
- 17
electrons
around nucleus
of 17 protons
+18 neutrons
hydrogen chloride molecule
1 atom of hydrogen (H) bonded to 1 atom of chlorine (Cl):
HCl
Water molecule, 2 atoms of hydrogen bonded to 1
atom of oxygen:
H2 O
O
H
H
Sugar (glucose) molecule, six atoms of carbon, twelve atoms of hydrogen and six atoms
of oxygen, bonded together: C6H12O6
The number of known molecules is huge.
A molecule has physical and chemical properties that are different from those of the
constituent atoms.
Examples of molecules.
Diatomics: HCl, O2, H2, N2, CO carbon monoxide, NO nitric oxide
Tiatomics: H2O, CO2 carbon dioxide, O3 ozone, H2S hydrogen sulphide (rotten eggs)
Polyatomics - millions of them – NH3 ammonia, H2SO4 sulphuric acid,
A molecule is electrically neutral. The positive charges on the nuclei are balanced by the
negative charges on the electrons.
The Chemical Bonding of atoms in molecules is due to the sharing or exchange of the
electrons of the constituent atoms. Covalent Bonding.
Chemical bonding does not change the nuclei or the electrons of the atoms. The nuclei
and the electrons of the atoms in the molecule are present in the molecule.
Chemical bonding rearranges the distribution of the electrons around the nuclei.
Historically, two ‘laws’ were identified:
1) Law of conservation of mass. Mass cannot be created or destroyed by a chemical
process. (Of course: the mass is almost entirely in the nuclei.)
2) Law of simple proportions. Atoms combine in simple (i.e. integer) proportions to form
molecules. (Of course: molecules contain integer numbers of atoms – no ‘bits’ of atoms
are possible.)
Molecular mass and molecular formula
Use information on the mass of atoms in a compound to determine the molecular
formula.
e.g. the mass of water is found to be due to 11.11% hydrogen and 88.88% oxygen.
Therefore 100 amu of water contains 11.11 amu of H and 88.88 amu of O.
Convert from amu to numbers of atoms by dividing by atomic mass, A, and simplifying:
(11.11/1) atoms of H combine with (88.88/16) atoms of O
11.11 atoms of H combine with 5.55 atoms of O
2 atoms of H combine with 1 atom of O
to form water.
Formula of water: H2O
What is the %age by mass of nitrogen in N2O5?
Atomic masses: N 14am
O 16amu
Molecular mass of N2O5 = {2x14 + 5x16}
= 108amu
Therefore the %age by mass of N = 100 x (2x14) / 108
= 25.92%
As a measure of the mass of molecules the amu is too small a unit of mass. The following
defines the mole which is the bulk scale equivalent of molecular mass.
Define Avogadro’s Number, NA = 6.022 x 1023
Define 1 mole – a mole of X contains NA of X.
The molecular mass of a compound in grams contains 1 mole of that compound.
1 mole of water has a mass of 18 g mol-1
360 g of sugar is 1 mole of sugar, contains NA molecules of sugar.
(C6H12O6: 6x12+12x1+6x16 = 360 amu)
1 gram of hydrogen contains NA atoms of hydrogen.
Hydrogen has an atomic mass of 1 amu =1.672 x 10-27 kg,
NA atoms of hydrogen have a mass of
6.022 x 1023 x 1.672 x 10-27 kg = 1.00 x 10-3 kg = 1 g mol-1
1 mole of HCl molecules contains 1 mole of H atoms and 1 mole of Cl atoms.
(Try removing the words ‘mole of ’.)
1 mole of N2O5 molecules contains 2 moles of N atoms and 5 moles of O atoms.