Chemistry Reference
In-Depth Information
3
Atomic structure and bonding
Inside atoms
Proton number and nucleon number
Ions
Isotopes
The mass spectrometer
Relative atomic mass
The arrangement of electrons in atoms
Covalent bonding
Other covalent compounds
Covalent structures
Properties of covalent compounds
Allotropy
Allotropes of carbon
Glasses and ceramics
Glasses
Ceramics
Ionic bonding
Ionic structures
Properties of ionic compounds
Formulae of ionic substances
Oxidation states
Metallic bonding
Properties of metals
Checklist
Additional questions
We have already seen in Chapter 2 that everything
you see around you is made out of tiny particles
called atoms (Figure 3.1). When John Dalton
developed his atomic theory, about 200 years ago
(1807/1808), he stated that the atoms of any one
element were identical and that each atom was
'indivisible'. Scientists in those days believed that
atoms were solid particles like marbles.
●
Inside atoms
The three sub-atomic particles are found in distinct
and separate regions. The protons and neutrons are
found in the centre of the atom, which is called the
nucleus
. The neutrons have no charge and protons
are positively charged. The nucleus occupies only a
very small volume of the atom but is very dense.
The rest of the atom surrounding the nucleus is
where electrons are most likely to be found. The
electrons are negatively charged and move around
very quickly in
electron shells
or
energy levels
. The
electrons are held within the atom by an
electrostatic
force of attraction
between themselves and the
positive charge of protons in the nucleus (Figure 3.2).
nucleus, containing
neutrons and protons
region where electrons
are found
Figure 3.1
A micrograph of atoms.
However, in the last hundred years or so it has been
proved by great scientists, such as Niels Bohr, Albert
Einstein, Henry Moseley, Joseph Thomson, Ernest
Rutherford and James Chadwick, that atoms are in
fact made up of even smaller 'sub-atomic' particles.
The most important of these are
electrons
,
protons
and
neutrons
, although 70 sub-atomic particles have
now been discovered.
Figure 3.2
Diagram of an atom.
About 1837 electrons are equal in mass to the mass
of one proton or one neutron. A summary of each
type of particle, its mass and relative charge is shown
in Table 3.1. You will notice that the masses of all
these particles are measured in
atomic mass units
(amu)
. This is because they are so light that their
masses cannot be measured usefully in grams.
