2. State the relative charges and approximate relative
masses of protons, neutrons and electrons.
Define proton number (atomic number) as the number
of protons in the nucleus of an atom.
Define nucleon number (mass number) as the total
number of protons and neutrons in the nucleus of an
atom.
Use proton number and the simple structure of atoms
to explain the basis of the Periodic Table (see section
9), with special reference to the elements of proton
number 1 to 20
3. Atoms are the smallest
particles of matter, that
we can not break down
further by chemical
means.
Atoms consist of a
nucleus containing
protons and neutrons,
surrounded by
electrons in shells.
4. An atom has a central
nucleus.
This is surrounded by shells
like orbits around the sun in
solar system.
The nucleus is tiny compared
to the atom as a whole.
Subatomic particles
The nuclei of all atoms
contain subatomic particles
called protons. The nuclei of
most atoms also contain
neutrons.
This is surrounded by
electrons arranged in shells.
Shells are also called energy
levels.
5. The masses of subatomic particles are very tiny.
Instead of writing their actual masses in
kilograms, we often use their relative masses.
The relative mass of a proton is 1, and a particle
with a relative mass smaller than 1 has less
mass.
The mass of an electron is very small compared
to a proton or a neutron. Since the nucleus
contains protons and neutrons, most of the mass
of an atom is concentrated in its nucleus.
Protons and electrons have electrical charges that
are equal and opposite.
6. Subatomic
particle
Relative mass Relative charge
Proton 1 +1
Neutron 1 0
Electron
Very small
(I/1840)
-1
Remember that Protons are Positive, and
Neutrons are Neutral.
7. Atomic number
The number of protons in an atom of an element
is its atomic number. Remember that:
all atoms of a given element have the same
number of protons
atoms of different elements have different
numbers of protons
An atom contains equal numbers of protons and
electrons. Since protons and electrons have equal
and opposite charges, this means that atoms are
have no overall electrical charge. Atoms are
overall NEUTRAL
For example, the atomic number of sodium is
11. Every sodium atom has 11 protons and 11
electrons. It has 11 positive charges and 11
negative charges. So overall charge is zero.
8. The mass number of an atom is its total
number of protons and neutrons in the
nucleus.
Atoms of different elements usually have
different mass numbers, but they can be the
same. For example, the mass number of
argon atoms and calcium atoms can both be
40.
9. The symbol for an atom can be written to show its
mass number at the top, and its atomic number at
the bottom.
To calculate the numbers of subatomic particles in
an atom, use its atomic number and mass number:
number of protons = atomic number
number of electrons = atomic number
number of neutrons = mass number - atomic
number
10. The atomic number of a
sodium atom is 11 and
its mass number is 23.
Calculate the number of
protons, neutrons and
electrons it contains.
11. An electron arrangement is the way in which
electrons are arranged in an atom.
Electrons in shells
Electrons occupy different energy levels, or shells.
Each shell can hold a certain maximum number of
electrons.
Moving through the elements in the periodic table,
each atom has one more electron than the last
because the number of electrons is the same as the
atomic number.
Electrons occupy the shells in order, starting with the
shell that is nearest the nucleus. They begin to
occupy the next shell only when this shell becomes
full.
13. The electron arrangement of an atom
can be worked out from its atomic
number. For example, the atomic
number of sodium is 11. Sodium
atoms have 11 protons and so 11
electrons.
2 electrons occupy the first shell
8 electrons occupy the second shell
1 electron occupies the third shell
This electron arrangement can be
written as 2.8.1 (each dot separates
one shell from the next). It can also
be shown as a diagram. In these
diagrams:
each shell is modelled as a circle
each electron is modelled as a dot or
a cross
14. The electron arrangement of an element is
related to its position on the periodic table.
Electronic structure feature Link to the periodic table
Number or numbers of shells Period number
Number of electrons in
outermost shell
Group number
Total number of electrons in
all shells
Atomic number
The electron arrangement of sodium (2.8.1) shows that sodium, Na:
is in period 3
is in group 1
has an atomic number of (2 + 8 + 1) = 11
16. We have learnt about the relative charges and
approximate relative masses of protons, neutrons and
electrons.
We learnt to Define proton number (atomic number) as
the number of protons in the nucleus of an atom and that
nucleon number (mass number) is the total number of
protons and neutrons in the nucleus of an atom.
We learnt how to use proton number and the simple
structure of atoms to explain the basis of the Periodic
Table, with special reference to the elements of proton
number 1 to 20