Structure of Atoms.
Originally scientists thought that atoms
could not be split into anything smaller.
We now know that atoms are made up
of smaller particles called:
• Protons
• Electrons
• Neutrons
Properties of protons, electrons & neutrons
•Protons have a charge of 1+ (one positive),
and a mass of 1 a.m.u. (one atomic mass unit).
•Electrons have a charge of 1- (one negative)
and are so light we give them a mass of 0 amu.
•Neutrons have no charge (neutral) and have a
mass of 1 a.m.u.
Where in the atom are the protons, neutrons
and electrons?
The protons and the
neutrons are found in
the centre of the
atom – in a place
called the nucleus.
The nucleus is very
small.
The electrons spin
round the nucleus.
Summary so far.
Particle
Charge
Mass (amu)
Where in atom?
Proton
1+
1 amu
Inside nucleus
Electron
1-
0 amu
Circling nucleus
Neutron
0
1 amu
Inside nucleus
Atoms are neutral.
Atoms do not have an overall charge – they
are neutral.
This means that atoms must have the same
number of protons (positive charges) and
electrons (negative charges).
nucleus with
4 electrons
spinning round
the nucleus
4 protons
Atomic Number.
The atomic number tells us the number of
protons in the nucleus of an atom.
As atoms are neutral, the atomic number also
gives the number of electrons in an atom.
It is the atomic number which identifies
which element is present e.g.
Hydrogen atoms have an atomic number of 1
Helium atoms have an atomic number of 2
Mass number.
The mass number tells us the number of
protons + the number of neutrons in an atom.
Showing atomic and mass numbers.
Chemists can show the atomic and mass numbers of
an element in the following way:
Mass number
23
11
Atomic number
Symbol of the element
Na
Calculating the number of protons, neutrons
and electrons in an atom.
Number of neutrons = 80 – 35 = 45
Mass number = 80
80
35
Br
Atomic Number = 35
Number of protons = 35
Number of electrons = 35
Arrangement of electrons in atoms.
The electrons in atoms are arranged in
energy levels round the nucleus.
The first energy level (lowest energy) can
hold up to 2 electrons.
1 electron
Nucleus with
1 proton
in first
energy level
1
1
H
2 electrons
Nucleus with
2 proton +
in first
energy level
2 neutrons
4
2
He
The second electron energy level.
The second electron energy level in an atom can hold
up to 8 electrons.
The diagram shows an atom of
2 electrons
in first
energy level
7 electrons
in second
energy level
19
fluorine, 9
F
Nucleus with
9 protons +
10 neutrons
The electron arrangement in
this atom would be written as
2,7
The third electron energy level.
The third electron energy level in an atom can also
hold up to 8 electrons.
The diagram below shows an atom of
31
15
P
Nucleus containing
15 protons and 16 neutrons.
2 electrons in first energy level
5 electrons in
third energy
level
8 electrons in second energy level
The electron arrangement is shown as 2,8,5
Isotopes.
Isotopes are atoms having the same atomic number
(number of protons) but different mass numbers
(number of neutrons).
Shown below are the three isotopes of hydrogen.
Nucleus with
1 proton and
Nucleus with
1 proton and
Nucleus with
1 proton and
no neutrons
1 neutron
2 neutrons
1
1
H
2
1
H
3
1
H