Junior Certificate Chemistry
Chapter 26 – The Atom
An atom is made up of protons, neutrons
and electrons. The protons and neutrons
are found in a small, dense nucleus in the
centre of the atom with the electrons
orbiting the nucleus in circular orbits
rather like the planets orbiting the sun.
This is often referred to as the Bohr
model of the atom as it was Neils Bohr (a
Danish scientist) who first proposed it.
Name
Found
Proton
Neutron
Electron
Nucleus
Nucleus
Cloud
Mass
[amu]
1
1
1/1850
4. The electrons are arranged in
circular orbits around the nucleus.
These orbits are sometimes called
shells. The electrons are arranged
in these shells as follows.
1st
2nd
3rd
4th
2 electrons
8 electrons
8 electrons
Remainder
Example; Draw an atom of sodium
given the following information,
23
11
Charge
shell
shell
shell
shell
Na
From the two numbers given with this
element we can get all of the information
we need to draw an atom of sodium.
+1
0
-1
You must be able to draw a simple
diagram of an atom if given the
necessary information. The information
you need to know can be summarised as
follows.
1. Atomic Number is the number of
protons in the nucleus of an atom.
This number can be obtained from
the periodic table. Every element
has two numbers assigned to them
in this table, the smaller one is
the atomic number.
2. Mass number is the number of
protons and neutrons in the
nucleus of an atom. This number is
the bigger number beside the
element in the P.T.E.
3. Number of protons = number of
electrons in a neutral atom.
Number of protons in
nucleus
11
Number of electrons
11
Number of neutrons in
the nucleus
23 – 11 = 12
We can now draw the atom of sodium in
the following manner. The nucleus is
drawn at the centre with the number of
protons and neutrons indicated. The
electrons are drawn as dots and are
arranged in circles (also called orbits or
shells) with two electrons in the first
shell, eight in the second and the
remaining one in the third shell.
1
G. Nugent
Junior Certificate Chemistry
The electron configuration can written
very simply as,
Cl; 2, 8,7
The configurations for the first twenty
elements can be written in a similar
fashion.
11P
12N
Element
Symbol
Configuration
Hydrogen
H
1
Helium
He
2
Lithium
Li
2,1
Beryllium
Be
2,2
Boron
B
2,3
Carbon
N
2,4
Nitrogen
C
2,5
Oxygen
O
2,6
Fluorine
Fl
2,7
Neon
Ne
2,8
Sodium
Na
2,8,1
Magnesium
Mg
2,8,2
Aluminium
Al
2,8,3
Silicon
Si
2,8,4
Phosphorus
P
2,8,5
Sulphur
S
2,8,6
Chlorine
Cl
2,8,7
Argon
Ar
2,8,8
Potassium
K
2,8,8,1
Calcium
Ca
2,8,8,2
Isotopes
Not all atoms of an element are identical.
The number of protons and electrons
present in an atom never varies for an
atom of an element but, sometimes,
there are small differences in the
number of neutrons in the nucleus.
Isotopes are atoms of the same
element which have different numbers
of neutrons in their nuclei.
This type of drawing of an atom is often
referred to as the Bohr model of the
atom.
The electronic configuration of an
element
The arrangement of electrons in an atom
is called its electronic configuration.
This configuration may be shown using
‘circle-and-dots’ diagrams like the one
above but a simpler method is often
used.
Example; Write the electronic
configuration of chlorine.
Solution;
Chlorine is element number 17 in the
periodic table. This means that it
contains 17 electrons.
The electrons will be arranged as
follows,
Shell number
1
2
3
Number of
electrons
2
8
7
2
G. Nugent