Ato m s a n d e l e m e n t s
3.1 Atoms and elements
Atoms
Sodium is made of tiny particles
called sodium atoms.
Diamond is made of carbon atoms –
different from sodium atoms.
Mercury is made of mercury
atoms – different again!
Atoms are the smallest particles of matter, that we cannot break
down further by chemical means.
Single atoms are far too small to see. Perhaps a million sodium atoms
could fit in a line across this full stop. So you can see sodium only if there
are enough sodium atoms together in one place!
In fact atoms are mostly empty space. Each consists of a nucleus and a
cloud of particles called electrons that whizz around it. This drawing
shows how a sodium atom might look, magnified many millions of times.
nucleus
electron
cloud
The elements
Sodium is made of sodium atoms only, so it is an element.
An element contains only one kind of atom.
Around 90 elements have been found in the Earth and atmosphere.
Scientists have made nearly 30 others in the lab. Many of the ‘artificial’
elements are very unstable, and last just a few seconds before breaking
down into other elements. (That is why they are not found in nature.)
Symbols for the elements
To make life easy, each element has a symbol. For example the symbol for
carbon is C. The symbol for potassium is K, from its Latin name kalium.
Some elements are named after the people who discovered them.
 This painting shows Hennig Brand,
who discovered the element phosphorus,
in 1669. It glows in the dark!
 Collecting the element sulfur from
a volcano crater in Indonesia. It is used
as an ingredient in many cosmetics.
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Ato m s a n d e l e m e n t s
The Periodic Table
Group
2
3
II
7
Li
3
9
Be
4
lithium
39
K
19
potassium
5
40
Ca
20
45
Sc
21
calcium
rubidium
strontium
48
Ti
22
51
V
23
scandium
titanium
vanadium chromium manganese
89
Y
39
91
Zr
40
93
Nb 96
Mo 99
Tc
41
42
43
zirconium
niobium molybdenum technetium ruthenium
yurium
6
133
Cs 137
Ba 139
La 178.5
Hf 181
Ta
55
56
57
72
73
7
223
Fr
87
caesium
francium
barium
V
VI
11
B
5
12
C
6
14
N
7
16
O
8
27
13
The transition elements
magnesium
88
Sr
38
IV
boron
beryllium
85
Rb
37
III
hydrogen
23
Na 24
Mg
11
12
sodium
4
1
I
lanthanium
hafnium
tantalum
0
Group
1
H
1
52
Cr
24
55
Mn 56
Fe
25
26
59
Co 59
Ni
27
28
iron
cobalt
101
Ru 103
44
45
rhodium
184
W 186
Re 190
Os 192
74
75
76
77
tungsten
rhenium
osmium
nickel
Rh 106
Pd
46
Ir
iridium
palladium
195
Pt
78
platinum
64
Cu 65
Zn
29
30
copper
zinc
Al
cadmium
mercury
oxygen
32
16
S
fluorine
He
helium
20
Ne
10
neon
35.5
Cl 40
17
18
Ar
silicon
phosphorus
sulfur
chlorine
argon
70
Ga
31
73
Ge
32
75
As
33
79
Se
34
80
Br
35
84
Kr
36
germanium
arsenic
selenium
gallium
In
indium
197
Au 201
Hg 204
79
80
81
gold
Si
nitrogen
31
P
15
19
F
9
4
2
aluminium
108
Ag 112
Cd 115
47
48
49
silver
carbon
28
14
VII
Tl
thallium
119
50
Sn 122
Sb 128
Te
51
52
tin
207
82
antimony
Pb 209
Bi
83
lead
bismuth
tellurium
210
84
Po
polonium
bromine
127
53
I
iodine
210
85
At
astatine
krypton
131
54
Xe
xenon
222
86
Rn
radon
226
Ra 227
Ac
88
89
radium
actinuim
Lanthanides
Actinides
140
Ce 141
Pr
58
59
cerium
144
Nd 147
Pm 150
Sm 152
Eu 157
Gd 159
60
61
62
63
64
65
praseodymium neodymium promethium
232
Th 231
Pa 238
U
90
91
92
thorium
protactinium
uranium
samarium
237
Np 244
93
94
europium gadolinium
Tb 162
Dy 165
Ho 167
Er
66
67
68
terbium
dysprosium holmium
Pu 243
Am 247
Cm 247
Bk 251
Cf
95
96
97
98
neptunium plutonium americium
curium
252
99
Es
berkelium califormium einsteinium
erbium
257
100
169
69
Tm 173
Yb
70
thutium
ytterbium
175
71
Lu
lutetium
Fm 258
Md 259
No 262
Lw
101
102
103
fermium mendelevium nobelium lawrencium
The table above is called the Periodic Table.
 It gives the names and symbols for the elements.
The column and row an element is in gives us lots of clues about it.
For example, look at the columns numbered I, II, III …
The elements in these form families or groups, with similar properties.
So if you know how one element in Group I behaves, for example, you
can make a good guess about the others.
 The rows are called periods.
Look at the zig-zag line. It separates metals from non-metals, with the
non-metals on the right of the line, except for hydrogen. So there is a
change from metal to non-metal, as you go across a period.
Now look at the small numbers beside each symbol. These tell us a lot
about the atoms of the element, as you will soon see.
Q
1 What is: a an atom? b an element?
2 If you could look inside an atom, what would you see?
3 The symbols for some elements come from their
Latin names. See if you can identify the element whose
Latin name is:
a natrium b ferrum c plumbum d argentum
 The element chlorine is a poisonous
gas. It was used as a weapon in World
War I. This soldier was prepared.
4 Which element has this symbol? a Ca b Mg c N
5 See if you can pick out an element named after the famous scientist Albert Einstein.
6 From the Periodic Table, name
a three metals a
three non-metals
that you expect to behave in a similar way.
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Ato m s a n d e l e m e n t s
3.2 More about atoms
Protons, neutrons, and electrons
Atoms consist of a nucleus and a cloud of electrons that move around
the nucleus. The nucleus is itself a cluster of two kinds of particles,
protons and neutrons.
All the particles in an atom are very light. So their mass is measured in
atomic mass units, rather than grams. Protons and electrons also have
an electric charge:
Particle in atom
Mass
Charge
proton
1 unit
positive charge (11)
neutron
1 unit
none
electron
almost nothing
negative charge (12)
Since electrons are so light, their mass is usually taken as zero.
How the particles are arranged
The sodium atom is a good one to start with. It has 11 protons,
11 electrons, and 12 neutrons. They are arranged like this:
 The nucleus is very tiny compared
with the rest of the atom. If the atom
were the size of a football stadium, the
nucleus would be the size of a pea!
the protons and neutrons cluster
together in the centre, forming
the nucleus; this is the heavy
part of the atom
the electrons circle very fast
around the nucleus, at
different energy levels from it;
these energy levels are called
shells
Note
!
Since they make up the atom,
protons, neutrons and electrons are
often called sub-atomic particles.
Proton number
A sodium atom has 11 protons. This can be used to identify it, since only
a sodium atom has 11 protons. Every other atom has a different number.
You can identify an atom by the number of protons in it.
The number of protons in an atom is called its proton number.
The proton number of sodium is 11.
How many electrons?
The sodium atom also has 11 electrons. So it has an equal number of
protons and electrons. The same is true for every sort of atom:
Every atom has an equal number of protons and electrons.
So atoms have no overall charge.
Look at the box on the right. It shows that the positive and negative
charges cancel each other, for the sodium atom.
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The charge on a sodium atom:
11 protons
Each has a charge of 11
Total charge 111
××××
11 electrons
××××
Each has a charge of 12
×××
Total charge 112
Adding the charges: 111
112
0
The answer is zero.
The atom has no overall charge.
!
Ato m s a n d e l e m e n t s
Nucleon number
Protons and neutrons form the nucleus, so are called nucleons.
The total number of protons and neutrons in an atom is called
its nucleon number.
The nucleon number for the sodium atom is 23. (11 1 12 5 23)
So sodium can be described in a short way like this:
23
​ ​Na.
11
You can describe any element in a
short way like this:
nucleon number
proton number
The lower number is always the proton number. The other number is the
nucleon number. So you can tell straight away that sodium atoms have
12 neutrons. (23 2 11 5 12)
!
Try it yourself!
symbol
For example: 16
​ 8​O
The atoms of the first 20 elements
In the Periodic Table, the elements are arranged in order of increasing
proton number. Here are the first 20 elements, shown as a list:
Element
Symbol
Proton number
Electrons
Neutrons
Nucleon number
(protons 1 neutrons)
hydrogen
H
1
1
0
1
helium
He
2
2
2
4
lithium
Li
3
3
4
7
beryllium
Be
4
4
5
9
boron
B
5
5
6
11
carbon
C
6
6
6
12
nitrogen
N
7
7
7
14
oxygen
O
8
8
8
16
fluorine
F
9
9
10
19
Ne
10
10
10
20
neon
sodium
Na
11
11
12
23
magnesium
Mg
12
12
12
24
aluminium
Al
13
13
14
27
silicon
Si
14
14
14
28
phosphorus
P
15
15
16
31
sulfur
S
16
16
16
32
chlorine
Cl
17
17
18
35
argon
Ar
18
18
22
40
potassium
K
19
19
20
39
calcium
Ca
20
20
20
40
So the numbers of protons and electrons increase by 1 at a time – and are
always equal. What do you notice about the number of neutrons?
Q
1 Name the particles that make up the atom.
2 Which particle has:
a a positive charge? b no charge? c almost no mass?
3 An atom has 9 protons. Which element is it?
4 Why do atoms have no overall charge?
5 What does this term mean? b nucleon number
a proton number
6 Name each of these atoms, and say how many protons,
electrons, and neutrons it has:
16
64
12
24
27
​ 6​C ​
​O ​
​Mg ​
​Al ​
​Cu
8
12
13
29
33