IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5))
Revision Notes
(1.9) Atomic Structure
All substances are made up of atoms - atoms are very small - 1/1,000,000,000 cm (one billionth)
Substances with only one type of atom are elements
However:
Atoms are not the smallest particles
Atoms are actually made up of sub-atomic particles arranged in a structure with a
nucleus and an outer shell
In the Nucleus:
Protons - Positively charged (+ve)
Neutrons - Not charged (neutral)
together these are
called Nucleons
In the outer shell(s):
Electrons - Negatively charged (-ve)
LEARN THIS:
(1.10 / 1.11 / 2.4) Atomic Number, Mass Number, Relative Atomic Mass & Isotopes:
• Atoms are usually neutral - the number of protons (+ve) is equal to the number of electrons (-ve)
• The atomic number (or proton number) of any element is the number of protons in the nucleus
of the element - it is fixed and unique - only one element has that atomic number
• The mass number (or nucleon number) of a particular atom of an element is equal to the
number of protons plus the number of neutrons in the nucleus (i.e. the total number of
nucleons) - this may vary in different isotopes of the element (see below)
• The number of neutrons in the nucleus is roughly equal to the number of protons but this may
vary to produce different isotopes of the same element
• The relative atomic mass (RAM or Ar) of any element is the weighted average mass of the
isotopes of the element - based on the carbon-12 scale, where the isotope carbon-12 has a mass
of exactly 12 units (calculations of Ar are based on the percentages of different isotopes in any
given sample of the element - see below (see also: Edexcel Chemistry p177)
• Electrons orbit at a relatively large distance from the nucleus (atoms are mainly empty space) - their
mass is roughly 1/2000 (1/1836) that of a proton (2,000 electrons have the same mass as 1 proton)
the mass of the electrons is therefore negligible and can be ignored
• The electrons are arranged in concentric shells orbiting the nucleus:o The inner shell can contain a maximum of two electrons
this 2, 8, 8 rule only applies
o The outer shells can contain a maximum of eight electrons
to the first 20 elements
• (2.4) The number of electrons in the outer shell (Valence Electrons) gives the element’s group
number (vertical column) and explains why elements in the same group have similar properties
• The number of shells orbiting the nucleus gives the element’s period number (horizontal row)
• (1.13 / 2.1) The discovery of this atomic structure of the elements allows them to be arranged in
the Periodic Table, where the vertical columns are Groups and the horizontal rows are Periods
IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5))
Revision Notes
(1.14 / 1. 15) Deductions based on Atomic Configuration & the Periodic Table:
1. Look at the diagram of the atom above:
There are two outer ‘shells’ orbiting the nucleus (2, 5) - this element is in period 2
There are five electrons in the outer shell (2, 5) - this element is in group 5
The number of electrons (7) equals the number of protons - the atomic number is 7
The element must be nitrogen (group 5, period 2)
14
Relative Atomic Mass (Ar)
N
Element symbol
7
And... Because the atomic mass is 14
and the atomic number is 7
there must be 7 neutrons in the nucleus (14 - 7 = 7)
Atomic Number
2. Look at the Periodic Table - you should know the symbols of the elements in bold
You need to be able to “read” the information about each element as follows:
Atomic number: 11 - there are 11 protons in the nucleus
(Proton Number: Z)
- there are 11 electrons arranged in three shells: 2,8,1
- therefore, it’s in period 3 / group 1
Atomic Mass: 23
- there must be 12 neutrons in the nucleus (23 - 11)
23
Na
11
(Nucleon Number: A)
OR:
Atomic Number: 17 - there are 17 protons in the nucleus
- there are 17 electrons arranged in three shells: 2,8,7
- therefore, it’s in period 3 / group 7 (it’s Chlorine)
Atomic Mass: 23
- there must be 18 neutrons in the nucleus (35 - 17)
35
X
17
(2.2)
Periodic Table of the Elements - Metals & Non-metals
I
VIII
1
HALOGENS
2
He
II
III
IV
V
VI
VII
7
9
11
12
14
16
19
20
Li
Be
B
C
N
O
F
Ne
3
ALKALI EARTH METALS
4
5
23
TRANSITION METALS
Na Mg
11
6
24
12
7
8
9
28
31
32
35
40
Al
Si
P
S
Cl
Ar
79
17
80
14
15
75
16
40
45
48
51
52
55
56
59
59
63
65
70
73
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
26
101
27
103
28
105
29
108
30
112
31
115
32
119
33
122
34
128
35
127
36
131
20
85
Rb
37
88
Sr
38
21
89
Y
39
22
23
91
Zr
40
24
93
25
96
Nb Mo
41
42
99
Tc
43
Ru
44
Rh
45
Pd
46
Ag
47
Cd
48
In
49
Sn
50
Sb
51
Te
52
53
I
84
Xe
133
137
139
178
181
184
186
190
192
195
197
200
204
207
208
2O9
210
222
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
55
223
56
226
57
227
72
261
73
262
74
266
75
264
76
277
77
268
78
281
79
272
Fr
Ra
Ac
Rf
Sg
Bh
Hs
Mt
Ds
Rg
87
ALKALI
METALS
88
89
104
Db
105
Metals
106
107
108
109
110
81
82
83
84
85
5
6
86
NOBLE GASES
111
‘Between’ Metals (Metaloids)
4
54
Cs
80
3
18
39
19
2
10
27
13
1
Non-metals
The elements are arranged in order of their Atomic Number - i.e. the number of Protons in the
nucleus, which is equal to the total number of Electrons in the outer shells - You need to know the
element families: Alkali Metals (group 1), Alkali Earth Metals (group 2), Transition Metals
(between groups 2&3), Halogens (group 7) & Noble Gases (group 8) – These are also called the
Inert Gases (inert = unreactive – because they have a ‘full’ outer shell – 2.5)
7
PERIODS
1
4
GROUPS
H
IGCSE Chemistry (1.9 to 2.2 & 2.4 & 2.5))
Revision Notes
(1.11) More on Isotopes and their Uses
Isotopes are varieties of an element that have different numbers of neutrons in the nucleus
They have the same Atomic Number but different Mass Numbers
In other words, they have different atomic masses and may be unstable or radioactive
However, the atomic number and the number of protons in the nucleus are the same
For Example:
6
7
Li
3
8
Li
3
Li
3
- different atomic masses:
3 Isotopes of Lithium
- 4, 3, & 5 neutrons in the nuclei (‘Lithium-7’, ‘Lithium-6’
- same atomic number
& ‘Lithium-8’)
(‘Common’ Lithium)
Different isotopes of an element may have different properties and therefore different uses
For Example:
Carbon dating: Carbon-14 (14C) is a radioactive isotope (radioisotope) - its nucleus is unstable radiation is released from the nucleus as it changes to carbon-13 - (12C & 13C nuclei are stable and
not radioactive)
Nuclear Power Generation, Nuclear Bombs and Nuclear Reactors in submarines and aircraft
carriers make use of radioactive Uranium-235 (235U) and Plutonium-238 (238Pu)
Radioisotopes are widely used in medicine for the diagnosis and treatment of diseases (notably
cancer) and injected or ingested before body scans to make certain tissues more easily visible
Radioactive tracers are widely used in industry to trace blockages and leaks; to show cracks in
metals and faults in engines; in mining, oil and gas industries to search for deposits
In Agriculture radiation is used to kill bacteria, moulds and other microorganisms in strawberries,
onions, potatoes, meats, and spices, preventing the food from spoiling, and making it safer to eat.
Radiation in Consumer Products: smoke detectors / photocopiers (to prevent static buildup) /
sterilisation of contact lenses, baby powder and cosmetics
See also:
http://www.raesystems.com/sites/default/files/downloads/ap-232-radioisotopes-in-everyday-life.pdf
(1.12) Calculations of Relative Atomic Mass (Ar / RAM)
Based on the percentage abundance of an element’s isotopes in any sample of the element
For example:
A typical sample of Chlorine contains:
75% 35Cl (chlorine-35) + 25% 37Cl (chlorine-37)
i.e. in 100 atoms there would be:
75 35Cl atoms + 25 37Cl atoms
The total mass of the 100 atoms would be: (75 x 35) + (25 x 37) = 3550 (Notice: No units it’s all relative)
The average mass of one atom would be:
3550 / 100 = 35.5 (the RAM of chlorine)
Or:
Magnesium:
78.6% 24Mg + 10.1% 25Mg + 11.3% 26Mg
100 atoms total mass: (78.6 x 24) + (10.1 x 25) + (11.3 x 26) = 2432.7
RAM:
2432.7 / 100 = 24.3 (to 3 significant figures)