Chemistry Reference
In-Depth Information
Relative atomic mass
The average mass of a large number of atoms of an
element is called its
relative atomic mass
(symbol
A
r
). This quantity takes into account the percentage
abundance of all the isotopes of an element which
exist.
In 1961 the International Union of Pure and
Applied Chemistry (IUPAC) recommended that
the standard used for the
A
r
scale was carbon-12.
An atom of carbon-12 was taken to have a mass of
12 amu. The
A
r
of an element is the average mass
of the naturally occurring atoms of an element on a
scale where
12
C has a mass of exactly 12 units:
A
r
=
average mass of isotopes of the element
It is not possible to give the exact position of an
electron in an energy level. However, we can state
that electrons can only occupy certain, defi nite energy
levels and that they cannot exist between them. Each
of the electron energy levels can hold only a certain
number of electrons.
●
First energy level holds up to two electrons.
●
Second energy level holds up to eight electrons.
●
Third energy level holds up to 18 electrons.
There are further energy levels which contain
increasing numbers of electrons.
The third energy level can be occupied by a
maximum of 18 electrons. However, when eight
electrons have occupied this level a certain stability is
given to the atom and the next two electrons go into
the fourth energy level, and then the remaining ten
electrons complete the third energy level.
The electrons fi ll the energy levels starting from
the energy level nearest to the nucleus, which has the
lowest energy. When this is full (with two electrons)
the next electron goes into the second energy level.
When this energy level is full with eight electrons,
then the electrons begin to fi ll the third and fourth
energy levels as stated above.
For example, a
1
8
O atom has a proton number
of 8 and therefore has eight electrons. Two of the
eight electrons enter the fi rst energy level, leaving
six to occupy the second energy level, as shown in
Figure 3.8. The electron confi guration for oxygen
can be written in a shorthand way as 2,6.
1
12
× mass of 1 atom of carbon-12
Note:
1
12
of the mass of one carbon-12 atom = 1 amu.
For example, chlorine has two isotopes:
3
17
Cl
37
17
Cl
% abundance
75
25
Hence the 'average mass' or
A
r
of a chlorine atom is:
(75
×
35)
+
(25
×
37)
100
= 35.5
A
r
=
35.5
1
= 35.5 amu
Questions
1
Calculate the number of neutrons in the following atoms:
a
27
13
Al
b
31
15
P
c
262
107
Uns
d
190
76
Os
2
Given that the percentage abundance of
20
10
Ne is 90% and
that of
2
10
Ne is 10%, calculate the
A
r
of neon.
8p
8n
●
The arrangement of
electrons in atoms
The nucleus of an atom contains the heavier sub-
atomic particles - the protons and the neutrons. The
electrons, the lightest of the sub-atomic particles,
move around the nucleus at great distances from the
nucleus relative to their size. They move very fast in
electron energy levels very much as the planets orbit
the Sun.
Figure 3.8
Arrangement of electrons in an oxygen atom.
There are 118 elements, and Table 3.5 shows the
way in which the electrons are arranged in the fi rst
20 of these elements. The way in which the electrons
are distributed is called the
electronic structure
(or electron confi guration). Figure 3.9 shows the
electronic structure of a selection of atoms.
