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Answers
Answer:
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Explanation:
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wyjwgjsgjsgjhdhdhrrhrgrgdhdhdhgfddsdddssdyjbgfruhcdsryhhuhickfjfifi26 PRINCIPLES OF CHEMISTRY
HINT
Be careful! When you are writing
symbols with two letters, the first is a
capital letter and the second must be
lower case. If you write CO you are
talking about carbon monoxide, not
cobalt.
6 protons
6 neutrons
o proton
6 protons
7 neutrons
e neutron I
14c
6
o•o• .'1'.0 •o.•o 6 protons
8 neutrons
' I
For any particular atom, this information can be shown as, for example:
mass number counts -----59
protons + neutrons C
0 - symbol for element
atomic number counts--27
the number of protons
This particular atom of cobalt contains 27 protons. To make the total number
of protons and neutrons up to 59, there must also be 32 neutrons.
You can see from this that:
number of neutrons = mass number - atomic number
ISOTOPES
The number of neutrons in an atom can vary slightly. For example, there are
three kinds of carbon atom called carbon-12, carbon-13 and carbon-14.
They all have the same number of protons (because all carbon atoms have 6
protons, its atomic number), but the number of neutrons varies. These different
atoms of carbon are called isotopes.
Isotopes are atoms (of the same element) which have the same atomic number
but different mass numbers. They have the same number of protons but
different numbers of neutrons.
6 Figure 3.
5 The nuclei of the three isotopes of The fact that they have varying numbers of neutrons makes no difference
carbon to their chemical reactions. The chemical properties (how something reacts)
are controlled by the number and arrangement of the electrons, and that is
identical for all three isotopes.
KEY POINT
The number above each symbol in
the International GCSE Periodic Table
papers is a relative atomic mass and
not a mass number. However, in most
cases the relative atomic mass stated
is the same as the mass number of
the most common isotope. The only
exceptions to this are chlorine (35.5)
and copper (63.5).
RELATIVE ATOMIC MASS
You might have seen the following in a Periodic Table:
35.5
Cl
chlorine
17 7 20 x 1 0.
The total mass of the 80 atoms with mass 11 is 80 x 11.
The total mass of all the atoms in the sample is 20 x 10 + 80 x 11.
There are 1 00 atoms so we can work out the average by dividing the total
mass by the total number of atoms (100):
relative atomic mass = 20 x 10
1
~
0
80 x 11 10.8
Therefore, the relative atomic mass of boron is 10.8.
Even if there are three or four different isotopes, you still do the calculation
in the same way: calculate the total mass of 100 atoms, then divide the
answer by 100.
THE ELECTRONS
Atoms are electrically neutral (they have no overall charge). The charge on
a proton (+1) is equal but opposite to the charge on an electron (- 1), and
therefore in an atom:
number of electrons = number of protons
So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have
8 electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must
also have 17 electrons.
You will see that the key feature in this is knowing the atomic number. You can
find the atomic number from the Periodic Table.
The number of protons in an atom is equal to the number of electrons.
However, the atomic number is defined in terms of the number of protons
because the number of electrons can change in chemical reactions, for
example when atoms form ions (see Chapter 7).