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ATOMS
Francine
Taylor Campbell, Contributor
POINTS
TO NOTE
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Matter consists of particles which
may be atoms, molecules or ions
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An atom is the smallest part into
which an element can be divided
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An atom consists of 3 main particles;
protons, neutrons and electrons
From
last week's lesson we learnt that
matter is all around us whether in
the form of gases, liquids or solids.
This matter is made of particles with
an atom being the smallest part. An
atom has two main sections:
1.
A nucleus which is the centre and
holds the protons and electrons
2.
Shells which contain the electrons.
(Please see figure 1)
These
3 sub-atomic particles have different
properties. (Please see figure 1)
| PARTICLE |
RELATIVE
MASS |
RELATIVE
CHANGE |
| Proton |
1
|
+1 |
| Neuton |
1 |
Neutral |
| Electron |
1/1840 |
-1 |
| |
Figure
2
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Please
note that the overall charge on an
atom is zero (-1 + 1 = 0) hence atoms
are uncharged. The mass of an electron
is 1/1840 of a proton. This mass is
very small hence we generally say
that the mass of an electron is negligible
or it has no mass. This is why the
mass of an atom is derived from the
number of protons and neutrons. Mass
number = Protons + Neutrons.
Remember
the masses given are not the actual
masses but the relative mass when
all three are compared. The relative
atomic mass (Ar) is the average mass
of one atom of an element compared
to the mass of one atom of carbon-12.
As
was stated earlier, the protons (P)
and neutrons (N) are in the nucleus
and this gives the nucleus a positive
charge, while the electrons are moving
in the shells with a negative charge.
In
an atom the number of protons in the
nucleus is called the atomic number
and this is equal to the number of
electrons in the shells.
#
of protons = # of electrons This is
why the charges cancel out.
The
element oxygen, for example, is written
like this:
Mass
number = P + N = 16
Atomic
number = # protons = 8
Therefore,
the # neutrons = 16 - 8 = 8
In
a neutral atom # P = # E = 8
1.
Try this one 35
Cl
17
How
many electrons and neutrons are present
in chlorine?
It
is important now for you to look at
the first 20 elements of the Periodic
Table. These elements are arranged
in order of increasing atomic number.
The first element is Hydrogen 1
H
1
Note that hydrogen has 1 proton and
therefore 1 electron, but since the
mass number = 1 (P + N) this means
that H has no neutrons.
All
the other elements can be written
in this form.
Please
review the symbols and atomic and
mass numbers of the first 20 elements.
Next
week we will examine how the electrons
are arranged in the shells.
*
Francine Taylor-Campbell is
an independent contributor.
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