The nucleus
The nucleus is at the
centre of the atom and contains the protons and neutrons. Protons and neutrons
are collectively known as nucleons.
Virtually all the mass
of the atom is concentrated in the nucleus, because the electrons weigh so
little.
Working out the numbers
of protons and neutrons
No of protons = ATOMIC NUMBER of the atom
The atomic number is
also given the more descriptive name of proton number.
No of protons + no of neutrons = MASS NUMBER of the atom
The mass number is also
called the nucleon number.
This information can be
given simply in the form:
How many protons and
neutrons has this atom got?
The atomic number counts
the number of protons (9); the mass number counts protons + neutrons (19). If
there are 9 protons, there must be 10 neutrons for the total to add up to 19.
Periodic Table and
therefore the number of protons defines what sort of element you are talking
about. So if an atom has 8 protons (atomic number = 8), it must be oxygen. If
an atom has 12 protons (atomic number = 12), it must be magnesium.
Similarly, every
chlorine atom (atomic number = 17) has 17 protons; every uranium atom (atomic
number = 92) has 92 protons.
Isotopes
The number of neutrons
in an atom can vary within small limits. For example, there are three kinds of
carbon atom 12C, 13C and 14C. They all have
the same number of protons, but the number of neutrons varies.
|
protons
|
neutrons
|
mass number
|
|
|
carbon-12
|
6
|
6
|
12
|
|
carbon-13
|
6
|
7
|
13
|
|
carbon-14
|
6
|
8
|
14
|
These different atoms of
carbon are called isotopes. The fact that they have varying
numbers of neutrons makes no difference whatsoever to the chemical reactions of
the carbon.
Isotopes are atoms which
have the same atomic number but different mass numbers. They have the same
number of protons but different numbers of neutrons.
The electrons
Working out the number
of electrons
Atoms are electrically
neutral, and the positiveness of the protons is balanced by the negativeness of
the electrons. It follows that in a neutral atom:
no of electrons = no 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
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