In a previous video we learnt that the nucleus
is at the centre of an atom, and that it is
made up of protons and neutrons, and around
this central point we find the electrons.
And we know that in a neutral atom the number
of electrons is always equal to the number
of protons in that atom.
So in a neutral atom of Carbon there are 6
protons, which mean there must be 6 electrons.
The electrons in the atom are organised around
the nucleus in different ENERGY LEVELS or
SHELLS. Each Shell can only hold a certain
amount of electrons. The first and innermost
shell can only hold a maximum 2 electrons.
The shells which come after this can hold
a maximum of 8 electrons.
Let's look at some arrangements.
Lithium has 3 protons, therefore it has 3
electrons, the first 2 electrons are placed
in the first shell, closest to the nucleus,
and the 3rd electron is placed in the 2nd
shell. 2 + 1 = 3
These diagrams are called dot and cross diagrams
and are used to show the electronic configuration
of an element. You need to be able to draw
these diagrams, as well as being able to write
the electronic configuration in a written
form. For lithium it would be written as Li,
2, 1
In the diagram the nucleus is represented
by the element Symbol or a central black dot.
The energy levels are represented by circles
around the nucleus, and the electrons are
represented by dots or crosses.
Let look at some more examples
Carbon has 6 protons, therefore it has 6 electrons,
the first 2 electrons are placed in the first
shell, and the next 4 are placed in the 2nd
shell. 2+ 6 = 6 This would be written as C,
2, 4
Magnesium has 12 protons therefore it has
12 electrons the first 2 are placed in the
first shell, the next 8 are placed in the
2nd shell, the next 2 are placed in the 3rd
shell, 2 + 8 + 2 . This would be written as
Ca, 2, 8, 2
The organisation of these energy levels is
what gives elements their different reactivities
The number of electrons in the outer shell
of an atom tells you the group number to which
they belong in the periodic table. Members
of the same group, with the same number of
electrons in their outer shell have similar
reactivity.
So, electrons are arranged in shells or energy
levels around the nucleus. Each energy level
has a maximum number of electrons it can hold.
The lowest energy level is always filled first,
and can only hold 2 electrons, while the levels
after that can hold a maximum of 8. The number
of electrons in the outer shell of an atom,
determines which group the element belongs
on the periodic table, and elements in the
same group have similar relativities.
