[Music]
you'll need to be familiar with the
structure of an atom before going any
further and you may wish to look at the
lesson parts of an atom their charge and
their mass first
electrons are arranged in energy levels
or shells around the nucleus of an atom
the actual orbit shapes are a bit too
complicated to cover in such a short
lesson
but on average the orbital radius
increases as the energy levels increase
in our diagrams the shell nearest the
nucleus is going to represent the lowest
energy level or shell and we draw a
circle to depict each shell
we use a dot or a cross to represent
each electron
and we'll represent the nucleus by the
chemical symbol
each electron in an atom is in a
particular shell and the electrons must
occupy the lowest available shell
nearest the nucleus
so when we are drawing the electron
configuration we have to fill up each
shell in turn starting with the lowest
we'll take a lithium atom as an example
with an atomic number of three it must
have three electrons to balance the
positive charge of the three protons in
the nucleus
it's worth remembering that the atomic
number tells you the total number of
electrons in a neutral atom
so
we put the first electron into the first
shell
and the second
however this shell can only contain a
maximum of two electrons that's one of
the rules that you need to remember
the third electron therefore must go
into the next shell which we draw as a
larger circle around the outside of the
first
and that completes the electronic
configuration of a lithium atom
the same process of filling up shells
applies to larger atoms you always start
with the lowest available shell and
whenever a shell is full the next
electron to be added goes into the next
shell
and that's pretty much it
all you need to know is how many
electrons can occupy each shell and at
this level you only need to know the
rules for the first 20 elements
the maximum numbers are as follows
the first shell can contain a maximum of
two electrons
the second shell
up to eight electrons
the third shell also has a maximum of
eight electrons
and the 19th and twentieth electrons go
into the fourth shell
and this would actually represent a
calcium atom
note that as we fill up each level we've
been drawing the electrons evenly spaced
out on each shell
this is to keep the diagrams tidy and
make it easier to count the electrons
and what's more there's a shorthand way
of writing this down
two
eight
two the first number represents the
inner shell or lowest energy level
the full stop shows how the electrons
are separated between the shells
the second number tells us how many
electrons are in the next shell followed
by the third and fourth shells
and so now it's your turn
i'd like you to draw the electron
configuration of a carbon atom
pause the video find a piece of paper
and have a go
and here is what it should look like it
has a full first shell and four
electrons in the outer shell
did you get it right
now how would you write the electron
configuration in numbers for this atom
pause the video again or rewind if you
need to have a think about this
the answer is that it would be written 2
dot 4
and now that you have covered the basics
of drawing electron configurations
you'll be ready to tackle our lesson
electron configurations of the first 20
elements
you