Chem143 Valence Electrons, Electron Dot Symbols and Periodic Trends

Valence electrons, electron-dot symbols for elements, and periodic trends in the periodic table.

>> In this video
we're going to talk

about what valence
electrons are,

how to write an electron
dot symbol, and a few trends

in periodic properties
in the periodic table.

So first of all,
valence electrons.

A valence electron is any
electron in an atom that's

in the highest occupied level.

That is, all of the electrons

that have the highest
n that's occupied.

For example, in sulfur here,
the electron configuration

for sulfur is 1s22s22p63s23p4.

The highest level
or the highest n

that has electrons
is the n equals 3.

And the 3s and the 3p electrons
are all valence electrons.

There's 2 plus 4 is 6 valence
electrons and so forth.

Now, in niobium, its electron
configuration using core

notation is krypton 5s24d3.

Well even though we put the 4d
electrons in last, the highest n

or the highest level that
has electrons is the n equals

to 5 energy level.

And so these two 5s electrons
are the only valence electrons

so niobium only has
two valence electrons.

Now, for the representative
elements, remember,

that's the first two
columns, alkaline metals

and alkaline earth metals
and from boron over here

through the noble gasses,
this block here, you can look

at the periodic table and just
tell how many valence electrons

they have.

Everything in the first
column has 1 valence electron,

just the one.

Everything in the second
column has 2 valence electrons.

Skipping the transition
metals, everything here

in the 3A has 3valence
electrons, 4, 5, 6, 7.

Now here, helium only
has 2 electrons total.

So helium only has
2 valence electrons,

otherwise every noble gas
has 8 valence electrons.

In terms of the transition
metals,

most of those only have
2 valence electrons

because the highest occupied
end would be the 4s for here,

the 5s for here,
6s and 7s and so

on because these are
3d, 4d, and so on.

Except, watch out
for the exceptions;

remember chromium is 4s13d5 so
there's only 1 valence electron

for chromium, likewise
for copper, 4s13d10.

Electron dot symbols.

All they are is writing
the symbol for the element

and for every valence
electron put a dot.

Now, usually we only put no more
than 2 dots per side but other

than that you can arrange
them anyway you want to.

You can imagine that there's
a square around this symbol.

There's 4 sides, each side
can have no more than 2 dots.

Sulfur has 6 valence electrons
so you could write them 2 here,

2 here, 2 here, 2 here,
2 here, 1 here, 1 here,

or any other arrangement
that gets us 6 dots,

no more than 2 to a side.

Calcium has 2 valence electrons,
it's an alkaline earth metal.

It's in the second
column so either like this

or 2 up here or however
you like.

Now, periodic trends, the signs
of the neutral atoms, okay,

the atomic signs or
the atomic radius.

Okay, what this it's
basically how far

out from the nucleus the outer
most electrons can extend

or do extend.

And the trend in
the periodic table,

this is basically what you have
to know about at this point

about atomic size
or atomic radius,

and that is that the atoms
get smaller, in general,

as you go up and to the
right in the periodic table.

So following this trend,
helium would be the smallest.

Francium would be the largest.

There are some minor exceptions

but you don't have
to really know those.

You don't have to know those.

Just know the trend.

Atomic size or atomic radius
decreases, gets smaller,

up and to the right
in the periodic table.

The next trend is called
first ionization energy

and this is how much
energy it takes

to remove a valence
electron from a neutral atom.

So sodium, this is the
electron dot symbol for sodium.

Sodium has 1 valence electron.

If we take that electron
away now we're left

with 1 more proton than
we have electrons, right?

Because neutral sodium
starts out with 11 electrons,

if we take 1 away it still has
11 protons but only 10 electrons

and has a positive
charge overall.

We'll see this in a little bit.

And so we remove the
electron, this process,

the energy required to do this,

is called the first
ionization energy.

Now really, all you need to
know about this at this point is

that it increases as you
go up and to the right

in the periodic table.

It gets harder to
remove an electron

as you go up and to the right.

So it would be hardest to
remove an electron from helium

and easiest from francium.

Metallic character is
the next periodic trend.

This is how much it
acts like a metal.

What do metals do?

Well, they tend to be shiny,
tend to be able to bend them,

pull them into wires,

they conduct electricity,
they conduct heat.

And the more something acts

like a metal the more
metallic character it has.

And you can see this
in the periodic table.

Remember this line right
here, remember to the left

of that are metals, to
the right are non-metals.

So you can probably
guess that it's more,

the elements are more metallic
as you go to the left and less

as you go to the right.

And so the trend as you
go up and to the right is

that the metallic character
decreases in the periodic table.

Helium is the least metallic,

francium would be
the most metallic.

So, if we wanted to arrange the
following elements, selenium,

germanium, and chlorine in order

of increasing atomic radius
first ionization [inaudible]

metallic character it
would look like this

because radius gets smaller
as you go up and to the right

but if you look at the periodic
table you see that chlorine is

up and to the right the
most so it's the smallest,

selenium is in the middle and
then germanium is the largest.

First ionization energy
increases as you go up

and to the right, so chlorine
would have the highest first

ionization energy,
germanium the lowest,

it would go germanium then
selenium then chlorine.

Metallic character
decreases up and to the right

so chlorine is the furthest
up and to the right,

it would have the least
metallic character then selenium

and the most metallic of the
these three would be germanium.