This is Rebecca from ChemistryIsMyJam.com.
In this video we are going to be looking at
periodic trends.
These are the trends on the periodic table
that help us understand the behavior of elements.
We are going to look at ionization energy,
electronegativity, atomic radius, and then
the size of ions.
These trends, along with much of what we see
in chemistry, can be explained by forces between
charged particles.
You have particles with the same charge that
are trying to get away from each other, and
particles with opposite charges that are trying
to get close together.
These forces of attraction and forces of repulsion
can be increased two different ways.
First of all, you can increase the number
of charged particles.
For us, that is going to mean increasing the
number of protons and electrons within the
atom.
The other way that you can increase these
forces is to decrease the distance between
the particles.
The closer they are together, the more they
are going to feel each other.
These two concepts are huge in understanding
the trends that we will be looking at today.
The first trend for us to talk about is something
called ionization energy.
This is the amount of energy required to remove
an electron from an atom.
When you remove an electron from an atom,
you are creating an atom with a charge.
You’re creating an ion.
So measuring that measures the amount of energy
required to ionize the atom.
Typically when you just see the term “ionization
energy,” that is talking about removing
the first electron from an atom, and that
electron would be coming from an outside energy
level.
In the case of aluminum, the ionization energy
would refer to how much energy is required
to remove that electron from the atom.
If you were to then go further and remove
a second electron from an atom, we would call
it the second ionization energy.
And then you could have the third, and the
fourth ionization energy.
But when you just see “ionization energy”
you are typically talking about removing one
electron from the atom.
On the periodic table, ionization energy typically
increases from left to right and increases
from bottom to top.
The overall trend could be represented with
an arrow going from the bottom left to the
top right of the periodic table.
Helium has the highest ionization energy on
the chart, which makes sense because helium
is a very small atom.
It’s electrons are very close to the nucleus.
They are very attracted to that positive charge
inside the nucleus, which is going to make
them difficult to remove.
We saw in our last video that the atoms on
the left side of the periodic table will normally
give away electrons when they become ions.
That makes sense because they have low ionization
energies.
It does not take much energy to remove those
electrons.
So ionization energy increases from left to
right, and increases from bottom to top.
Let’s take a look at what causes this trend.
As you go from left to right across the periodic
table, each element adds a proton to its nucleus.
It is adding a positively charged particle
to its nucleus.
By having more protons in their nucleus, these
elements on the right hand side of the periodic
table are able to hold on tighter to their
electrons.
It takes more energy – more ionization energy
– to remove those electrons.
Those protons in the nucleus, we call that
the nuclear charge.
So whenever you hear nuclear charge, that
is just a way of referring to the number of
protons within the nucleus.
Ionization energy decreases from top to bottom
on the periodic table because each time you
go down a row on the periodic table, you are
adding an energy level.
So you add this energy level and that causes
the electrons to be farther away from that
positive charge in the center.
That makes them easier to remove.
So the electrons that are farther away from
the nucleus have a lower ionization energy.
That explains why every time you go down a
row on the periodic table, you are lowering
the ionization energy.
While we are looking at the effect of energy
levels on these electrons, let’s look at
a concept called shielding.
An electron in the outside energy level is
feeling two major forces.
It is feeling attracted to the protons inside
the nucleus of the atom.
However, it is feeling repelled by the electrons
that are between it and the nucleus.
This makes an electron on the outside energy
level fairly easy to remove.
It is farther away from the nucleus and it
is being repelled by the inner electrons.
The term that is often used to describe this
concept is “shielding.”
These electrons in the outside energy level
are shielded from the nucleus by these electrons.
And it makes these electrons easier to remove.
In contrast, an electron that is on a lower
energy level, like this one, would be extremely
difficult to remove.
It is very close to the nucleus and the electrons
that are farther out are actually pushing
it towards the nucleus.
So one that is on an inner energy level would
be extremely difficult to remove.
The second trend on the periodic table that
we need to discuss is called electronegativity.
Electronegativity is the ability of an atom
in a molecule to attract shared electrons
to itself.
We are talking about two atoms that are bonded
together, one of them has the ability to hog
the electrons – it pulls the electrons closer
to itself.
The one that is hogging the electrons is said
to have a higher electronegativity.
When we look at the trend for electronegativity,
you’re going to notice that it does not
include the noble gases, and that is because
electronegativity deals with bonded atoms,
and the noble gases do not bond.
So we will be ignoring the noble gases when
we talk about this trend.
The element with the highest electronegativity
value on the chart is fluorine.
Here you can see the trend for electronegativity
on the periodic table.
Electronegativity increases from left to right.
It increases from bottom to top.
This trend ignores the noble gases because
the noble gases never bond with any other
atoms.
And the highest value for electronegativity
on the periodic table is fluorine.
The reasons behind this trend are very similar
to the ones we just looked at for ionization
energy.
As you go from left to right across the periodic
table, you are adding protons to the nucleus
and those protons are able to pull electrons
closer to themselves.
As you go from top to bottom on the periodic
table, energy levels are being added.
The electrons are getting further away from
the nucleus.
That is increasing the distance between one
atom’s nucleus and another atom’s electrons.
That is going to decrease the amount of pull
that they can have on those electrons.
So electronegativity increases from left to
right and it increases from bottom to top.
The third trend on the periodic table that
we need to discuss is atomic radius.
We are basically looking now at the size of
the atom.
One thing to point out is that the atom does
not have an outer barrier so this is something
that is difficult to measure.
The way scientists measure this is by taking
two identical atoms that are bonded together.
They can tell where the nuclei are for those
two atoms.
They measure the distance between the two
nuclei, divide that distance in half and it
gives you the atomic radius.
So two identical bonded atoms are used to
determine the atomic radius.
In general, the atomic radius increases as
you go from right to left on the periodic
table.
It increases as you go down the periodic table.
If you are looking at the main body elements
on the periodic table, francium should have
the largest atomic radius.
The reasons behind this trend are very similar
to the reasons that we just saw.
Atomic radius increases as you go down the
periodic table because each time you are adding
an energy level, and energy levels take up
space.
Atomic radius decreases as you go from left
to right across the periodic table because
you are increasing the nuclear charge by having
more protons in the nucleus.
The protons are able to have more pull on
the electrons, so they pull those electrons
closer to themselves and shrink the atom as
you go from left to right across the periodic
table.
Reactivity is another important trend on the
periodic table.
This is referring to how easy it is to get
an element to react with another element.
The most reactive metals are on the lower
left portion of the periodic table.
The electrons for francium are extremely easy
to remove.
It is a big atom with low ionization energy.
Those electrons are easy to remove making
that a very reactive element.
The trend is the opposite for the nonmetals.
The most reactive nonmetals are in the upper
right corner.
Fluorine would be the most reactive nonmetal.
This trend ignores the noble gases, because
they are not reactive at all.
The final trend for us to discuss is ion size.
This is comparing the size of an ion to the
size of it’s neutral atom.
When you will find is that cations are smaller
than their parent atoms and anions are larger
than their parent atoms.
Take sodium for example.
When sodium becomes an ion, it actually gives
away an electron.
It loses an entire energy level.
So sodium, when it becomes an ion takes a
positive charge, and gets smaller.
Cations are smaller than their parent atoms.
The opposite of that would be something like
bromine.
Bromine is going to gain electrons when it
becomes an ion.
This is going to cause the bromine ion to
actually get larger.
Anions are larger than their parent atoms.
On the periodic table, ion size increases
as you go down a group.
It decreases from left to right across the
periodic table.
So you may notice that this just followed
the same trend as atomic radius.
The only exception is that when you switch
from positive to negative charges, you should
notice a sharp increase in size when you switch
from those positive metals one the left side
of the periodic table to those negative nonmetals
on the right side of the periodic table.
I hope that this unit on the periodic table
has shown you what a powerful tool that it
can be.
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