Video Transcript
In this video, we will be looking
closely at how electrons are arranged within atoms. And how this arrangement is
responsible for the fundamentals of an entire field of science, the field of
chemistry. But of course, in this video, we’ll
be looking at it from the perspective of physicists. So let’s get started.
Let’s start by recalling that an
atom is made up of three main types of fundamental particles. Firstly, in the nucleus of an atom,
we know that protons and neutrons are clumped together. And for the purposes of this video,
in many cases, we’ll be drawing the nucleus as one big pink lump, as opposed to made
up of individual protons and neutrons. And secondly, we can recall that
surrounding the nucleus are particles known as electrons. Those are represented by these blue
dots here. And that is the basic structure of
an atom. But the question is just how do
these electrons surround the nucleus. Are they just randomly scattered
around the nucleus? The answer to that question is
no. Electrons within atoms are
restricted in terms of just where they can be. Specifically, they are restricted
to so-called shells or energy levels. Well, we will be representing
energy levels with these black lines. And as we said earlier, within an
atom, electrons can only be found at these energy levels. And we’ll label each one of these
black lines or rather black circles as an energy level or a shell.
Now, over here in this diagram,
we’ve drawn each energy level as a circle. And each of these circles is
centred around the nucleus. However, in real life, these energy
levels are actually spheres because they exist not only as these circles. But rather they come out of the
screen and go into the screen as well. But for the sake of convenience, we
draw them as circle so we can see them clearly on a two-dimensional surface, such as
a piece of paper or a screen. So what we can gather from this
diagram then is that electrons can only exist very specific distances from the
nucleus. In other words, in a stable atom,
we will not find an electron here, for example. But we could find an electron this
distance away from the nucleus, for example. Because at that distance from a
nucleus, the electron would be on an energy level.
Now, there’s a couple of key points
that we need to know about these energy levels. Firstly, we label these energy
levels in a very specific way. The energy level nearest to the
nucleus of the atom is labelled as the first energy level. The next one out is labelled as the
second. The next one out is labelled as the
third and so on and so forth. Now, there’s an infinite number of
energy levels. But in most of diagrams, we only
draw the first few. And the reason for this is that the
energy level nearest to the nucleus, that is, the first energy level, is lowest in
energy. And the further away from the
nucleus we go, the energy increases. Now, what we technically mean by
this is that electrons found in the energy level nearest to the nucleus will have
the lowest energy. And any electron found in an energy
level further out will have higher energy.
Now, another thing that we need to
know is that electrons will try and get to the lowest possible energy level that is
available to them. And this is so that the overall
energy of the atom is as low as possible. Let’s recall everything in our
universe tends to the lowest energy that is possibly available to it. And so what we’re saying here is
that all electrons will try and get to the lowest possible energy level that is
available to them. But then, does that mean that every
single electron in the atom will simply find its way down to the lowest energy
level? Well, the answer to that question
is no. And there’s a very particular
reason for this. It’s because every energy level has
a maximum number of electrons that it can hold. Once the energy level in question
contains that number of electrons, then it is considered full. And any of remaining electrons that
aren’t found in that energy level must then fill the energy level above.
So the first energy level, the one
nearest to the nucleus, can hold up to two electrons. And then it’s full. The second energy level can hold up
to eight electrons. And then, it’s full. Now, from the third energy level,
things get a little bit more complicated, namely, that eight electrons fill the
third energy level before the fourth one starts filling. But the third energy level can
actually hold up to 18 electrons. Now, we won’t go into too much
detail about why things get weird from the third energy level onwards. But what’s important to realise is
that energy levels further away from the nucleus, for example, the third energy
level, can hold more electrons, compared with an energy level, let’s say the first
energy level, that is nearer to the nucleus. So, for example, the first one can
only hold two electrons. And so the further out we go from
the nucleus, the energy levels can hold more and more electrons.
Now, the reason that each energy
level holds these particular numbers of electrons is not really important. And it’s quite complicated. So instead, what we’re going to do
is to consider some neutral atoms and the way in which electrons are structured
within these atoms. In other words, we’ll be looking at
what’s known as the electronic configuration, which is basically just a fancy way of
saying the distribution of electrons in an atom or molecule. And we’ll be thinking about the
distribution of electrons in atoms. So let’s go about doing that.
If we look at the very beginning of
the periodic table of the elements, then we will see that the simplest possible atom
that we can find is one of hydrogen. And we also see from the periodic
table that the atomic number of hydrogen is one, where the atomic number is the
number of protons found in the nucleus of the atom that we’re considering. So starting with a hydrogen atom,
we know that there’s going to be one proton in the nucleus. And actually, the number of
neutrons in the nucleus is currently not relevant to us. And we’ll see why that is in a
second.
But let’s start by drawing our
hydrogen nucleus, which we know contains one proton. Now, we’re going to be considering
a neutral hydrogen atom. In other words, the atom itself
overall has no charge. And what this means is that because
there’s one proton in the nucleus of the hydrogen atom, there must be one electron
in the energy levels surrounding this nucleus. Because, this way, the number of
protons which are positively charged is equal to the number of electrons which are
negatively charged. And so the overall charge on the
atom is zero.
So let’s start by drawing a couple
of energy levels around the nucleus of a hydrogen atom, where these energy levels
are the positions that electrons can occupy around the nucleus. And then, because there’s one
proton in the nucleus, we know that there’s going to be one electron in the energy
levels. And that electron will be found in
the lowest possible energy level, which is the first energy level, the one nearest
to the nucleus. So that is the electronic
configuration of a neutral hydrogen atom. Because we’re not doing anything to
this hydrogen atom, the electron will be found in the lowest possible or lowest
available energy level. Where the lowest available energy
level is the first energy level or the one nearest to the nucleus. So that’s a hydrogen atom.
Let’s now move on to looking at
helium. The periodic table tells us that
helium has an atomic number of two, which means that there are two protons in the
nucleus of a helium atom. Now, the only reason the number of
protons in the nucleus is relevant right now is because we’re considering neutral
atoms. And in neutral atoms, the number of
protons in the nucleus is equal to the number of electrons found in the energy level
surrounding the nucleus. And this is also why the number of
neutrons is not relevant to us right now. Because regardless of the number of
neutrons in the nucleus, the number of electrons in the neutral atom is not going to
change. But anyway, so let’s now say that
the atom that we’ve drawn is a helium atom. In other words, it’s got two
protons in its nucleus. And the two electrons surrounding
this nucleus are both going to be found in the lowest possible energy level, which
is the first energy level. That’s the one nearest to the
nucleus. And hence, this is the electronic
configuration of a neutral helium atom. Nothing too interesting! But then, we get to a lithium
atom.
This time, lithium has an atomic
number of three, meaning that there are three protons in its nucleus. So if we say that this is now a
lithium atom, let’s write down three protons in the nucleus. So then, for a neutral helium atom,
the three electrons surrounding this nucleus will be found as follows. The first one will be in the energy
level nearest to the nucleus. Another one will be found in the
same energy level. But then, the third one cannot
occupy this energy level. And that’s because we can recall
from earlier that the maximum number of electrons that the first energy level can
hold is two. And so the remaining electron that
we haven’t yet considered for this lithium atom must go in the next lowest available
energy level, which is energy level number two. And at this point, the first energy
level is full completely because it’s got two electrons. And the second energy level is
starting to be filled up. And remember that the second energy
level can hold up to eight electrons.
So if we then look at beryllium and
boron and carbon and nitrogen and so forth, we see that a beryllium atom which has
four protons in its nucleus has two electrons in its inner shell and two more in the
second energy level. Moving onto boron which has five
protons in its nucleus, that one has two electrons in the inner shell once again and
three in the second shell. And that gives us a total of five
electrons to match the five protons in the nucleus, thus giving us a neutral boron
atom. A neutral carbon atom has six
protons in its nucleus, which means two electrons in the first energy level and the
remaining four in the second energy level.
Nitrogen, which has seven protons
in its nucleus, has two electrons in the first energy level and five in the second
energy level and so on and so forth. We do this until we reach neon,
which has 10 electrons altogether. So two electrons in the first
energy level and eight electrons in the second energy level. At which point, both the first and
the second energy levels are full. And then the element after neon,
which will have 11 protons in the nucleus, will therefore have two electrons in the
first energy level, eight electrons in the second energy level. And then, the remaining one
electron will start filling the third energy level. And so that is how we can work out
the electronic structure of neutral atoms of specific elements.
And it’s actually this electronic
structure, this distribution of electrons in energy levels around the nucleus of an
atom, that determines how the atom in question behaves in a chemical reaction. Because for some atoms, the
outermost shell that has got electrons in it is completely full whereas, for some
atoms, the outermost shell with an electron in it is nearly empty. And in some other cases, these
electron shells are nearly full. And so each type of atom will
behave differently in chemical reactions.
Now, interestingly, we said earlier
that electrons will go into the lowest available energy levels. And that’s how we’ve been
calculating the electronic structures of atoms like this. In this case, we’ve got two
electrons in the innermost energy level and five in the second energy level. So altogether, that’s seven
electrons. And if this is a neutral atom, then
there are seven protons in the nucleus, which means this is a nitrogen atom. Now, when the electronic structure
is like this, when electrons are in the lowest available energy levels, these
electrons are said to be in their ground state. Which basically means that the
electrons have filled up the energy levels, from the lowest numbered energy level
all the way to the highest numbered energy level, without leaving any gaps in any of
the energy levels. And in this particular case, the
inside energy level, the first one, is completely full. And only then has the second energy
level started filling.
However, if we provide energy to
this atom, then electrons can absorb this energy and move to higher energy levels,
leaving behind a gap in the energy level that they were in so that the electronic
configuration is no longer in the lowest possible energy state. So what do we mean by this? Well, first of all, we can provide
energy to these electrons by shining light on these atoms or for that matter any
other form of electromagnetic radiation. And each electron has the
capability to absorb one photon of light, where a photon is basically just a
particle of light. And so in this case, we could say
that this electron here is absorbing that photon. Now when this happens, when the
electron absorbs the photon, the electron itself gains the energy from the
photon. Hence, the electron now has more
energy. And it cannot stay in this energy
level. It has to go up to a higher energy
level, for example, energy level number three.
So here’s our electron now. And it’s moved up to the third
energy level. In this case, we say that the other
electrons, the ones that haven’t moved up to a higher energy level, are still in the
ground state. However, the one that has moved up
to a higher energy level is said to be in an excited state. And what this means is that we’ve
started filling the third energy level in this case, without completely filling up
the second energy level. And as we said earlier, the higher
the number of the energy level or in other words the further away the energy level
from the nucleus, the more energy the electrons will have in that energy level. And this makes sense, right. The electron has gained energy
because it absorbed a photon. And a photon is basically a packet
of light, which carries energy. And so that extra energy from the
photon itself means that the electron has now moved to a higher energy level.
Now, eventually, after sometime, if
we stop shining light on this atom, then the electron will go back down to its
original position. It will return back to the lower
energy level. But in doing so, it will lose
energy because it’s going to a lower energy level. But then, where does that energy go
when the electron loses the energy and returns back to level two, in this case? Well, that energy is released by
the electron. Or, in other words, the electron
emits another photon. And so releasing that energy allows
the electron to return back to level two in this case. So now that we’ve learnt about
electronic configurations as well as how electrons can move between energy levels,
let’s attempt an example question.
The diagram shows electrons in
different electron shells in an atom. The atom is electrically
neutral. What element is this an atom
of?
Okay, so in this question, we’ve
been given a diagram, which shows what appears to be the nucleus at the centre of
the atom as well as the electronic energy levels, which are represented in
black. Now, these electronic energy levels
are also known as shells. And we can see that occupying these
shells there are one, two, three blue dots representing electrons. In other words, there’re three
electrons in this atom.
Now, we’ve been told that this atom
in question is electrically neutral. This means that the overall
electric charge on the atom is zero, because it’s neutral. Now, to understand the significance
of this, we should recall that electrons are negatively charged particles. And that atoms are neutral if the
number of electrons in the atom is equal to the number of positively charged protons
found in the nucleus. And so what this diagram is telling
us is that there are three electrons in this atom. And in order for it to be neutral,
there must be three protons in the nucleus of the atom. And then, we can recall that an
element is defined by the number of protons found in the nucleus of an atom of that
element.
And so in order to find the answer
to our question, we will need to look on the periodic table. Specifically, we can recall that
the periodic table of the elements shows all of the elements arranged in order of
atomic number, where the atomic number of an atom is simply the number of protons
found in the nucleus of that atom. And so considering the periodic
table is arranged in order of increasing atomic number, we want to find the element
in the periodic table, which has an atomic number of three. Because there are three protons in
this particular atom’s nucleus. So the element that has three
protons in its nucleus is lithium. And hence, that is the answer to
our question.
So now that we’ve answered this
question, let’s summarise what we’ve talked about in this lesson. Firstly, in this video, we saw that
electrons can be found in atoms surrounding the nucleus and arranged in energy
levels or shells. Secondly, we saw that energy levels
are numbered one, two, three, four, and so on, with the energy level nearest to the
nucleus being shell one. Thirdly, we saw that each electron
will occupy the lowest available energy level to minimize the overall energy of the
atom unless we provide energy from an external source. And this is done by shining light
or other forms of electromagnetic radiation onto the atom. And finally, we saw the reason why
all electrons don’t just fall into the first energy level. It’s because each energy level has
a limit to the number of electrons it can hold. We also saw that shells, further
away from the nucleus, can hold more electrons. For example, we saw that the
nearest energy level to the nucleus, the first energy level, can hold only two
electrons whereas the second one can hold up to eight electrons. And the third can hold up to 18
electrons.
