Video Transcript
In this video, we will learn about the
elements that make up our universe and about atoms, which are the simplest unit of those
elements. We’ll look at what happens when atoms
come together and at how we write and describe the combinations.
To start off, let’s have a quick look at
the structure of atoms. Atoms are extremely small. A piece of coal that fits in the palm of
your hand contains about 10 million billion billion atoms. That’s 10 to the power of 25 atoms. Every atom is made of a nucleus and some
electrons. The nucleus is the densely packed core of
an atom containing protons and neutrons. And the electrons fill the surrounding
space.
One proton has the mass of about one
unified atomic mass unit. That’s one twelfth of the mass of a
carbon-12 atom. We’ll take a look at exactly what a
carbon-12 atom is in a moment. Neutrons also have a mass of about one
unified atomic mass unit. So, for most purposes, we treat protons
and neutrons as if they have the same mass. Electrons are much lighter, with a mass
of 0.00055 unified atomic mass units. You’ll sometimes see this expressed as a
fraction, one divided by about 1840. This presentation tells us that it takes
about 1840 electrons to have the same mass as a single proton or neutron.
The other key property of these particles
is their charge. Particles can either have no charge, or
positive charge, or negative charge. Particles with the same type of charge
repel each other. So, positive particles repel positive
particles, and negative particles repel negative particles. But two particles with opposite charge,
one with positive charge and one with negative charge, will attract each other. Protons are positively charged, electrons
are negatively charged, and neutrons have no overall charge.
To make things simpler, we say that a
proton has a single positive charge. And electrons have equal and opposite
charge, so they have a 1− charge. Atoms can have different numbers of
protons, neutrons, and electrons. For instance, a carbon-12 atom is an atom
with six protons, six neutrons, and six electrons. Having more neutrons makes an atom
heavier. And having more protons also makes an
atom heavier. But crucially, adding more protons makes
the nucleus of an atom more positively charged.
The more positively charged the nucleus,
the more electrons it can hold on to. By definition, atoms are neutral
overall. So, we need exactly the same number of
electrons as we do have protons in order to classify it as an atom. All this boils down to a very simple
definition of the atom. An atom is simply a nucleus with enough
bound electrons to make it neutral overall. So, the number of electrons for an atom
is equal to the number of protons in its nucleus. So, the number of protons in an atom’s
nucleus dictates the number of electrons it has. These two factors together determine the
chemistry for that atom.
The number of neutrons does matter. It does contribute extra mass to the
atom. But apart from some very special cases,
the number of neutrons in an atom doesn’t affect how the atom reacts. This is why the very first way we
classify atoms is by talking about elements. And then, we use the number of neutrons
to break elements down further into isotopes. But in this video, I’ll just be looking
at elements.
An element is simply a type of atom based
on the number of protons in the atom’s nucleus. So, let’s imagine we’ve got an atom. We look inside its nucleus. And we count the number of protons. If there’s one proton, we say the atom is
of the element hydrogen. If there are two protons, then we say
it’s an atom of helium, and so on. There are, at the moment, 118 confirmed
chemical elements. But only about 100 of these have
practical applications at the moment.
The heavier elements are very unstable
and have to be created in super colliders. And they break down into lighter elements
in fractions of a second. The table shows us all the elements from
the lightest hydrogen with one proton per atom to the heaviest oganesson with 118 protons
per atom. You can use the periodic table to find
out lots of information about an element, including its name, symbol, and atomic number. The atomic number is simply the number of
protons in atoms of that element.
Now that we’ve had an overview of the
different types of atom, let’s see what happens when we combine them together. We’ve already talked about how the type
or element of an atom might affect how it reacts. If we were to separate out all the
particles in the atmosphere, about one percent of them would be atoms of argon. What this means is that these atoms have
18 protons in their nuclei. And these atoms of argon also have 18
electrons each.
For reasons that are beyond the scope of
this video, argon atoms are unreactive. And so, they go around in the atmosphere
as an atomic gas. What this means is when argon atoms
interact with other atoms, they just bounce off. On the other hand, another substance to
be found in Earth’s atmosphere is oxygen. When we look at oxygen gas, we see the
oxygen atoms going around in pairs. The two atoms share their electrons to
some degree. And they bind together in a chemical
bond. This is what we call a molecule. So, oxygen in this form is a molecular
gas.
You might see the oxygen molecule as two
of the letter O with two lines between them. The two lines indicate the number of
electrons that are being shared. Or you may see them as two balls
connected by a line, or two balls stuck together, or O with a little two next to it. The bond between these two oxygens is an
example of a covalent bond. What this means is that the atoms are
sharing their outer electrons. The nuclei are attracted to these shared
electrons, generating the bond.
When we have two or more atoms bonded
together in this fashion, we call them a molecule. Molecules can have two, three, four,
five, 100, or even a million atoms. And they don’t even need to be the same
type of atom. Now, you may be aware that atoms can lose
or gain electrons to form things that we call ions. Ions form ionic structures where positive
and negative ions alternate in all directions. It’s important to realize that these
structures are not molecules. Now that we know what molecules are and
are not, let’s have a look at how molecules interact.
Molecules can interact with each other
and stick together to some degree. That’s how we get solids like ice and
liquids like drinking water. But the forces between molecules are much
weaker than the bonds inside molecules. We call the forces between molecules
intermolecular forces. And we call the forces inside molecules
intramolecular forces. To help you remember these, you can think
about an interview, a discussion between two people that maybe results in somebody getting a
job. And you can think about an intravenous
injection that goes into someone.
Now, why are intermolecular forces
important? It’s because they control whether a
substance is going to be a solid, a liquid, or a gas at a given temperature. Let’s look at the intermolecular forces
in ice, which is made of molecules of H₂O, one oxygen atom bound to two hydrogen atoms. These intermolecular forces are strong
enough to hold the molecules in place making ice a solid. But if we heat up the ice above zero
degrees Celsius, we’re going to melt it. And the individual particles of water can
move over each other in a fluid way.
If we heat the liquid water so that it
boils, we’re giving the molecules so much energy that they completely overcome the
attractive intermolecular forces and they fly away as free particles filling up the
surrounding space. The stronger the intermolecular forces,
the more energy we need to turn a molecular substance into a gas. Now, let’s have a look at how we might
write down elements, atoms, and molecules.
As you learn more about chemistry, you’ll
see elements, atoms, and molecules written in many different ways. For instance, you might see the word
oxygen, which means the element oxygen. Or you might see oxygen atom, which means
an atom with eight protons in its nucleus, an atom of the element oxygen. Or you may see the words oxygen molecule,
which refers to a molecule that contains only oxygen atoms. We would normally expect an oxygen
molecule to contain two oxygen atoms.
Now, it can get a little bit confusing
when we’re talking about the symbols because the symbol for the element oxygen is the same
as the symbol we’d use for a single atom. You’ll have to use the context to figure
out what’s being described. But we already know that the symbol for
the oxygen molecule is O₂. You can think of this as two oxygens. You may see these formulas appearing with
state symbols, which are symbols that indicate the state, or phase, of the substance.
So, you may see symbols like that, where
we have the oxygen molecule in solid form, in liquid form, or in gas form. And you may also see the symbol aq, which
indicates a substance is dissolved in water. These symbols will crop up all over the
place, for instance, in chemical reaction equations. Here, we have a carbon atom reacting with
an oxygen molecule to form an entirely new substance called carbon dioxide. This new substance has different
intermolecular forces and different intramolecular forces. We’ve gone over a lot about atoms,
molecules, and elements. It’s about time we had some practice.
Which of the following is not a
molecule? A) CO₂, B) O₂, C) NO, D) N₂, or E)
Na.
A molecule is two or more atoms
covalently bonded together. An example of this would be a molecule of
water, or H₂O, where we have one oxygen atom covalently bound to two hydrogen atoms. We can use the periodic table to work out
what the symbols in all our formulas mean.
If we look up the letter C on the
periodic table, we’ll see that it stands for the element carbon. So, when we look at our formulas, we can
see whenever we see a capital C like that, it indicates we’ve got one carbon atom. O is the symbol for oxygen. So, where we see O, we know we’re talking
about an oxygen atom. And when we see a little two, that means
we have two of that thing. So, O₂ means two oxygen atoms. This is the symbol for an oxygen
molecule, two oxygen atoms covalently bonded together. So, we know it’s not the correct answer
because we’re looking for something that’s not a molecule.
The last symbols are N for nitrogen and
Na for sodium. Elements’ symbols are always a capital
letter or a capital letter followed by a lowercase letter. So, the N in Na isn’t for nitrogen; it’s
just the beginning of Na for sodium. Now, the main difference between sodium
and nitrogen is that sodium is a metal and nitrogen is a nonmetal.
Now, we can still get nitrogen atoms and
sodium atoms, but nonmetals tend to form covalent bonds and form molecules, while metals
form metallic structures that continue in all directions and have very different bonding to
the covalent bonding in molecules. So, CO₂, O₂, NO, N₂ all describe a
molecule containing two or more atoms covalently bonded together. But Na describes a single atom. And it’s a single atom of a metal that
would be unlikely to form molecules in the first place. So, of our five candidates, the only one
that is not a molecule is Na.
Now, let’s have a look at a slightly
tougher question.
What is the difference between a sulfur
atom and a sulfur molecule? A) Sulfur atoms are reactive, while
sulfur molecules are not. B) Sulfur atoms consist of multiple
sulfur molecules bonded together. C) A sulfur molecule consists of multiple
sulfur atoms bonded together. Or D) sulfur atoms can be divided into
smaller units, while sulfur molecules cannot.
The first thing we need to identify is
that sulfur is a chemical element. As with all elements, we can look up
sulfur on the periodic table. We find sulfur in group 16, otherwise
known as group six, just below oxygen. If we zoom in a little closer, we can see
a little bit more information about the element sulfur. The number 16 in the box for sulfur is
the atomic number of sulfur. That number tells us that if we have a
sulfur atom, it will contain 16 protons in its nucleus.
Now, a molecule is when we have two or
more atoms, it doesn’t matter what type, bonded together covalently. So, a sulfur molecule is simply a
molecule that contains only sulfur atoms bonded together, like S₂ or S₈. sulfur, unlike
oxygen, is a solid at room temperature and generally forms eight-membered rings. Now that we’ve recapped, let’s have a
look at our statements.
The first statement suggests that sulfur
atoms are reactive while sulfur molecules are not. Well, the truth is that the normal form
for sulfur is the S₈ molecule. You’ll see it as a vibrant yellow
powder. If you set light to sulfur, it will react
with the oxygen in the air producing a beautiful blue flame and sulfur dioxide gas. sulfur
atoms, if we had them free, would do the same. So, the first statement is false and an
incorrect answer.
The second statement is that sulfur atoms
consist of multiple sulfur molecules bonded together. The truth is, it’s the other way around.
sulfur molecules contain sulfur atoms. We see things the correct way round in
the third statement. A sulfur molecule consists of multiple
sulfur atoms bonded together. This statement correctly describes the
difference between a sulfur atom and a sulfur molecule. But just in case, let’s check the last
statement.
The first part of this statement is that
sulfur atoms can be divided into smaller units. This is, of course, true. Electrons can be taken off or added to
sulfur atoms, forming sulfur ions. And if we try really hard, we can add or
remove protons and neutrons from the nucleus. Although, in general, we do think about
the atom as being fairly fundamental because splitting the atom is quite difficult and
releases tremendous amounts of energy. Adding or taking away electrons is much
less worrisome.
The second part of the statement is that
sulfur molecules cannot be divided into smaller units. This is false because we know that sulfur
molecules are made of sulfur atoms. So, sulfur molecules can be divided into
smaller units that are atoms or even smaller units that are protons, neutrons, and
electrons.
To round it all off, let’s have a look at
the key points. An atom is a positively charged, densely
packed nucleus with enough bound electrons to make it neutral overall. An element is just the type of an
atom. And it’s based on the number of protons
in the atom’s nucleus. There are currently 118 different
confirmed chemical elements. And their information is stored on the
periodic table. In the future, we may discover more. And a molecule is two or more atoms
covalently bonded together. And lastly, intermolecular forces, the
forces between molecules, determine how molecular substances behave, as solids, liquids, or
gases.
