Video Transcript
In this video, we will learn how to
describe elements, compounds, and molecules, distinguish between them, and identify
chemical elements from their symbols.
Atoms make up the matter that is
all around us. Atoms were discovered by scientists
over 100 years ago. As technology got better over time,
the mass of atoms could be measured. The mass of an atom is incredibly
small. Scientists have also determined
that atoms are electrically neutral. This means atoms are not positively
charged or negatively charged. They are uncharged.
An atom is the smallest unit of
ordinary matter that forms a chemical element. For example, a piece of the element
gold is made up of atoms of gold. And if we filled a balloon with
pure helium gas, the balloon would contain atoms of helium. An atom is the most basic building
block of matter. Atoms make up all of the elements
in the matter we find around us in our world.
So then, what exactly are
molecules? A molecule is the smallest unit of
matter that can exist freely with well-defined physical and chemical properties. A molecule is an electrically
neutral group of two or more atoms held together by chemical bonds.
Oxygen gas is composed of oxygen
molecules. Each oxygen molecule contains two
oxygen atoms bonded together. Water is another substance that is
made up of molecules. Each water molecule is composed of
two hydrogen atoms and one oxygen atom. We can see from these examples that
the molecules of one substance are all the same. In other words, the molecules of a
substance have the same number and type of atoms. All oxygen molecules are composed
of two oxygen atoms, and all water molecules are composed of one oxygen atom and two
hydrogen atoms. Oxygen gas and water have some
important differences. Chemists classify oxygen gas as an
element, but they classify water as a compound. Let’s find out why.
To help us understand the
differences between elements and compounds, let’s take a look at two elements: gold
and oxygen gas. Gold and oxygen gas are both
classified as chemical elements. Chemical elements are substances
that consist of one type of atom. Gold is made up of only gold
atoms. Oxygen gas is made up of oxygen
molecules, but each oxygen molecule contains the same type of atom. Both of the atoms in the molecule
are oxygen atoms. We call the molecules that make up
an element elemental molecules. Water is a compound. Compounds are substances that
contain the atoms of two or more different elements. Let’s look more closely at the
molecules that make up water.
Each water molecule contains one
atom of the element oxygen and two atoms of the element hydrogen. Therefore, water is a compound
because its molecules contain atoms of two different elements. These examples have helped us to
see that chemical elements may be composed of atoms or molecules, but they only
contain one type of atom. In contrast, compounds contain the
atoms of at least two different elements. Many, but not all, compounds are
composed of molecules. For example, salt compounds are not
composed of molecules.
We’ve discussed the types of matter
that are made of molecules. But how do molecules form? First of all, atoms are the
smallest unit of matter that can undergo a chemical reaction. Two or more atoms may combine
during a chemical reaction and form a molecule. Let’s take a look at a few
examples. When one oxygen atom reacts with
another oxygen atom, an oxygen molecule forms. The oxygen molecule is an elemental
molecule because it contains atoms of the same element. When a hydrogen atom reacts with a
chlorine atom, a hydrogen chloride molecule forms. The hydrogen chloride molecule is
composed of atoms of two different elements. So, it is the molecule of a
compound.
Molecules may contain more than two
atoms. When two hydrogen atoms react with
one oxygen atom, a water molecule forms. When three hydrogen atoms react
with one nitrogen atom, an ammonia molecule forms. These molecules contain the atoms
of two different elements. So, they are molecules of
compounds. It’s hard to imagine what the size
of an atom or molecule is because they are so incredibly small. Let’s say we had a piece of wood
that is one meter in length. Then, we cut the wood into 10,000
pieces of equal size. Then, if we took just one of those
tiny pieces and cut it into one million pieces, we’d have a piece that is about the
size of an atom.
Molecules, which are made up of
atoms, are also amazingly small. One raindrop holds about 1.5
sextillion molecules of water. 1.5 sextillion is the number shown
here, which is a one and a five followed by 20 zeros. This number is billions of times
larger than the entire population of the world. Atoms and water molecules are so
small that they cannot be seen by the naked eye or with a light microscope.
Even though molecules are so small,
scientists know quite a lot about their properties. First of all, molecules are
continuously moving. In a solid, molecules move
slowly. But in liquids and gases, molecules
move much more quickly. Molecules are separated by
intermolecular space. For example, there is a lot of
empty space between the molecules of a gas. There are also intermolecular
forces of attraction between molecules. These forces tend to pull molecules
closer together. Energy is needed to weaken these
forces and separate the molecules.
Now that we have a better
understanding of the sizes and properties of atoms and molecules, let’s discuss how
chemists represent them with symbols. There are now over 100 known
elements. Scientists have created a chemical
symbol to represent each element. A chemical symbol is an
abbreviation that represents a chemical element. Some chemical symbols are one
uppercase letter. For example, the chemical symbol of
the element oxygen is uppercase O. Other elements that have one
uppercase letter for their chemical symbol are hydrogen, nitrogen, and carbon. The chemical symbol of hydrogen is
H, for nitrogen it is N, and for carbon it is C.
The chemical symbols of most
elements are two letters. These chemical symbols are written
as one uppercase letter followed by a lowercase letter. For example, the chemical symbol of
magnesium is uppercase M followed by lowercase g. Other elements that have chemical
symbols with two letters include aluminum, helium, and chlorine. The chemical symbol of aluminum is
Al, for helium it is He, and for chlorine it is Cl.
The chemical symbols of all of the
elements we have discussed so far come from the English names of the elements. But some chemical symbols come from
the Latin names of the elements. The Latin name of sodium is
natrium. Its chemical symbol is Na, not S or
So like we might think. The Latin name of potassium is
kalium. Its chemical symbol is K, not P or
Po like we might think.
Chemical symbols are very
useful. Chemists combine chemical symbols
to make chemical formulas. For example, an oxygen molecule is
composed of two oxygen atoms. So, its chemical formula is O2. A water molecule is composed of two
hydrogen atoms and one oxygen atom. So, the chemical formula of water
is H2O. The oxygen molecule is an elemental
molecule. The chemical formula of an element
has only one chemical symbol. Water is a compound. The chemical formula of a compound
has two or more different chemical symbols.
We can also write the chemical
formulas of salt compounds. For example, the chemical formula
of the salt compound, sodium chloride, is NaCl. Na is the chemical symbol of the
element sodium, and Cl is the chemical symbol of the element chlorine. The salt compound magnesium
chloride has the chemical formula MgCl2. The chemical symbol of the element
magnesium is Mg. And we know the chemical symbol of
the element chlorine is Cl.
Before we summarize our learning,
let’s take a look at a question.
The diagram below shows an atom of
oxygen, an atom of nitrogen, and an atom of hydrogen. Which of the following combinations
is an example of an elemental molecule?
The diagrams given in the answer
choices are all molecules. Molecules are made of two or more
atoms bonded together. We need to determine which of these
molecules is an elemental molecule. Chemical elements are substances
that consist of one type of atom. So, an elemental molecule contains
two or more of the same type of atom. Compounds are substances that
contain the atoms of two or more different elements. So, a molecule of a compound
contains two or more different atoms.
To help us solve this problem,
let’s find out which of the answer choices are elements and which are compounds. The molecule in choice (E) contains
one hydrogen atom, one oxygen atom, and one nitrogen atom. So, this molecule contains atoms of
three different elements. This is a molecule of a
compound. The molecule in choice (D) is made
of two oxygen atoms and one nitrogen atom. Oxygen and nitrogen are different
elements. So, this molecule is also a
molecule of a compound.
The molecule in choice (C) contains
two hydrogen atoms and one oxygen atom. Hydrogen and oxygen are different
elements. So, this is also a molecule of a
compound. The molecule in choice (B) contains
one atom of the element nitrogen and three atoms of the element hydrogen. So, it is also a molecule of a
compound. Finally, the molecule in choice (A)
contains two atoms of the element nitrogen. This molecule contains only one
type of atom, so it must be an elemental molecule. Choice (A) is a molecule of the
element nitrogen and is not a molecule of a compound. Therefore, the combination that is
an example of an elemental molecule is choice (A).
Now, let’s summarize our learning
with some key points. An atom is the smallest unit of
ordinary matter that forms a chemical element. Chemical elements are substances
that consist of one type of atom. Elemental molecules contain the
atoms of only one element. Compounds contain the atoms of at
least two different elements. Atoms and molecules cannot be seen
by eye or with a light microscope. All molecules have three basic
properties: continuous motion, intermolecular space, and intermolecular forces. A chemical symbol is an
abbreviation that represents an element.
