Video Transcript
In this video, we will learn how to
describe the composition of the atom, determine its atomic number, and calculate its
mass number.
Our world is made up of matter. The air we breathe, the water we
drink, and the clothes we wear are all matter. If we could zoom in on the matter
around us, we would find that all of this matter is made up of atoms. Atoms are the smallest unit of
ordinary matter that forms a chemical element. We sometimes refer to atoms as the
building blocks of matter.
Atoms are incredibly small, many,
many times smaller than a meter. To describe the size of an atom,
scientists typically use the unit angstroms. There are 10,000 million angstroms
in one meter. The radius of an atom is only about
0.3 angstroms. To put this in perspective,
mechanical pencil lead ordinarily has a diameter of 0.5 millimeters. If we could line up atoms right
beside each other across the tip of the pencil lead, over eight million atoms would
fit in a row.
Even though atoms are so small,
there are even smaller pieces of matter that make up atoms. The smaller pieces of matter that
can form an atom are called subatomic particles. There are three types of subatomic
particles. The first two subatomic particles
are protons and neutrons. Protons are positively charged,
while neutrons are neutrally charged. Protons and neutrons are found in
the central region of an atom. We call this central region the
nucleus. As the nucleus contains positively
charged protons and neutrally charged neutrons, the overall charge of the nucleus is
positive.
The third type of subatomic
particle is the electron. Electrons are negatively
charged. In an atom, electrons are usually
found in areas outside of the nucleus called energy levels. The negatively charged electrons
remain in the atom because they are attracted to the positively charged nucleus.
Despite containing both positively
charged particles and negatively charged particles, an atom is neutral overall. In order for this to be true, atoms
must contain the same number of protons and electrons. So if this atom contained two
positively charged protons, it would also have to contain two negatively charged
electrons in order to have an overall neutral charge.
When we compare the three types of
subatomic particles, we find that protons and neutrons are relatively large and
heavy, while electrons are relatively small and have negligible mass. This means that most of an atom’s
mass is found in the nucleus.
While the general structure of all
atoms is the same, not all atoms have the same number of protons, neutrons, and
electrons. Chemists can describe the
composition of atoms using a notation that looks like this. The letters are the chemical symbol
of an element. This indicates the type of
atom.
The bottom-left number is called
the atomic number. The atomic number is the number of
protons in the atom. This value is unique for each
element. For example, all atoms of helium
have two protons and therefore an atomic number of two. But all atoms of lithium contain
three protons and have an atomic number of three.
The number to the top left is
called the mass number. The mass number is the sum of the
number of protons and neutrons in the atom. In other words, the mass number is
equal to the number of protons plus the number of neutrons in the nucleus of an
atom.
Using the given notation, we can
determine the number of protons and neutrons in this atom of helium. We know from the atomic number that
this atom of helium contains two positively charged protons. We can use the mass number equation
to determine the number of neutrons by substituting the mass number and number of
protons into the equation. The mass number is four, and the
number of protons is two. If we subtract two from both sides
of the equation, we find that the atom of helium contains two neutrally charged
neutrons.
We can also determine the number of
electrons by remembering that atoms contain the same number of protons and
electrons. As this atom contains two protons,
it must also contain two electrons.
Now let’s take a look at three
atoms of hydrogen. Since all three atoms are of the
same type of element, they all contain the same number of protons. This means that all three atoms
have an atomic number of one. As each atom contains one proton,
it must also contain one electron in order to be neutrally charged overall.
But despite being the same type of
element and containing the same number of protons and electrons, each of these atoms
contains a different number of neutrons. This means that each of these atoms
has a different mass number. So, atoms of the same type of
element will have the same number of protons and the same number of electrons. But the number of neutrons could be
the same as the number of protons, less than the number of protons, or greater than
the number of protons.
Before we summarize what we’ve
learned in this video, let’s take a look at a few questions.
Which two types of particles can be
found in the nucleus of an atom?
An atom is the smallest unit of
ordinary matter that forms a chemical element. There are two main regions in an
atom: the nucleus in the center of the atom and energy levels outside of the
nucleus. These regions of the atom are where
we can find subatomic particles. Subatomic particles are smaller
pieces of matter that can form an atom. There are three types of subatomic
particles. The three types are positively
charged protons, neutrally charged neutrons, and negatively charged electrons. The protons and neutrons are found
in the nucleus in the center of the atom, while electrons are ordinarily found
outside of the nucleus in energy levels.
We now know that the two types of
particles that can be found in the nucleus of an atom are the protons and
neutrons.
Which of the following best
describes an electron? (A) A positively charged particle
with a much smaller mass than that of the nucleus. (B) A negatively charged particle
with a much smaller mass than that of the nucleus. (C) A positively charged particle
with a much greater mass than that of the nucleus. (D) A negatively charged particle
with a much greater mass than that of the nucleus. (E) A neutral particle with a mass
equal to that of the nucleus.
An electron is a subatomic
particle. Subatomic particles are pieces of
matter that can form an atom. There are three types of subatomic
particles: protons, neutrons, and electrons. Protons are positively charged,
neutrons are neutrally charged, and electrons are negatively charged. This question asked, which of the
statements best describes an electron? Knowing that an electron is
negatively charged, we can eliminate answer choices (A), (C), and (E).
In comparison to one another,
protons and neutrons are relatively large and heavy, while electrons are relatively
small and have a negligible mass. Protons and neutrons are found in
the central region of the atom called the nucleus, while electrons are ordinarily
found outside of the nucleus in energy levels. As the nucleus contains heavy
protons and neutrons, it contains most of an atom’s mass. When compared to the nucleus, an
electron has a much smaller mass.
In conclusion, the statement that
best describes an electron is answer choice (B), a negatively charged particle with
a much smaller mass than that of the nucleus.
Which of the following equations
can be used to calculate the mass number of an atom? (A) Mass number equals number of
protons plus number of neutrons plus number of electrons. (B) Mass number equals number of
neutrons divided by number of protons. (C) Mass number equals number of
protons plus number of neutrons. (D) Mass number equals number of
protons plus number of electrons. (E) Mass number equals number of
protons times number of neutrons.
An atom is a very small piece of
matter that consists of even smaller pieces of matter called subatomic
particles. The subatomic particles found in
the center of the atom, known as the nucleus, are positively charged protons and
neutrally charged neutrons. The negatively charged subatomic
particles, ordinarily found outside the nucleus in energy levels, are called
electrons.
Not all atoms have the same number
of protons, neutrons, and electrons. We can describe the makeup of an
atom using atomic number and mass number.
The atomic number is equal to the
number of protons in the nucleus of an atom. Since atoms are neutrally charged
overall, the number of positively charged protons must equal the number of
negatively charged electrons. So for atoms, the atomic number
also equals the number of electrons in the atom. The atom shown in the diagram has
one proton and one electron and therefore an atomic number of one.
The mass number of an atom is equal
to the number of protons plus the number of neutrons in the nucleus of an atom. The atom shown in the diagram has
one proton and one neutron. Therefore, it has a mass number of
two.
The question asked, which of the
equations can be used to calculate the mass number? The mass number is the sum of the
number of protons and neutrons. So, the equation that can be used
to calculate the mass number of an atom is the equation shown in answer choice (C):
mass number equals number of protons plus number of neutrons.
An atom of phosphorus has the
chemical symbol 31 15 P. What is the mass number of this
atom?
Let’s take a closer look at the
provided chemical symbol.
The capital P is the chemical
symbol of the element phosphorus. In this notation, the number
written to the bottom left of the chemical symbol is the atomic number. The atomic number indicates the
number of protons in the nucleus of the atom. So this atom of phosphorus has 15
protons.
The number written to the top left
of the chemical symbol is the mass number. The mass number is the number of
protons and neutrons in the nucleus. We can express this as the equation
mass number equals number of protons plus number of neutrons. This atom has a mass number of 31
and 15 protons. By subtracting 15 from both sides
of the equation, we find that this atom of phosphorus has 16 neutrons.
In conclusion, the mass number of
the phosphorus atom given is 31.
Now let’s wrap up this video by
reviewing what we’ve learned. Atoms are the smallest unit of
ordinary matter that form a chemical element. Atoms are usually measured in
angstroms, a unit 10 billion times smaller than a meter. Atoms are composed of positively
charged protons, neutrally charged neutrons, and negatively charged electrons. The nucleus in the center of the
atom contains the protons and neutrons, while electrons ordinarily stay outside of
the nucleus in energy levels.
Atoms do not have an overall
electric charge. This is because they contain the
same number of positively charged protons and negatively charged electrons. The atomic number represents the
number of protons in an atom. The mass number is equal to the sum
of the number of protons and neutrons in an atom. The mass number and atomic number
can be included as part of an atom’s chemical symbol.