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Atomic Number and Mass Number
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Early Atomic Theory: Dalton, Thomson,
Rutherford and Millikan
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1. Atomic Number and Mass Number
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2. Early Atomic Theory: Dalton,
Thomson, Rutherford and Millikan
3. Isotopes and Average Atomic Mass
0:25 The Atom 3:58 Neutrons 8:29 Summary 4. Avogadro's Number: Using the Mole
to Count Atoms
1:13 Protons 6:59 Electrons
5. Electron Configurations in Atomic
Energy Levels
Did You Know…
6. Four Quantum Numbers: Principal,
This lesson is part of a free course that leads to real college credit accepted by 2,900 colleges. Angular Momentum, Magnetic & Spin
Learn how simple it is.
7. The Bohr Model and Atomic Spectra
Atoms are the basic building blocks of everything around you. In order to really understand how atoms
combine to form molecules, it's necessary to be familiar with their structure. In this lesson, we'll dissect CHEMISTRY 101: GENERAL CHEMISTRY
atoms so we can see just what really goes into those little building blocks of matter. COURSE INFO
Subatomic Particles View all 30 Videos
Did you know that you are made up of mostly empty space? That is not meant to be an insult; more of an
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example of how you are made up of atoms and most of the volume of an atom is completely empty
space. An atom is the smallest particle of an element that still has the same properties of that element. Enter your email Go
Remember that an element is a pure substance made up of only one type of atom. Thomson,
Rutherford, and Millikan (all scientists) discovered that atoms were made up of even smaller particles
called subatomic particles.
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This lesson is going to focus on the three main subatomic particles: the proton, neutron, and electron. As
we move through this lesson, I would highly recommend having a periodic table in front of you as a Biology 101: Intro to Biology
reference. The periodic table is an organized reference of all of the known elements arranged
Biology 102: Basic Genetics
according to their properties. You will almost always have access to one of these so it would be a good
idea to learn how to use it. Biology 105: Anatomy & Physiology
Protons Business 101: Principles of
Protons are probably one of the single most important parts of an atom. The number of protons an atom Management
has determines what type of element it is. For example, all atoms that have only one proton are going to
Business 102: Principles of Marketing
be atoms of hydrogen. Find hydrogen on the topleft side of your periodic table. You should notice a few
2. things about this box. First, it may have a large H printed in it. This is the chemical symbol for hydrogen. Business 103: Introductory Business
Each element has a different chemical symbol. You should also notice that the number one is Law
prominently displayed in the box. This indicates the number of protons that atoms of that element have.
Business 104: Information Systems and
You may quickly notice that this number is increasing by one as you move from left to right and top to Computer Applications
bottom on the periodic table. So, helium has the number two, lithium has the number three, and so on.
This means that all atoms of helium have two protons and all atoms of lithium have three protons. The Chemistry 101: General Chemistry
number of protons is called the atomic number and it is really special because it gives the atom its
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identity. Keep in mind that not all periodic tables are exactly the same so some may have the atomic
number for each element in a different location. English 101: English Literature
Another interesting feature of the proton is that it carries an electrical charge of positive one. Don't worry English 102: American Literature
about the units of this charge just remember that it has a charge of positive one. Protons are also
English 104: College Composition
relatively heavy. They have a mass of about one amu, or atomic mass unit. Because atoms are so tiny,
they require a different type of unit to measure their mass. We will be using the atomic mass unit (or GED Math: Algebra, Geometry, &
amu) when we discuss the masses of atoms and molecules. The final feature of a proton is that it is Arithmetic
tucked away inside the nucleus of the atom. The nucleus is the very tiny, dense region in the center of
an atom. The nucleus is so tiny, that if the nucleus were the size of a basketball, the rest of the atom GED Reading: Language Arts
would be the size of a large city. One thing that gets a little confusing is that the word nucleus can mean GED Science: Life, Physical and
different things depending on which branch of science you are in. In chemistry, the word nucleus refers to Chemical
the nucleus (or center) of an atom. In biology, the word nucleus refers to the nucleus of a cell. To give
you a visual idea of the difference between an atom and a cell, there are trillions upon trillions of atoms GED Social Studies: History,
Geography, Economics & Civics
that make up each living cell. And atoms are not living they are the building blocks of everything.
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Neutrons
The next particle inside the atom is called the neutron. Just like protons, neutrons are also located inside History 101: Western Civilization I
the nucleus of the atom. Neutrons also have a mass of about one amu. So protons and neutrons are
both located inside the nucleus and they both have a mass of about one amu. Unlike protons, however, History 103: U.S. History I
neutrons do not carry an electrical charge. They are neutral particles. This should be very easy to History 104: US History II
remember because the words neutron and neutral are so similar.
Humanities 101: Intro to the Humanities
Remember that all atoms of the same element have the same number of protons. Does this mean that
they have to have the same number of neutrons? Nope! Atoms of the same element can have different Math 101: College Algebra
numbers of neutrons. These atoms with the same number of protons, but different numbers of neutrons,
Math 102: College Mathematics
are called isotopes. Think of isotopes like siblings they are related, but have some differences. For
example, all carbon isotopes have the same number of protons: six. However, some carbon atoms are Math 103: Precalculus
going to have six neutrons, some will have seven, and some may have eight neutrons. These isotopes
will all chemically react the same way, but they will have differences in their nuclear stability, which means Math 104: Calculus
that some will be able to undergo a nuclear reaction more easily than others. This is because they all
Psychology 101: Intro to Psychology
have different amounts of neutrons in their nucleus.
Psychology 102: Educational
So, how do we tell the difference between the different isotopes of carbon? For this, we use nuclear Psychology
symbols. The letters in a nuclear symbol represent the element. Recall that each element has its own
abbreviation. In this example we have a C, which is the chemical symbol for carbon. The numbers tell us Psychology 103: Human Growth and
what is located inside the nucleus. The number on the lower left of the symbol is the atomic number (or Development
the number of protons). You will also notice that all of these have the same atomic number because they
Science 101: Intro to Natural Sciences
are all carbon atoms. The top number equals the number of protons plus the number of neutrons. This is
called the mass number. So on the first one, the mass number is 12 indicating that the total number of Sociology 101: Intro to Sociology
protons and neutrons is 12. Below that is the number six, which is just the number of protons. So, how
many neutrons does this atom have? If you answered six, you are correct. You can find the number of
neutrons by subtracting the top number minus the bottom number. Try the next one. How many neutrons
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does this carbon atom have? This one has seven and the next one has eight. In this example, the carbon
with eight neutrons has the most unstable nucleus. This is the isotope that is used in carbon dating to
estimate how old an ancient artifact is.
Electrons
The last particle on our journey inside of the atom is the electron. Electrons are very different from
protons and neutrons. First of all, they are not located inside the nucleus. They are found in a very large
electron cloud outside of the nucleus. Remember if the nucleus were the size of a basketball, the rest of
the atom would be the size of a city. As you can see, the electrons have a very large space to move
around in; however, because they are so small, most of this large space is completely empty. That's why
you are made up of mostly empty space! Later on, we'll also find that these speedy little guys play a key
role in determining the chemical properties of an atom.
Also, every electron carries a negative one charge. This exactly equal and opposite to the charge of a
proton, so one proton and one electron would completely cancel out in charge. Atoms are electrically Meet all 53 of our instructors
neutral, which means that they will have the same number of protons and electrons. So, back to our
