Atomic Number and Mass Number

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  • 0:19 The Atom
  • 1:07 Protons
  • 3:52 Neutrons
  • 6:53 Electrons
  • 8:23 Summary
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Lesson Transcript
Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

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 atoms so we can see just what really goes into those little building blocks of matter.

Subatomic Particles

The periodic table of elements
Periodic Table Image

Did you know that you are made up of mostly empty space? That is not meant to be an insult; more of an 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. 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.

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 reference. The periodic table is an organized reference of all of the known elements arranged 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.


Protons are probably one of the single most important parts of an atom. The number of protons an atom has determines what type of element it is. For example, all atoms that have only one proton are going to be atoms of hydrogen. Find hydrogen on the top-left side of your periodic table. You should notice a few things about this box. First, it may have a large H printed in it. This is the chemical symbol for hydrogen. Each element has a different chemical symbol. You should also notice that the number one is prominently displayed in the box. This indicates the number of protons that atoms of that element have. You may quickly notice that this number is increasing by one as you move from left to right and top to 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 number of protons is called the atomic number, and it is really special because it gives the atom its 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.

Each box in the periodic table shows the chemical symbol and atomic number of the element
Periodic Table Hydrogen

Another interesting feature of the proton is that it carries an electrical charge of positive one. Don't worry about the units of this charge - just remember that it has a charge of positive one. Protons are also 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 amu) when we discuss the masses of atoms and molecules. The final feature of a proton is that it is 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 would be the size of a large city. One thing that gets a little confusing is that the word nucleus can mean different things depending on which branch of science you are in. In chemistry, the word nucleus refers to 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 that make up each living cell. And atoms are not living - they are the building blocks of everything.


The next particle inside the atom is called the neutron. Just like protons, neutrons are also located inside 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, neutrons do not carry an electrical charge. They are neutral particles. This should be very easy to remember because the words neutron and neutral are so similar.

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 numbers of neutrons. These atoms with the same number of protons, but different numbers of neutrons, 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 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 that some will be able to undergo a nuclear reaction more easily than others. This is because they all have different amounts of neutrons in their nucleus.

The mass number is equal to the atomic number plus the number of neutrons
Isotope Examples

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Additional Activities

Atomic Number and Mass Number

The periodic table has all known elements listed on it. In each element's box are three very important pieces of information, the element's chemical symbol, its atomic number, and its atomic mass. The atomic number and atomic mass tell us the number of protons (+ charge) and electrons (- charge) in a neutral atom and the atomic mass tells us the sum of the protons and neutrons (0 charge) in an atom, respectively. In this activity, we are going to "build" atoms with beans and lentils.


  • A bag each of dried black beans, dried white beans, and dried green lentils.
  • A periodic table.
  • Calculator, if necessary.


  1. Pick an element from hydrogen to iron.
  2. Use the periodic table to locate the element's atomic number. Place that many black beans in a pile. They represent the number of protons. Calculate how many white beans (neutrons) you will need to add to the pile to get the total number of beans in the pile equal to the rounded value of that element's atomic mass.
  3. Place the atomic number of green lentils symmetrically spaced around the pile of black and white beans. The green lentils represent the electrons in a neutral atom of that element.
  4. Repeat this procedure for a total of 5 elements from hydrogen to iron.

Follow Up Questions:

  1. Why do you think we rounded the atomic mass from the values on the periodic table?
  2. Why are atoms electrically neutral?
  3. What mathematical equation can you write for the atomic mass of an atom?


  1. We rounded the values because we can't have fractions of a subatomic particle.
  2. There are equal numbers of protons and electrons. These charges cancel each other out.
  3. Atomic mass = protons + neutrons, or Z = p + n

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