Counting the nucleons
Mass number A.
How heavy is the nucleus?
Electrons weigh so little that we can ignore them. Most of an atom's mass is in the nucleus.
Which particles should we count to get a whole-number measure of an atom's mass?
What this lesson covers
The idea
The mass number A is the total number of protons and neutrons (nucleons) in the nucleus: mass number = number of protons + number of neutrons.
How heavy is the nucleus?
Electrons weigh so little that we can ignore them. Most of an atom's mass is in the nucleus.
Which particles should we count to get a whole-number measure of an atom's mass?
- Protons and neutrons
- Protons and electrons
- Electrons only
Build a nucleus
Use the buttons to set the protons and neutrons.
Protons plus neutrons
The protons and neutrons together are called nucleons. Their total is the mass number A. Electrons have almost negligible mass, so they are ignored.
The mass number A is the total number of protons and neutrons (nucleons) in the nucleus: mass number = number of protons + number of neutrons.
Notes
Mass number A = protons + neutrons, the total number of nucleons in the nucleus.
Check yourself
An atom has 6 protons and 6 neutrons. What is its mass number?
Answer: 12
6 + 6 = 12.
An atom has mass number 14 and 7 protons. How many neutrons does it have?
Answer: 7
14 − 7 = 7 neutrons.
What are nucleons?
Why can electrons be ignored when finding the mass number?
- Electrons and protons. Electrons are outside the nucleus, so they are not nucleons.
- Protons and neutrons in the nucleus — correct. Yes!
- Only the neutrons. Nucleons are the protons and the neutrons together.
- They have no charge. Electrons have a charge of −1.
- They are inside the nucleus. Electrons move around the nucleus, outside it.
- Their mass is almost negligible compared with protons and neutrons — correct. Yes!