Atomic Number and Mass Number
Atomic number is the number of protons; mass number is protons plus neutrons.
About this topic
The atomic number (Z) is the number of protons in the nucleus, and it identifies the element: all carbon atoms have Z = 6. The mass number (A) is the total number of protons and neutrons. The number of neutrons is A minus Z, and in a neutral atom electrons equal protons.
Nuclide notation writes the mass number as a superscript and the atomic number as a subscript before the symbol, for example 23 over 11 Na. Atomic number is the true identity card of an element, which is why the periodic table is ordered by it. Mass number is always a whole number, unlike the average atomic mass shown on the table, which is usually a decimal.
Worked example: chlorine-35 has Z = 17 and A = 35, so it has 17 protons, 18 neutrons and, if neutral, 17 electrons. Chlorine-37 has the same 17 protons but 20 neutrons. For an ion such as Mg2+ with Z = 12, there are 12 protons but only 10 electrons.
These numbers help chemists identify unknowns, balance nuclear equations, and understand isotopes. A common error is mixing up the two numbers or using the decimal atomic mass as the mass number; always round to find the most common mass number, and remember that changing neutrons never changes the element.
Key ideas
- Z identifies the element
- A = Z + N
- Atoms are neutral when p = e
Quick check
Use cases: where Atomic Number and Mass Number helps
- Homework: review the key idea behind Atomic Number and Mass Number before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Atomic Number and Mass Number without notes.
- Visual learning: play with the interactive lab to see how Atomic Number and Mass Number changes when you adjust its inputs.
- Teaching a friend: explain Atomic Number and Mass Number in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Atomic Number and Mass Number?
Atomic number is the number of protons; mass number is protons plus neutrons.
What will I learn in Atomic Number and Mass Number?
Z identifies the element. A = Z + N. Atoms are neutral when p = e.
Is Atomic Number and Mass Number part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Atomic Number and Mass Number lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Atomic Number and Mass Number?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.