Beginner Topic 20 of 1102 Atomic Structure & Periodicity
Mass Number
The total count of protons and neutrons in a nucleus, symbol A.
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In-depth explanation of Mass Number
The mass number, symbol A, is the total count of protons and neutrons in a nucleus. It is written as a superscript before the element symbol, as in carbon-14. The number of neutrons equals A minus Z, so knowing A and Z gives the full nuclear composition. Mass number is always a whole number, which distinguishes it from the average atomic mass listed in the periodic table. Electrons contribute negligibly to the mass, so they are not counted in the mass number.
Going deeper: Mass Number
Key ideas: Mass number A = protons + neutrons, a whole number written as the superscript left of the symbol. Neutrons = A - Z. For 35Cl, Z = 17, so 18 neutrons. A is not the atomic mass in u, though it is close to it.
Common mistakes and connections: Students confuse mass number (an integer for one isotope) with the average atomic mass on the periodic table (a weighted decimal). In nuclear equations, A and Z must each balance on both sides. It leads into isotopes, isobars and nuclear decay.
Visualization of Mass Number
Mass number counts protons plus neutrons; average atomic masses for Na to Ar rise roughly in step with the mass numbers of their common isotopes. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Uranium-238 has 92 protons and 238 - 92 = 146 neutrons.
Frequently asked questions about Mass Number
What is Mass Number?
The total count of protons and neutrons in a nucleus, symbol A.
Is Mass Number a beginner, intermediate or advanced topic?
Mass Number is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Mass Number?
Study these earlier topics in Atomic Structure & Periodicity first: Rutherford's Gold Foil Experiment, Nuclear Atom Model, Atomic Number.
What comes after Mass Number?
The next topics in Atomic Structure & Periodicity are Isotopes and Isobars.
Can I practise Mass Number with quizzes?
Yes. The interactive lesson includes a self-check question on Mass Number, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Mass Number fits in Atomic Structure & Periodicity
Mass Number is topic 20 of 115 in Atomic Structure & Periodicity.