The mass number of an atom is the sum of its protons and neutrons. The mass of electrons is negligibly small and is typically ignored. The atomic number represents the number of protons in an atom, which is also equal to the number of electrons in a neutral atom.

These relationships are commonly tested in chemistry, especially when studying the structure of an atom. Typically, two values are provided, and students are tasked with finding the missing ones.

This is where our calculator comes in! Simply enter at least two known values (e.g., protons and neutrons, or atomic number and mass number), click the Calculate button, and the calculator will determine the remaining values for you. It’s that easy and convenient!

Problem Examples

1. The table below gives some information about five different atoms. Determine the missing figures. (The letters do represent the actual symbols of letters)

Atom	Atomic Number	Number of Protons	Number of Neutrons	Mass Number
A	11	(a)	(b)	23
B	(c)	15	(d)	31
C	6	(e)	(f)	12
D	(g)	18	22	(h)

2. Uranium-235 is an important isotope used in nuclear fission reactions for nuclear power and nuclear weapons. With an atomic number of 92 and atomic mass of 235, what are the numbers of protons, neutrons, and electrons in a neutrally charged atom of uranium-235?

To find the specific number of protons, neutrons, electrons, atomic number, and mass number for any element in the periodic table, use this calculator.