Determining the Number of Neutrons in an Atom

ATOMIC NUMBER: The number of protons in a given atom. It is also the order in which the atom is listed in the periodic chart.

ATOMIC MASS (or WEIGHT): The mass (or weight) of an atom given in "atomic mass units."

The mass or weight of a proton or neutron is "very close to one atomic unit." Therefore, the number of neutrons in an atom can be determined by subtracting the ATOMIC NUMBER from the ATOMIC MASS (or WEIGHT).

Note: Atoms that have the same atomic number (and therefore the same number of protons) but differ in their number of neutrons are said to be ISOTOPES of each other, or isotopes of the atom. The atomic mass is usually given as the average mass of all of the isotopes of the atom as they are found to exist in nature. We use the atomic mass rounded off to the nearest whole number in these exercises. (For example the vast majority of hydrogen atoms have no neutrons. A very, very few of them have one neutron, and still far fewer have two neutrons. Their average atomic mass is found to be 1.0079. So we use the whole number 1 as the atomic mass of the hydrogen atom in these exercises.)


The following exercises check your ability to calculate the number of NEUTRONS in a given atom. (Works best on an HTML standard 4.0 or later.)

Selection type: in order or at random

Atomic
Mass
- Atomic
Number
= Number
of
Neutrons
- =