Final answer:
An atom with a different number of neutrons than typically found on the periodic table is called an Isotope. These isotopes have the same number of protons but vary in their number of neutrons, altering their mass numbers but not their chemical properties.
Step-by-step explanation:
The term used to describe an atom with a number of neutrons that is higher or lower than what's indicated on the periodic table is Isotope. Isotopes have the same number of protons (which defines the element) but different numbers of neutrons, resulting in different mass numbers. The mass number (A) is the sum of the numbers of protons and neutrons in the nucleus of an atom. Isotopes of an element have identical chemical properties because they contain the same number of protons and electrons. The differences between isotopes of the same element lie solely in their atomic mass. For example, hydrogen has three isotopes: protium (¹H) with no neutrons, deuterium (²H) with one neutron, and tritium (³H) with two neutrons.