# What are TWO substances that have the same atomic number, but different atomic masses?

Jan 21, 2017

$\text{Hydrogen and deuterium?}$

#### Explanation:

$\text{Protium}$, ""^1H, and $\text{deuterium}$, ""^2H, are two of the most abundant substances in the universe. Of course, they are isotopes that differ in the number of neutrons present in their nuclei. If we consider the atoms, ""^1H, and, ""^2H, clearly these each have the one electron, and the one $\text{nucular}$ proton (which defines them as hydrogen isotopes), but ""^2H has an extra NEUTRON in its nucleus, which gives rise to its isotopic status.

All isotopes fit your proposed criteria.

Jan 21, 2017

Two atoms with the same number of protons but different number of neutrons are called isotopes, examples are C^12:C^14 ; U^235: U^238

#### Explanation:

Some ratios of protons to neutrons are more stable than other ratios. Carbon 12 is the most common form of Carbon. Carbon 12 is very stable. Carbon 14 is a very rare form of Carbon. Carbon 14 is radioactive, it breaks down into a beta particle and forms Nitrogen 14.

All forms of Uranium are radioactive, but some are more radioactive than others. ${U}^{235}$ is highly radioactive and was used to make the first atomic bomb. ${U}^{238}$ is a more common and less radioactive form of Uranium.

All atoms of the same element have the same number of protons (and electrons in non ionic forms) but can vary in the number of neutrons. These variations are called isotopes.