Isotopes

4 minAS Level1.2.11.2.21.2.31.2.4

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can define an isotope, read the notation ${}^{x}_{y}\text{A}$ , and explain why isotopes of an element react the same way but have different masses and densities.

Definition

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons.

Reading the notation

${}^{x}_{y}\text{A}$

x (top left) is the mass number (nucleon number): protons + neutrons.
y (bottom left) is the atomic number (proton number).
A is the element's symbol.

isotope	protons	neutrons	electrons (atom)
${}^{35}_{17}\text{Cl}$	17	18	17
${}^{37}_{17}\text{Cl}$	17	20	17
${}^{1}_{1}\text{H}$	1	0	1
${}^{2}_{1}\text{H}$ (deuterium, D)	1	1	1

You'll also see isotopes named in words: chlorine-37 means mass number 37.

Quick check

DefinitionQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Answer the 3 questions above to continue.