What are isotopes?
Atoms of the same element can hold different numbers of neutrons.
Their atomic number — the number of protons — is the same, because that is what makes them the same element. But the mass number is protons + neutrons, so a change in the neutrons changes the mass number.
Isotopes
Atoms of the same element having different mass numbers are called the isotopes of that element.
What stays and what changes
| Between isotopes of one element | |
|---|---|
| Protons | Same |
| Atomic number (Z) | Same |
| Electrons | Same |
| Neutrons | Different |
| Mass number (A) | Different |
What this shows
Notice that the electrons are unchanged. Since chemistry is decided by the electrons, isotopes of an element behave the same way chemically — they differ only in mass.