# Isotopes and ions

### Isotopes

of an have the same number of , but different numbers of in their . Isotopes of an element have:

• the same atomic number
• different mass numbers

### Three isotopes of hydrogen

All hydrogen atoms contain one proton (and one electron), but they can contain different numbers of neutrons. Hydrogen-1 is the most isotope of hydrogen.

Three isotopes of hydrogen, and their symbols, and numbers of protons, neutrons and electrons

All three isotopes of hydrogen have identical chemical . This is because the number of determines chemical properties, and the atoms of all three isotopes have one electron.

### Ions

are charged particles formed when atoms, or groups of atoms, lose or gain electrons:

• positively charged ions form when electrons are lost
• negatively charged ions form when electrons are gained

The charge on an ion is shown as a superscript. For example, an aluminium ion Al3+ carries 3 positive charges. It forms when an aluminium atom loses 3 electrons.

When you calculate the number of subatomic particles in an ion, you need to take into account the number of electrons lost or gained.

Question

The atomic number of a chloride ion, Cl-, is 17 and its mass number is 35. Calculate the number of protons, neutrons and electrons it contains.

• number of protons = 17
• number of electrons = 17 + 1 = 18
• number of neutrons = 35 - 17 = 18
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