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# How to Determine Number of Protons and Electrons in Ions The number of protons and electrons in an atom or molecule determines its charge and whether it is a neutral species or an ion. This worked chemistry problem demonstrates how to determine the number of protons and electrons in an ion. For atomic ions, the key points to keep in mind are:

• A neutral atom has the same number of protons and electrons. This number is the atomic number of the element.
• A positively-charged ion or cation has more protons than electrons. The proton number is the atomic number of the element, while the electron number is the atomic number minus the charge.
• A negatively-charged ion or anion has more electrons than protons. Again, the number of protons is the atomic number. The number of electrons is the atomic number added to the charge.

### Protons and Electrons Problem

Identify the number of protons and electrons in the Sc3+ ion.

#### Solution

Use the Periodic Table to find the atomic number of Sc (scandium). The atomic number is 21, which means that scandium has 21 protons.

While a neutral atom for scandium would have the same number of electrons as protons, the ion is shown to have a +3 charge. This means it has 3 fewer electrons than the neutral atom or 21 - 3 = 18 electrons.