# 1.17: Ions

As discussed in the previous section, the number of protons in an atom determines to which element an atom belongs.  If the number of protons changes (as you will in see in unit 2 can happen during nuclear reactions) the identity of the atom changes.  In a neutral atom, the number of protons and electrons are equal to each other.  However, the number of electrons in an atom can change.  If the number of electrons changes compared to the number in a neutral atom, the resulting particle is charged.  Why?  Consider the following examples.

 Element Protons Electrons Net Charge Particle Symbol Potassium atom 19 19 0 Neutral atom K Potassium ion 19 18 +1 Cation K+ Sulfur atom 16 16 0 Neutral atom S Sulfur ion 16 18 −2 Anion S2−

Protons are positively charged particles and the number of protons is determined by the atomic number of the element.  Since an electron has the same magnitude of charge as a proton but opposite sign (electrons are negative), in order for an atom to be neutral, to have no net charge, the number of protons and electrons are equal to each other. If, however, there are fewer electrons than protons as in the case of the potassium ion, the net charge is positive since there is one more positive proton than negative electrons [(+19) + (−18) = +1]. Positive ions are cations. When there are more electrons than protons, as in the case of the sulfur ion, the net charge is negative [(+16) + (−18) = −2]. Negative ions are anions

Complete the following table and consider how you would explain your answers if you were teaching this concept to a peer.

 Element Protons Electrons Net Charge Particle Symbol Strontium atom 0 Sr Strontium ion +2 Cation Bromine atom Neutral atom Bromine ion Br−