Determine the number of protons and the number of electrons...

Determine the number of protons and the number of electrons in each ion.
$$\text { a. }\mathrm{Ni}^{2+} \quad \text { b. } \mathrm{s}^{2-} \quad \text { c. Br }^{-} \quad \text { d. } \mathrm{Cr}^{3+}$$

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Key Concepts

Atomic Number

The atomic number of an element represents the number of protons found in the nucleus of its atoms. It uniquely identifies the element and is used to determine its position in the periodic table. The atomic number directly informs us about the number of positive charges in a neutral atom or ion, and hence is essential when calculating the balance of electrons in ions.

Ions and Their Charges

An ion is an atom or molecule that has a net electrical charge due to the loss or gain of one or more electrons. When an atom loses electrons, it becomes a positively charged ion (cation), and when it gains electrons, it becomes a negatively charged ion (anion). The magnitude of the ion’s charge indicates by how many electrons the number differs from the number of protons, which is key to understanding its electron configuration compared to its neutral state.

Electron Count Determination

In determining the number of electrons in an ion, one starts with the number of electrons in the neutral atom (which is equal to the atomic number) and then adjusts for the ion’s charge. For cations, electrons are subtracted by the charge value, and for anions, electrons are added. This principle allows for understanding the electronic structure of ions which is critical in predicting their chemical properties and behavior.

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