Determine the number of protons and the number of electrons in each ion. a. \(\mathrm{Ni}^{2+} \quad\) b. \(\mathrm{s}^{2-} \quad\) c. Br \(^{-1} \quad\) d. \(\mathrm{Cr}^{3+}\)

The atomic number is a fundamental concept in chemistry that identifies each unique element. Understanding the atomic number of an element is crucial because it tells you the number of protons found in the nucleus of an atom. Since protons are positively charged particles, the atomic number also indicates the charge of the nucleus.

In a neutral atom, which has no overall charge, the atomic number is equal to the number of electrons as well. Electrons are the negatively charged particles that orbit the nucleus. The atomic number is typically located above the element symbol on the periodic table, making it easy to find. For instance, hydrogen, being the first element, has an atomic number of 1, which means it has one proton and, when neutral, one electron.

When it comes to ions, which are charged atoms, the number of electrons changes, but the number of protons remains the same. This constancy of proton number is what preserves the identity of the element, regardless of its ionic state.

To illustrate, a nickel ion with a 2+ charge (represented as \( \mathrm{Ni}^{2+} \) ) still has 28 protons, the same as neutral nickel. The 2+ charge indicates that it has lost two electrons, resulting in a total of 26 electrons instead of the 28 it would have if it were neutral. Conversely, a sulfur ion with a 2- charge (represented as \( \mathrm{S}^{2-} \) ) carries two extra electrons, having 18 instead of 16. This changes the ion's charge but not its basic elemental composition.

The periodic table is an organized display of chemical elements, arranged by increasing atomic number. It is not just a list but a powerful tool that reflects the electron configuration of elements and their recurring properties. The periodic table's vertical columns are called groups or families; elements within the same group have similar chemical behaviors.

This similarity arises because they have the same number of electrons in their outermost shell, a condition that dictates how they react with other substances. The horizontal rows, termed periods, reflect the disposition of elements' atomic orbitals and overall complexity as you move from left to right. Each element's position on the periodic table can provide a wealth of information, such as whether it is a metal, nonmetal, or metalloid, its relative reactivity, and the type of ions it is likely to form.