A student claims that the way to convert a neutral atom of \(\mathrm{Cl}\) into a \(\mathrm{Cl}^{-}\) anion is to decrease its atomic number by one while leaving the number of electrons unchanged. (a) Will this give the atom a \(-1\) charge and convert it into an anion? Explain. (b) Will the atom still be an atom of the element chlorine? Explain. (c) What should the student have done?

Understanding the atomic number is fundamental when it comes to the chemical identity of elements. The atomic number of an element is essentially the number of protons present in the nucleus of its atoms. This is a unique value for each element in the periodic table and is represented by the symbol 'Z'.

For instance, chlorine has an atomic number of 17, which means every atom of chlorine has 17 protons. If you change the atomic number, as the student suggested, you're no longer dealing with chlorine. Adjusting the atomic number by reducing or increasing it will transform the atom into a completely different element. Gaining or losing electrons, on the other hand, will form ions but the atomic number remains constant, keeping the element's identity.

Anions and cations are types of ions, which are atoms or molecules that have gained or lost electrons, thus obtaining a net charge. Cations are positively charged ions formed when an atom loses one or more electrons. Anions, conversely, are negatively charged ions formed when an atom gains one or more electrons.

Ionic Formation

In our chlorine example, to form a chloride ion (Cl-), a neutral chlorine atom (which is neither an anion nor a cation) needs to gain one electron. This additional electron results in there being more electrons than protons, giving the chlorine atom a net negative charge and turning it into an anion. This process is crucial for many chemical reactions, including the formation of salts.

The chemical identity of an element is defined by its atomic number (the number of protons) and, to a lesser extent, its isotopes (which are variations based on neutron number). Electrons do not define an element’s chemical identity, but they do play a significant role in chemical bonding and the formation of compounds.

Element vs. Ion

It is crucial to differentiate between changing the chemical identity and changing the ionic state of an element. When a chlorine atom becomes a chloride ion, its chemical identity as chlorine is preserved because the atomic number remains unchanged. Only the electron count (and thus the charge) is altered. This is a core concept in chemistry, as misinterpreting this could lead to confusion between different elements and ions.

The charge of ions is determined by the balance between the number of protons (positively charged particles in the nucleus) and electrons (negatively charged particles orbiting the nucleus).

Understanding Charge

When an atom has an equal number of protons and electrons, the charges cancel each other out, resulting in a neutral atom. When an atom gains electrons and has more electrons than protons, it becomes a negatively charged ion, or an anion. Conversely, when an atom loses electrons and has fewer electrons than protons, it becomes a positively charged ion, or a cation. In our chlorine example, adding one electron results in a chloride ion (Cl-) with one more electron than protons, which gives it a negative charge.