What do you call the gain of electrons by an element

We talked about the octet rule, which states that "all elements tend to want to gain or lose electrons so they have the same electron configuration as the nearest noble gas." Basically, what this means is that a neutral atom of any element other than a noble gas isn't very stable. As a result, it will gain or lose electrons until it attains the stable electron configuration of a noble gas. Atoms that have gained electrons are called anions and have negative charge, and atoms that have lost electrons are called cations and have positive charge. When anions stick to cations, the resulting chemical compound is called an ionic compound.

Figure 8.1Anions and cations are formed when an atom gains or loses electrons to achieve the electron configuration of the closest noble gas.

One question you may be asking yourself is, "How many electrons do different elements want to gain or lose?" The answer to this question, like many things in chemistry, can be found on the periodic table.

An anion is a negatively charged atom or group of atoms, while a cation is an atom or group of atoms with positive charge. When an anion sticks to a cation, the result is an ionic compound.

The best way to figure out how many electrons will be gained or lost by a specific element is to count forward from it in the periodic table until you reach the next noble gas, and then count backward from it in the periodic table until you reach the last noble gas. If the forward direction requires less counting than the backward direction, the element will gain the same number of electrons as you counted to the closest noble gas to form an anion. Likewise, if the backward direction requires less counting, the element will lose the same number of electrons as you counted to the nearest noble gas to form a cation. Now, this is very important: Skip over the transition metals when counting to the noble gases—otherwise, things won't work out the way you'd like.

Problem 1: What will the charges of the following elements be when they gain or lose electrons to gain the same electron configurations as the nearest noble gas? (a) magnesium (Mg) (b) aluminum (Al)

For example, oxygen forms ionic compounds as an anion with -2 charge because it needs to gain two electrons to have the same electron configuration as neon. Gallium, on the other hand, has a +3 charge because it needs to lose three electrons to have the same electron configuration as argon. If you're confused because it looks like you need to count backward by 13 rather than three, remember the rule to ignore the transition metals!

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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Chemistry: What's in a Name? Ionic Nomenclature

Chapter 1 • Chapter 2 • Chapter 3 • Chapter 4 • Chapter 5 • Chapter 6 • Chapter 7 • Chapter 8 Chapter 9 • Chapter 10 • Chapter 11 • Chapter 12 • Chapter 13 • Chapter 14

Learning Objectives

Define the two types of ions.

Most atoms do not have eight electrons in their valence electron shell. Some atoms have only a few electrons in their outer shell, while some atoms lack only one or two electrons to have an octet. In cases where an atom has three or fewer valence electrons, the atom may lose those valence electrons quite easily until what remains is a lower shell that contains an octet. Atoms that lose electrons acquire a positive charge as a result because they are left with fewer negatively charged electrons to balance the positive charges of the protons in the nucleus. Positively charged ions are called cations. Most metals become cations when they make ionic compounds.

A neutral sodium atom is likely to achieve an octet in its outermost shell by losing its one valence electron.

\[\ce{Na \rightarrow Na^{+} + e^{-}} \nonumber \]

The cation produced in this way, Na+, is called the sodium ion to distinguish it from the element. The outermost shell of the sodium ion is the second electron shell, which has eight electrons in it. The octet rule has been satisfied. Figure \(\PageIndex{1}\) is a graphical depiction of this process.

Figure \(\PageIndex{1}\): The Formation of a Sodium Ion. On the left, a sodium atom has 11 electrons. On the right, the sodium ion only has 10 electrons and a 1+ charge. Neutral sodium atom on left has 11 protons and 11 electrons. Sodium ion on right has 11 protons and 10 electrons, with a +1 overall charge.

Some atoms have nearly eight electrons in their valence shell and can gain additional valence electrons until they have an octet. When these atoms gain electrons, they acquire a negative charge because they now possess more electrons than protons. Negatively charged ions are called anions. Most nonmetals become anions when they make ionic compounds.

A neutral chlorine atom has seven electrons in its outermost shell. Only one more electron is needed to achieve an octet in chlorine’s valence shell. (In table salt, this electron comes from the sodium atom.)

\[\ce{e^{-} +Cl -> Cl^{-}} \nonumber \]

In this case, the ion has the same outermost shell as the original atom, but now that shell has eight electrons in it. Once again, the octet rule has been satisfied. The resulting anion, Cl−, is called the chloride ion; note the slight change in the suffix (-ide instead of -ine) to create the name of this anion. Figure \(\PageIndex{2}\) is a graphical depiction of this process.

Figure \(\PageIndex{2}\): The Formation of a Chlorine Ion. On the left, the chlorine atom has 17 electrons. On the right, the chloride ion has 18 electrons and has a 1− charge. Neutral chlorine atom on left has 17 protons and 17 electrons. Sodium ion on right has 17 protons and 18 electrons, with a -1 overall charge.

The names for positive and negative ions are pronounced CAT-eye-ons and ANN-eye-ons, respectively.

In many cases, elements that belong to the same group (vertical column) on the periodic table form ions with the same charge because they have the same number of valence electrons. Thus, the periodic table becomes a tool for remembering the charges on many ions. For example, all ions made from alkali metals, the first column on the periodic table, have a 1+ charge. Ions made from alkaline earth metals, the second group on the periodic table, have a 2+ charge. On the other side of the periodic table, the next-to-last column, the halogens, form ions having a 1− charge. Figure \(\PageIndex{3}\) shows how the charge on many ions can be predicted by the location of an element on the periodic table. Note the convention of first writing the number and then the sign on a ion with multiple charges. The barium cation is written Ba2+, not Ba+2.

Figure \(\PageIndex{3}\): Predicting Ionic Charges. The charge that an atom acquires when it becomes an ion is related to the structure of the periodic table. Within a group (family) of elements, atoms form ions of a certain charge.