2.4:
Ions and Ionic Charges
Atoms are neutral particles with equal numbers of protons and electrons. An element is defined by the number of protons, which does not change; however, many atoms can gain or lose one or more electrons to become a charged particle.
When an atom or molecule gains or loses electrons, it is called an ion. Electrons, being much smaller in mass than protons, do not contribute significantly to the atomic mass. Therefore, the atomic mass of an ion is essentially the same as the neutral atom’s atomic mass.
When atoms or molecules lose electrons, they are called cations. Cations have a positive charge because they have more protons, which have a positive charge, than electrons, which have a negative charge. Cations are given the same name as their element, followed by the word ion.
A calcium atom has 20 protons and 20 electrons. It loses two electrons to become a calcium ion with 20 protons and 18 electrons. Because it now has two more positive charges than negative charges, it has an overall charge of positive two.
The chemical symbol for an ion is written as the chemical symbol of the atom with the charge written on the right superscript position. The chemical symbol for a calcium ion is therefore written as Ca2+.
When atoms or molecules gain electrons, they are called anions. Anions have a negative charge because they have more electrons than protons. Anions are named by adding the suffix '-ide' to their element name.
A fluorine atom has 9 protons and 9 electrons. Fluorine gains one electron to become a fluoride ion with 9 protons and 10 electrons. Because it now has one more negative charge than positive charges, it has an overall charge of negative one.
The chemical symbol for fluoride is written as F−. The one is usually omitted for ions with a positive or negative one charge.
Atoms will lose or gain specific numbers of electrons when forming an ion. Many of the charges for ions formed by the main group elements can be predicted using the periodic table.
The metals, on the left side of the periodic table, will lose electrons. Group 1 alkali metals all lose one electron and have a positive one charge when they become an ion. Group 2 alkaline earth metals lose two electrons and have a positive two charge.
The nonmetals, on the right side of the periodic table, will gain electrons. Group 17 halogens gain one electron and have a negative one charge. Groups 16 elements gain two electrons and will have a negative two charge.
Many transition metals can form cations with different charges. Group 18 noble gases do not generally form ions.
2.4:
Ions and Ionic Charges
In ordinary chemical reactions, the nucleus — which contains the protons and neutrons of each atom and thus identifies the element — remains unchanged. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. The transfer and sharing of electrons among atoms govern the chemistry of the elements. During the formation of some compounds, atoms gain or lose electrons to form electrically charged particles called ions.
When an atom loses electrons, it forms a positive ion called a cation. Cations have more protons than electrons. Likewise, when an atom gains electrons, it forms a negative ion called an anion. Anions have more electrons than protons.
The periodic table helps to predict whether an atom will form an anion or a cation, and also the charge of the resulting ion. Moving from the far left to the right on the periodic table, atoms of main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. They form cations with a charge equal to the group number. To illustrate, an atom of an alkali metal (Group 1) loses one electron and forms a cation with a 1+ charge; an alkaline earth metal (Group 2) loses two electrons and forms a cation with a 2+ charge, and so on. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca2+. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca2+ is called a calcium ion.
Moving from the far right to the left on the periodic table, atoms of nonmetal elements generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. They form anions with a negative charge equal to the number of groups moved left from the noble gases. For example, atoms of group 17 elements (one group left of the noble gases) gain one electron and form anions with a 1− charge; atoms of group 16 elements (two groups left) gain two electrons and form ions with a 2− charge, and so on. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. This results in an anion with 35 protons, 36 electrons, and a 1− charge. It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br−.
The trend between the charge of the ion and position of the group on the periodic table can be used as a guide in many cases. However, its predictive value decreases when moving toward the center of the periodic table. Transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge.
This text is adapted from Openstax, Chemistry 2e, Section 2.6: Molecular and Ionic Compounds.
Suggested Reading
- Jensen, William B. "The proper writing of ionic charges." Journal of Chemical Education 89, no. 8 (2012): 1084-1085.
- Schmid, Roland. "The Noble Gas Configuration-Not the Driving Force but the Rule of the Game in Chemistry." Journal of chemical education 80, no. 8 (2003): 931.
- Gillespie, Ronald J. "Electron densities, atomic charges, and ionic, covalent, and polar bonds." Journal of Chemical Education 78, no. 12 (2001): 1688.