What are Ionic Compounds?
Atoms are electrically neutral because the number of protons (which carry a 1+ charge) in an atom’s nucleus equals the number of electrons (which carry a 1- charge). As a result, the total net charge for the atom is zero.
Most atoms can either gain or lose electrons; when this happens, the number of electrons in the nucleus differs from the number of protons. The resulting charged species is referred to as an ion.
Ionic compounds are formed when ions with opposing negative and positive charges form ionic bonds and form compounds, which are compounds made of ions. Since one atom’s loss or gain matches the loss or gain of the other, one atom essentially ‘donates’ an electron to the other atom with which it pairs up.
Table of Contents
- Writing Formulas of Ionic Compounds
- Nomenclature of Ions
- Nomenclature of Ionic Compounds
- Frequently Asked Questions – FAQs
Writing Formula of Ionic Compounds
- The monatomic or polyatomic anion is written first, followed by the cation.
- The formula’s subscripts must result in an electrically neutral formula unit. (In other words, the total positive charge must equal the total negative charge.)
- The subscripts should be the smallest possible set of whole numbers.
- If a formula contains only one polyatomic ion, do not use parentheses around it. For example, NaNO3, not Na(NO3).
- If the formula contains more than one polyatomic ion, put the ion in parentheses and the subscript after the parentheses. For example, Ca(OH)2, Ba3(PO4)2, etc.
Example-
| Cation | Anion | Formula |
|---|---|---|
| Na+ | Cl– | NaCl |
| Ca2+ | Br– | CaBr2 |
| Na+ | S2- | Na2S |
| Mg2+ | O2- | MgO |
| Na+ | SO42- | Na2SO4 |
| Cr3+ | O-2 | Cr2O3 |
| NH4+ | SO42- | (NH4)2SO4 |
Nomenclature of Ions
- A monatomic cation name is simply the element’s name followed by the word ion.
For example– Na+ denotes sodium ion, Al3+ denotes aluminium ion, Ca2+ denotes calcium ion, and so on.
- Different elements lose different numbers of electrons, resulting in ions with varying charges. For example– Iron can form two cations: Fe2+ and Fe3+.
This distinction can be made in two ways-
1. The positive charge of an ion is indicated by a Roman numeral in parentheses following the element name, followed by the word ion. Thus, Fe2+ is named as the iron(II) ion, whereas Fe3+ is named as the iron(III) ion. This system is only applicable to elements that have more than one common positive ion.
2. The second method employs two suffixes (-ic and -ous) that are attached to the stem of the element name. The -ic suffix denotes the higher of the two cation charges, while the -ous suffix denotes the lower. In many cases, the element name’s stem is derived from the Latin name of the element.
| Element | Stem | Charge | Name |
|---|---|---|---|
| Iron | ferr- | 2+ | Iron (II) ion or Ferrous ion |
| 3+ | Iron (III) ion or Ferric ion | ||
| Copper | cupr- | 1+ | Copper (I) ion or Cuprous ion |
| 2+ | Copper (II) ion or Cupric ion |
- The name of a monatomic anion is made up of the stem of element name, the suffix -ide, and the word ion.
For example- F– is Fluoride ion, Cl– is Chloride ion, N3– is Nitride ion and so on.
Nomenclature of Ionic Compounds
Binary Ionic Compounds Containing a Metal and a Non-metal
A binary compound is one that is made up of two different elements. Each element may or may not have more than one atom. A diatomic compound (or diatomic molecule) is made up of two atoms that are not necessarily the same.
Ionic compounds are formed when metals and nonmetals combine. When naming binary ionic compounds, start with the cation (specifying the charge if necessary), then move on to the nonmetal anion (element stem + -ide).
Prefixes should not be used to indicate how many of each element is present; this information is implied in the compound’s name.
For example-
| Formula | Name |
|---|---|
| CaCl2 | Calcium chloride |
| NaCl | Sodium chloride |
| AlF3 | Aluminium fluoride |
| KCl | Potassium chloride |
| Ca3P2 | Calcium phosphide |
Ionic Compounds with Polyatomic Ions
Ionic compounds are formed when metals combine with polyatomic ions.
The method for naming polyatomic ionic compounds is the same as for binary ionic compounds. The anion is named first, then the cation.
Prefixes should not be used to indicate how many of each element is present; this information is implied in the compound’s name.
For example-
| Formula | Name |
|---|---|
| Ca(NO3)2 | Calcium nitrate |
| (NH4)2SO4 | Ammonium sulfate |
| NH4F | Ammonium fluoride |
| CaCO3 | Calcium carbonate |
| Fe(OH)3 | Iron (III) hydroxide or ferrous hydroxide |
| Cr3(PO4)2 | Chromium (II) phosphate |
| CrPO4 | Chromium (III) phosphate |
Frequently Asked Questions on Naming Ionic Compounds
What are the rules in naming ionic compounds?
The rules in naming ionic compounds are:
- Rule 1: The cation is written first in the name, followed by the anion.
- Rule 2: When there are two or more of the same polyatomic ion in the formula unit, that ion is written in parentheses, with the subscript written outside the parentheses.
When naming a binary compound, what is the suffix used?
The suffix -ide is used while naming a binary compound.
How is an anion named?
The name of a monatomic anion is made up of the stem of the element name, the suffix -ide, and the word ion.
Why are ions so important in the naming of compounds?
Ionic compounds are made up of cations and anions that are attracted to one another and combine to form a compound with a neutral charge overall. The ions in an ionic compound are distinct species, which affects their behaviour and chemistry. Therefore, ions are important in the naming of compounds.
How are polyvalent caions named?
Polyvalent cations can be named in two ways-
The first way – The positive charge of an ion is indicated by a Roman numeral in parentheses following the element name, followed by the word ion.
The second way employs two suffixes (-ic and -ous) that are attached to the stem of the element name. The -ic suffix denotes the higher of the two cation charges, while the -ous suffix denotes the lower.
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