In this lesson we will learn about the formation and naming of ionic and covalent compounds.
By the end of this lesson you will be able to:
- Describe how ionic compounds and covalent compounds are formed.
- Write the chemical names and formulas for ionic and covalent compounds.
- Distinguish between ionic and covalent compounds based on their names or formulas.
- Draw structures for simple covalent compounds.
Ionic and Covalent Bonds
- Atoms can combine with other atoms to acquire full valence shells.
- Having full valence shells makes atoms more stable.
- There are two ways atoms can combine:
- They can form ionic bonds by gaining or losing electrons.
- They can form covalent bonds by sharing electrons.
- When metal and non-metal atoms combine, they form ionic bonds.
- When two non-metal atoms combine, they form covalent bonds.
Ionic and Covalent Compounds
- Compounds are pure substances that consist of two or more different types of atoms in a fixed arrangement.
- They can be divided into two main groups – ionic compounds and covalent compounds.
- This classification reflects the different types of chemical bonds between atoms.
(Image: creativefamily, Adobe Stock)
Formation of Ionic Compounds
- When a metal and a non-metal react to form a compound, the do so by forming ions.
- Ions are charged particles resulting from atoms that have lost or gained valence electrons.
- Metal atoms lose electrons to form positive ions (cations).
- Non-metal atoms gain electrons to form negative ions (anions).
- Atoms don’t just spontaneously lose or gain ions. The need to combine with other atoms to make it possible.
- When metals and non-metals combine to form ions, electrons are transferred from metal atoms to non-metal atoms.
- Atoms gain or lose the number of electrons necessary to have a full valence shell.
- Metals lose all of their valence electrons, so the next shell in becomes the valence shell.
- Non-metals gain electrons to complete a full valence shell.
- Ionic compounds are held together in rigid lattice structures, resulting from the strong ionic bonds (electrostatic attraction) between positive and negative ions.
- Because ions contain full valence shells, ionic compounds are more stable than the elements from which they are formed.
Monatomic and Polyatomic Ions
- The types of ions involved when metals react with non-metals are called monatomic ions, as they are derived from single atoms.
- For example, Na+ and Br–.
- Ionic bonds can also involve polyatomic ions, which are derived from multiple atoms.
- For example, NH4+ and OH–.
- Within a polyatomic ion, atoms are held together by covalent bonds.
Formation of Covalent Compounds
- When two non-metals react to form a compound, they do so by forming covalent bonds.
- Covalent bonds involve the sharing of electrons, so that each atom attains a full valence shell.
- For hydrogen, a full outer shell consists of two electrons.
- For all other non-metals, a full outer shell consists of eight electrons, known as an octet.
- When electrons are shared between atoms, they are attracted to both nuclei, which forms the bond between the two atoms.
- The number of covalent bonds an atom forms is usually equal to the number of electrons shared, which is generally equal to the number of electrons needed to attain a full valence shell.
- Atoms can often form different combinations of bonds with other atoms.
- Example 1
- Hydrogen has one valence electron. Since it only needs one more to have a full valence shell, it only needs to share one electron.
- It can pair up with another atom which also needs to share one electron, such as another hydrogen atom or a chlorine atom.
- Example 2
- Oxygen has six valence electrons. It needs to share two electrons to have a full octet.
- It can pair up with: another atom which also needs to share two electrons, such as another oxygen atom; or, with two atoms which need to share one electron each, such as two hydrogen atoms.
- Example 3
- Nitrogen has five valence electrons. It needs to share three electrons to have a full octet.
- It can pair up with: another atom which also needs to share three electrons, such as another nitrogen atom; or, with three atoms which need to share one electron each, such as three hydrogen atoms; or, with two atoms, one which needs to share one electron, such as hydrogen, and one which needs to share two electrons, such as oxygen.
- Example 4
- Carbon has four valence electrons. It needs to share all four electrons to have a full octet.
- It can pair up with: four atoms which need to share one electron each, such as four fluorine atoms; or, two atoms which need to share two electrons each, such as two oxygen atoms; or, two atoms, one which needs to share one electron, such as hydrogen, and one which needs to share three electrons, such as nitrogen. (Carbon atoms cannot pair with each other by sharing four electrons each.)
- Sometimes atoms contribute unequally to bonds between them. For example, in carbon monoxide, carbon atoms share two electrons while oxygen atoms share four electrons.
- There are also exceptions to the octet rule. For example, phosphorus and sulfur atoms often form an expanded octet, with more than eight electrons. Similarly, beryllium and and boron atoms can form an incomplete octet, with less than eight electrons.
Single, Double and Triple Bonds
- When two electrons (one pair) are shared between two atoms, they form a single bond.
- These are represented by one line.
- When four electrons (two pairs) are shared between two atoms, they form a double bond.
- These are represented by two lines.
- When six electrons (three pairs) are shared between two atoms, they form a triple bond.
- These are represented by three lines.
- Ionic compounds and covalent compounds have different naming systems.
- Therefore the first step in naming a compound is to identify whether the compound is ionic or covalent.
- If the compound contains a metal or the ammonium group (NH4), it is ionic.
- If the compound does not contain a metal or the ammonium group, it is covalent.
Naming Ionic Compounds
- Ionic compounds are named after the ions that form them.
- Their names consist of two parts:
- The first part of the compound name is the name of the positive ion.
- The second part of the compound name is the name of the negative ion.
Monatomic Positive Ions
- Monatomic positive ions have the same name as the metal atoms they are formed from.
- CaBr2 → “calcium…”
Al2O3 → “aluminium…”
Na3P → “sodium…”
Metals That Form More Than One Type of Ion
- Many transition and post-transition metals form more than one type of ion.
- Roman numerals corresponding to the size of the positive charge are used to distinguish the different ions.
- Common metal ions with more than one charge are shown below.
|Metal||Ion Charge||Ion Name|
Monatomic Negative Ions
- Monatomic negative ions have the same name as the non-metal atoms they are formed from, except the last part of the name is changed to “‑ide”, as shown below.
- CaBr2 → “…bromide”
Al2O3 → “…oxide”
Na3P → “…phosphide“
- The names of polyatomic ions (ions that contain more than one type atom) cannot be predicted from the atoms they contain.
- They need to be learned or looked up in a valency table.
- The names of common polyatomic ions are shown below.
|Ion Formula||Ion Name|
Full Names for Ionic Compounds
- To form the full name of an ionic compound, simply join the names of the positive and negative ions together.
- CaBr2 → “calcium bromide”
Al2O3 → “aluminium oxide”
Na3P → “sodium phosphide“
- Examples Involving Polyatomic Ions
- (NH4)2S → “ammonium sulfide”
K2CO3 → “potassium carbonate“
- For compounds containing metal ions that can have more than one charge, Roman numerals need to be included in the name, but not in the formula.
- SnF2 → “tin (II) fluoride”
Fe2O3 → “iron (III) oxide”
Pb(NO3)4 → “lead (IV) nitrate“
- Note that the names of ionic compounds do not include the numbers of each type of ion (which is denoted by the subscript numbers in the formulas).
- To determine the formula of an ionic compound from its name, you need to calculate the number of each type of ion, based on their charges, using the ‘drag-and-drop’ method.
Naming Binary Covalent Compounds
- Binary covalent compounds are compounds composed of two different non-metal atoms in fixed proportions.
- Their names consist of two parts:
- The first part of the compound name is derived from the first atom in the formula.
- The second part of the compound name is derived from the second atom in the formula.
The First Part of the Name
- The first part of the compound name contains the same name as the first atom in the formula.
- N2O3 → “nitrogen…”
P4S10 → “phosphorus…”
The Second Part of the Name
- The second part of the compound name contains the same name as the second atom in the formula, except the last part of the name is changed to “‑ide”.
- This is the same manner as for naming negative ions (see previous).
- N2O3 → “…oxide”
P4S10 → “…sulfide“
Indicating the Number of Each Type of Atom
- Unlike ionic compounds, the number of each type of atom in a covalent molecule is included in the compound name.
- This is because two non-metals can often form two different types of compounds, so their names need to be distinguishable.
- Prefixes are used to indicate the number of each type of atom, as shown in the following table.
|Number of Atoms||Prefix||Number of Atoms||Prefix|
Full Names for Covalent Compounds
- The full name of a covalent compound contains the names for the two types of atoms, as well as prefixes indicating how many of each type of atom there are.
- N2O3 → “dinitrogen trioxide”
P4S10 → “tetraphosphorus decasulfide“
Exceptions to the Rules
- There are some exceptions to the naming rules for binary covalent compounds.
- The prefix “mono” is only used for the second type of atom – if there is only one of the first type of atom, it has no prefix.
- SF6 → “sulfur hexafluoride”
PCl3 → “phosphorus trichloride“
- If the second type of atom is oxygen, the “‑o” or “‑a” is omitted from prefixes.
- CO → “carbon monoxide”
N2O5 → “dinitrogen pentoxide“
- Many covalent compounds have common names that are used instead of names derived from these rules.
- Water (H2O) is never referred to as dihydrogen monoxide.
Ammonia (NH3) is never referred to as nitrogen trihydride.
- Compounds can be divided into two main groups: ionic compounds and covalent compounds.
- Ionic compounds consist of positive ions and negative ions, joined by ionic
- Ionic bonds are electrostatic forces of attraction between positive and negative ions.
- Covalent compounds consist of non-metal atoms, joined by covalent bonds.
- Covalent bonds result from the sharing of valence electrons between atoms.
- A single covalent bond involves the sharing of one pair of electrons.
- A double covalent bond involves the sharing of two pairs of electrons.
- A triple covalent bond involves the sharing of three pairs of electrons.
- Ionic compounds are named after the ions that form them.
- The names of the positive and negative ions are joined together to form the compound name.
- Covalent compounds are named after the atoms that form them.
- The names of the two atoms are joined together to form the compound name.
- The ending of the second atom’s name is changed in the same way as for negative ions.
- The number of each type of atom is included in the name, using the prefixes mono, di, tri…
(Image: Benjah-bmm27, Wikimedia Commons)
(Header image: Benjah-bmm27, Wikimedia Commons)