Learning Objective

In this lesson we will learn about the formation and naming of ionic and covalent compounds.

Learning Outcomes

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.

 

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Introduction

  • Compounds are pure substances that consist of two or more different types of atoms in a fixed arrangement.
  • Compounds can be divided into two main groups: ionic compounds and covalent compounds.
    This classification reflects the different types of chemical bonds between atoms.

 

Ionic and Covalent Bonds

  • Atoms combine with other atoms in ways that result in full valence shells.
    Having full valence shells makes atoms more stable.
  • Atoms can combine in two different ways to achieve this:
    1. They can gain or lose electrons to form ionic bonds.
    2. They can share electrons to form covalent bonds.
  • Ionic bonds form when metals and non-metals react to form compounds.
    Covalent bonds form when two non-metals react to form compounds.

 

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 valence electrons to form positive ions (cations).
    Non-metal atoms gain valence 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 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.

 
formation of an ionic compound

Ionic compounds are formed when electrons are transferred between atoms.

 

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.

 
formation of a covalent compound

Covalent compounds are formed when electrons are shared between 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.

 

 hydrogen molecule h2 covalent lewis structure  hydrogen chloride molecule hcl covalent lewis structure

In hydrogen and hydrogen chloride molecules, hydrogen atoms attain full valence shells by forming one single bond.

 

    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.

 
oxygen molecule o2 covalent lewis structure

In oxygen molecules, oxygen atoms attain full valence shells by forming one double bond.

 
water molecule h2o covalent lewis structure

In water molecules, oxygen atoms attain full valence shells by forming two single bonds.

 

    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.

 
nitrogen molecule n2 covalent lewis structure

In nitrogen molecules, nitrogen atoms attain full valence shells by forming one triple bond.

 
ammonia molecule nh3 covalent lewis structure

In ammonia molecules, nitrogen atoms attain full valence shells by forming three single bonds.

 
nitroxyl molecule hno covalent lewis structure

In nitroxyl molecules, nitrogen atoms attain full valence shells by forming one single bond and one double bond.

 

    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.)

 
carbon tetrafluoride molecule cf4 covalent lewis structure

In carbon tetrafluoride molecules, carbon atoms attain full valence shells by forming four single bonds.

 
carbon dioxide molecule co2 covalent lewis structure

In carbon dioxide molecules, carbon atoms attain full valence shells by forming two double bonds.

 
hydrogen cyanide molecule hcn covalent lewis structure

In hydrogen cyanide molecules, carbon atoms attain full valence shells by forming one single bond and one triple bond.

 

  • Sometimes atoms contribute unequally to bonds between them. For example, in carbon monoxide, the carbon and oxygen atoms are joined by a triple bond in which two electrons are contributed by carbon and four electrons are contributed by oxygen.
  • 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 are shared between two atoms, they form a single bond.
    These are represented by a single line.

 
single covalent bond hydrogen

The atoms in a hydrogen molecule are joined by a single covalent bond, which involves the sharing of two electrons.

 

  • When four electrons are shared between two atoms, they form a double bond.
    These are represented by a two lines.

 
double covalent bond oxygen

The atoms in an oxygen molecule are joined by a double covalent bond, which involves the sharing of four electrons.

 

  • When three electrons are shared between two atoms, they form a triple bond.
    These are represented by a three lines.

 
triple covalent bond nitrogen

The atoms in a nitrogen molecule are joined by a triple covalent bond, which involves the sharing of six electrons.

 

Naming Ionic and Covalent Compounds

  • 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 ion (NH4+), it is ionic.
    If the compound does not contains metal atoms or the ammonium ion, it is covalent.

 
ionic or covalent compound classification

Ionic and covalent compounds can be distinguished by the types of atoms they contain.

 

Naming Ionic Compounds

  • Ionic compounds are named after the ions that form them.
    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 non-metal ion.

 

Monatomic Positive Ions

  • Monatomic positive ions have the same name as the metal atoms they are formed from.
    Examples
    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
    chromium Cr2+ chromium (II)
    chromium Cr3+ chromium (III)
    cobalt Co2+ cobalt (II)
    cobalt Co3+ cobalt (III)
    copper Cu+ copper (I)
    copper Cu2+ copper (II)
    gold Au+ gold (I)
    gold Au3+ gold (III)
    iron Fe2+ iron (II)
    iron Fe3+ iron (III)
    lead Pb2+ lead (II)
    lead Pb4+ lead (IV)
    manganese Mn2+ manganese (II)
    manganese Mn3+ manganese (III)
    tin Sn2+ tin (II)
    tin Sn4+ tin (IV)

 

Monatomic Negative Ions

  • Monatomic negative ions have the same name as the non-metal atoms they are formed from, except the last part of their name is changed to “-ide”, as shown below.

 

Atom Ion
fluorine fluoride
chlorine chloride
bromine bromide
iodine iodide
oxygen oxide
sulfur sulfide
nitrogen nitride
phosphorus phosphide

 

    Examples
    CaBr2 “…bromide
    Al2O3 “…oxide
    Na3P “…phosphide

 

Polyatomic Ions

  • The names of polyatomic ions (ions that contain more than one type atom) cannot be predicted.
    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
NH4+ ammonium
OH hydroxide
NO3 nitrate
HCO3 hydrogen carbonate
CH3COO acetate
SO42- sulfate
CO32- carbonate
PO43- phosphate

Click here for a downloadable valency table..

 

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.
    Examples
    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.
    Examples
    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 types of non-metal atoms.
    The first part of the compound name is derived from the first type of atom.
    The second part of the compound name is derived from the second type of atom.

 

The First Type of Atom

  • The first part of the compound name contains the same name as the first type of atom.
    Examples
    N2O3 nitrogen…”
    P4S10 phosphorus…”

 

The Second Type of Atom

  • The second part of the compound name contains the same name as the second type of atom, except the last part of their name is changed to “-ide”.
    This is the same manner as for naming negative ions (see above).
    Examples
    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.
  • The prefixes used to indicate the number of each type of atom are shown below.

 

Number of Atoms Prefix Number of Atoms Prefix
1 mono 6 hexa
2 di 7 hepta
3 tri 8 octa
4 tetra 9 nona
5 penta 10 deca

 

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.
    Examples
    N2O3 dinitrogen trioxide
    P4S10 tetraphosphorus decasulfide

 

Exceptions to the Rules

  • There are some exceptions to the naming rules for rules for binary covalent compounds.
    1. 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.
         Examples
         SF6 sulfur hexafluoride
         PCl3 phosphorus trichloride
    2. If the second type of atom is oxygen, the “-o” or “-a” is omitted from prefixes.
         Examples
         CO carbon monoxide
         N2O5 dinitrogen pentoxide
    3. Many covalent compounds have common names that are used instead of names derived from these rules.
         Examples
         Water (H2O) is never referred to as dihydrogen monoxide.
         Ammonia (NH3) is never referred to as nitrogen trihydride.

 

Summary

  • 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
    bonds.
    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…

 
ionic compound lattice

(Image: Benjah-bmm27, Wikimedia Commons)

 

(Header image: Benjah-bmm27, Wikimedia Commons)

 

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