- Know when covalent bonds occur.
- Differentiate between single, double, and triple bonds.
- Understand why the subscripts of the elements in the formula of any covalently bound compound can never be "reduced".
Covalent bonds occur when two atoms of similar electronegativity, usually two organic elements, bond by sharing electrons. When this occurs electrons in the bond freely move between the orbitals of both atoms.
A single bond occurs when a two electrons are shared between two nuclei/atoms. When this occurs, both electrons contribute their charges to both atoms. This happens because the electrons move within the orbitals of both atoms. Usually, each atom contributes one electron to the bond, but in some cases one atom contributes two electrons to the bond.
In addition to single bonds, there are double and triple bonds. These are similar to single bonds by how electrons are shared between the two nuclei/atoms. However, they differ from single bonds by the number of electrons shared. In double bonds, four electrons are shared. In triple bonds, six are shared.
One way to pictorially represent covalently-bonded molecules is with Lewis structures. These pictorially represent an atom's outer electron s and p-orbitals with dots or X's drawn in an octet around a chemical symbol. See below for an example.
This is a Lewis dot representation of elemental nitrogen. Notice how the number of dots corresponds to the number of s and p-orbital electrons (2+3=5 total).
In Lewis dot structures, the atoms form bonds when dots are shared between the atoms, as shown below.
In this Lewis dot representation, the central carbon is bonded to three hydrogens and one oxygen. This one oxygen is subsequently bonded to one hydrogen. Notice how each bond is made up of two dots representing electrons. As previously stated, each atom contributes one electron to this bond.
Practice Lewis Dot Structures
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Types of Covalent Bonds
The bonds formed by sharing of an electron pair between two atoms can typically be single, double or triple; that is, in covalent bonds, atoms may be linked together by single, double or triple bonds. In simplest terms, "single", "double", and "triple" refer to the number of shared electron pairs in the bond. For example, in the dioxygen (O2) molecule, each oxygen must share two of its electrons in order to obtain the noble gas configuration of 8 valence electrons, resulting in a double bond of two shared electron pairs. The valency of an atom is the maximum number of bonds it can form, usually the number of electrons required to reach a stable noble gas configuration (there are exceptions to this rule, called the octet rule as all noble gases except helium have 8 valence electrons, discussed further down). Each double bond counts as 2 bonds for this purpose, and each triple bond counts as 3.
Whether a bond is single, double, or triple is its bond order.
Single Covalent Bond
In some molecules, a shared pair of electrons are contributed between the atoms, thus creating a "single bond". For example, in a hydrogen molecule, the two atoms of hydrogen are bonded together by a single bond.
How? The valency of hydrogen is 1, that is, the number of electrons present in the outermost shell is 1 and it needs 1 more electron to attain a noble gas configuration, that is a completely filled outer shell.
H has a valency of 1 and it needs 1 more electron to become stable. So, it forms a bond with another hydrogen atom and shares its electron with it and the second hydrogen atom shares its 1 electron with the first one, (Hx) (xH) (Here, x shows the number of electrons present in the outer shell of a hydrogen atom.) Now, the first atom shares its one electron with the second hydrogen atom and the second atom shares its one electron with the first one and thus we get one hydrogen atom. (H(xx)H) Thus, 2 atoms of hydrogen share their electron to form a molecule of hydrogen, H2. This allows each hydrogen atom to attain the electronic configuration of the noble gas helium. The shared pair of electrons is said to constitute a single bond within the two hydrogen atoms. It is a single bond because only one pair of electrons are shared within the hydrogen atoms. A single bond is denoted by a single line between the atoms. For example, the covalent bond in a hydrogen molecule is represented by:-
Such single covalent bond is also formed in chlorine molecule, Cl2.