3a. Introduction to the covalent bond - bonding in small molecules

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes


(c) doc bCovalent bonding – electron sharing in small or big molecules!

What is a covalent bond? How is it formed?

Covalent bonds are formed by atoms sharing electrons to form bonds that hold the atoms together in a molecule.

This type of bond usually formed between two non–metallic elements. The molecules might be that of an element i.e. one type of atom only OR from different elements chemically combined to form a compound.

Note: The molecular formula is the summary of all the atoms in a molecule.

The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms forming the bond, and the SHARING of a pair of negative electrons between them.

A COVALENT BOND IS THE SHARING OF A PAIR ELECTRONS BETWEEN TWO ATOMS

It only involves electrons in the outer shell i.e. the outermost energy level containing 1–7 electrons, which can be shared between atoms to form a covalent bond.

Lewis dot and cross diagrams for a covalent bond single double triple

One single covalent bond is a sharing of 1 pair of electrons, two pairs of shared electrons between the same two atoms gives a double bond and it is possible for two atoms to share 3 pairs of electrons and give a triple bond.

From the diagram above, and as already stated, a covalent bond is formed by the mutual electrical charge attraction of two positive nuclei and the negative electrons in between.

The two nuclei are the centres of the two atoms bonded together.

The electrons reside where the two energy levels overlap so the electrons are shared in the formation of a covalent bond.

Note: In the examples of covalent bonding it is assumed YOU can work out the electron configuration (arrangement in shells or energy levels) given the atomic number from the Periodic Table.

This kind of bond or electronic linkage, does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why covalent molecules have a particular shape.

In the case of ionic or metallic bonding, the electrical attractive forces act in all directions around the particles involved.


Which electronic structures are the most stable?

because is this what atoms will try to get to electronically!

(c) doc b   (c) doc b   (c) doc b  symbol, (atomic number) and electron arrangement

When atoms SHARE ELECTRONS in a covalent bond, they try to attain the electron structure (electron configuration) of the electronically very stable atoms of the Group 0 Noble Gases.

The electron structures of helium neon and argon are shown above.

eg helium (2), neon (2.8) or argon (2.8.8), that is, a full outer shell of electrons (full highest energy level).

Hydrogen (1 proton, 1 electron) needs one shared electron to become electronically stable like helium.

Many other elements become stable by sharing electrons to give an outer shell of 8 electrons, like Ne and Ar.

Quite simply, this is because these are the most stable electron arrangements and have a full outer shell of electrons (full highest energy level).

The number of bonds formed depends on the number of electrons that needs to be shared so that any pair of atoms in a molecule forming a covalent bond attain the electron arrangement of a noble gas (i.e. 2, 2.8 or 2.8.8 etc.)

Note that hydrogen and helium only have one shell, so when referring to the full outer shell of hydrogen, it is the one and only shell, but the descriptive word 'outer' is much more crucial when describing the electronic structures of any element with at least two shells e.g. when describing covalent bonding in molecules containing carbon, oxygen, nitrogen and chlorine etc.

In advanced level chemistry you will encounter examples of electronic structures of atoms in covalent molecules that are NOT those of a Noble Gas.

 


An example of COVALENT BONDING – a covalent bond is formed by two atoms sharing electrons so that the atoms combine to form molecules.

The bond is usually formed between two non–metallic elements combine to form a molecular compound. The two positive nuclei (due to the positive protons in them) of both atoms are mutually attracted to the shared negative electrons between them forming the covalent bond in the molecule. They share the electrons in a way that gives a stable Noble Gas electron arrangement like helium (2) or neon (2.8) etc..

  • This kind of bond or electronic linkage does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why molecules have a particular shape.

  • (c) doc b Hydrogen and oxygen atoms share electrons to give covalent O–H bonds to form molecules of the covalent compound water

  • (c) doc b which has a 'bent' shape 2 bond pairs (2 non-bonding pairs)

 


What next?

Recommend next: How to draw diagrams of small covalent molecules and work out a formula from valencies

 

Sub-index for: Part 3. Covalent Bonding: small molecules & properties

 

Index for ALL chemical bonding and structure notes

 

Perhaps of interest?

Advanced A level Notes on molecular shape

 

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