Covalent Bonding & Structure (OCR A Level Chemistry)

Revision Note

Test yourself
Richard

Author

Richard

Last updated

Covalent Bonds & Strength

  • Covalent bonding occurs between two non-metals
  • A covalent bond involves the electrostatic attraction between nuclei of two atoms and the bonding electrons of their outer shells
  • No electrons are transferred but only shared in this type of bonding

Chemical Bonding Defining Covalent Bonds, downloadable AS & A Level Chemistry revision notes

The positive nucleus of each atom has an attraction for the bonding electrons shared in the covalent bond

  • Non-metals are able to share pairs of electrons to form different types of covalent bonds
  • Sharing electrons in the covalent bond allows each of the 2 atoms to achieve an electron configuration similar to a noble gas
    • This makes each atom more stable

Covalent Bonds & Shared Electrons Table

Chemical Bonding Table_8, downloadable AS & A Level Chemistry revision notes

Bond energy

  • The bond energy is the energy required to break one mole of a particular covalent bond in the gaseous states
    • Bond energy has units of kJ mol-1

  • The larger the bond energy, the stronger the covalent bond is
  • Average bond enthalpy is also used as a measurement of the strength of a covalent bond
    • The average bond enthalpy term is the average amount of energy needed to break a specific type of bond, measured over a wide variety of different molecules

Bond length

  • The bond length is the internuclear distance of two covalently bonded atoms
    • It is the distance from the nucleus of one atom to another atom which forms the covalent bond

  • The greater the forces of attraction between electrons and nuclei, the more the atoms are pulled closer to each other
  • This decreases the bond length of a molecule and increases the strength of the covalent bond
  • Triple bonds are the shortest and strongest covalent bonds due to the large electron density between the nuclei of the two atoms
  • This increase the forces of attraction between the electrons and nuclei of the atoms
  • As a result of this, the atoms are pulled closer together causing a shorter bond length
  • The increased forces of attraction also means that the covalent bond is stronger

Chemical Bonding Bond Lengths and Strengths, downloadable AS & A Level Chemistry revision notes

Triple bonds are the shortest covalent bonds and therefore the strongest ones


Examiner Tip

The values for bond length and bond enthalpy are often different depending on which source they are taken from

Dot & cross diagrams

  • Dot and cross diagrams are used to represent covalent bonding
  • They show just the outer shell of the atoms involved
  • To differentiate between the two atoms involved, dots for electrons of one atom and crosses for electrons of the other atom are used
  • Electrons are shown in pairs on dot-and-cross diagrams

Single covalent bonding 

Hydrogen, H2

Chemical Bonding Single Covalent Bonding Hydrogen, downloadable AS & A Level Chemistry revision notes

Covalent bonding in hydrogen

Chlorine, Cl2

Chemical Bonding Single Covalent Bonding Chlorine, downloadable AS & A Level Chemistry revision notes

Covalent bonding in chlorine

Ammonia, NH3

Chemical Bonding Single Covalent Bonding Ammonia, downloadable AS & A Level Chemistry revision notes

Covalent bonding in ammonia

Double covalent bonding

Oxygen, O2

Chemical Bonding Double Covalent Bonding Oxygen, downloadable AS & A Level Chemistry revision notes

Covalent bonding in oxygen

Carbon dioxide, CO2

Chemical Bonding Double Covalent Bonding CO2, downloadable AS & A Level Chemistry revision notes

Covalent bonding in carbon dioxide

Ethene, C2H4

Chemical Bonding Double Covalent Bonding Ethene, downloadable AS & A Level Chemistry revision notes

Covalent bonding in ethene

Triple covalent bonding

Nitrogen, N2

Chemical Bonding Triple Covalent Bonding, downloadable AS & A Level Chemistry revision notes

Covalent bonding in nitrogen

  • In some instances, the central atom of a covalently bonded molecule can accommodate more or less than 8 electrons in its outer shell
  • Being able to accommodate more than 8 electrons in the outer shell is known as ‘expanding the octet rule’
  • Some examples of this occurring can be seen with period 3 elements

Sulfur dioxide, SO2 – dot and cross diagram, downloadable AS & A Level Chemistry revision notes

Sulfur dioxide, SO2 – dot and cross diagram

Chemical Bonding Expanding the Octet Rule PCl5, downloadable AS & A Level Chemistry revision notes

Phosphorus pentachloride, PCl5 – dot and cross diagram

  • Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’

2-4-3-boron-trichloide-bcl3

Boron trichloride, BCl3 - dot and cross diagram

Examiner Tip

Covalent bonding takes place between nonmetal atoms.

Remember: Use the periodic table to decide how many electrons are in the outer shell of a nonmetal atom.

Dative Covalent / Coordinate Bonding

  • In simple covalent bonds, the two atoms involved share electrons
  • Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
    • An electron-deficient atom is an atom that has an unfilled outer orbital

  • So both electrons are from the same atom
  • This type of bonding is called dative covalent bonding or coordinate bonding
  • An example with a dative bond is in an ammonium ion
    • The hydrogen ion, H+ is electron-deficient and has space for two electrons in its shell
    • The nitrogen atom in ammonia has a lone pair of electrons which it can donate to the hydrogen ion to form a dative covalent bond

 Chemical Bonding Dative Covalent Bonding Ammonium ion, downloadable AS & A Level Chemistry revision notes

Ammonia (NH3) can donate a lone pair to an electron-deficient proton (H+) to form a charged ammonium ion (NH4+)

  • Aluminium chloride is also formed using dative covalent bonding
  • At high temperatures aluminium chloride can exist as a monomer (AlCl3)
    • The molecule is electron-deficient and needs two electrons to complete the aluminium atom’s outer shell

  • At lower temperatures the two molecules of AlCl3 join together to form a dimer (Al2Cl6)
    • The molecules combine because lone pairs of electrons on two of the chlorine atoms form two coordinate bonds with the aluminium atoms

Chemical Bonding Dative Covalent Bonding Aluminium chloride, downloadable AS & A Level Chemistry revision notes

Aluminium chloride is also formed with a dative covalent bond in which two of the chlorine atoms donate their lone pairs to each of the aluminium atoms to form a dimer

Examiner Tip

In dative covalent bonding, both electrons in the covalent bond are shared by one atom.

A dative covalent bond is drawn using an arrow from the donated pair of electrons to the electron-deficient atom.

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.