• Covalent?bonding occurs between two?non-metals
• A covalent bond involves the?electrostatic?attraction?between nuclei of two atoms and the electrons of their outer shells
• No?electrons?are?transferred?but only?shared?in this type of bonding
• When a covalent bond is formed, two?atomic orbitals?overlap and a?molecular orbital?is formed
• Covalent bonding happens because the electrons are more stable when attracted to two nuclei than when attracted to only one

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

• In a normal covalent bond, each atom provide one of the electrons in the bond. A covalent bond is represented by a short straight line between the two atoms, H-H
• Covalent bonds should not be regarded as shared electron pairs in a fixed position; the electrons are in a state of constant motion and are best regarded as?charge clouds

A representation of electron charge clouds. The electrons can be found anywhere in the charge clouds

• 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
• 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’
  • Accommodating?less?than 8 electrons in the outer shell means than the central atom is?‘electron deficient’
  • Some examples of this can be found in the section on Lewis structures

• The differences in Pauling electronegativity values can be used to predict whether a bond is?covalent?or?ionic?in character

Electronegativity & covalent bonds

• In?diatomic?molecules?the electron density is shared equally between the two atoms
  • Eg. H₂, O₂?and Cl₂
• Both atoms will have the same electronegativity value and have an?equal?attraction?for the bonding pair of electrons leading to formation of a?covalent?bond
• A difference of less than around?1.0?in electronegativity values will be associated with covalent bonds, although between 1.0 and 2.0 can be considered polar covalent:

You can use the Pauling scale to decide whether a bond is polar or nonpolar:

• 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 bond
• An example of 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

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

• More examples of coordinate bonding can be found in the section on?Lewis Structures

• Non-metals?are able to?share?more than one pair 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
• It is not possible to form a quadruple bond as the repulsion from having 8 electrons in the same region between the two nuclei is too great

Covalent Bonds & Shared Electrons Table

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

Bond length

• The?bond length?is?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

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

• Test your knowledge of covalent bonding:

Answer:

The correct option is?C.

• • To form a dative covalent bond one species must have a lone pair of electrons and the other must be electron deficient.
  • NH_3?has a lone pair and HF splits into H⁺?(electron deficient) and F^-

NH₃?+ ?HF?→?NH₄⁺?F^-