CIE A Level Chemistry复习笔记1.3.7 Covalent Bonding

• Covalent bonding occurs between two nonmetals
• 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

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

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

Covalent bonding in hydrogen

Chlorine, Cl₂

Covalent bonding in chlorine

Hydrogen Chloride, HCl

Covalent bonding in hydrogen chloride

Ammonia, NH₃

Covalent bonding in ammonia

Methane, CH4

Covalent bonding in methane

Ethane, C₂H₆

Covalent bonding in ethane

Double covalent bonding

Oxygen, O₂

Covalent bonding in oxygen

Carbon dioxide, CO₂

Covalent bonding in carbon dioxide

Ethene, C₂H₄

Covalent bonding in ethene

Triple covalent bonding

Nitrogen, N₂

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’
• Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
• Some examples of this occurring can be seen with Period 3 elements

Sulfur dioxide, SO₂ – dot and cross diagram

Phosphorus pentachloride, PCl₅ – dot and cross diagram

Sulfur hexafluoride, SF₆ – dot and cross diagram