Chemistry Ionic and Covalent Bonding

Ionic Bonding

• Ionic bonding occurs between a metal and a non-metal
• Electrons are transferred from the metal to the non-metal, creating ions with full outer shells
• Oppositely charged ions are attracted to each other through electrostatic forces, forming a giant ionic lattice
• The charge of an ion affects the strength of the ionic bond. Higher charges lead to stronger bonds
• Larger ions with a greater ionic radius have weaker attractions because the forces act over a greater distance
• Cations (+ve) and anions (-ve) can be represented using dot and cross diagrams
• The electrons being transferred from the cation can be seen on the outer shell of the anion

Covalent Bonding

• Covalent bonds form between two non-metals
• Electrons are shared between the atoms to achieve a full outer shell
• Multiple electron pairs can be shared to form multiple covalent bonds
• Dot and cross diagrams are used to represent the shared electron pairs in a covalent bond
• Two electrons (one from each atom) form a single bond;
  • Four electrons (two from each atom) form a double bond;
  • Six electrons (three from each atom) form a triple bond

Dative Bonding

• A dative (or coordinate) bond forms when both electrons in the shared pair come from the same atom
• Indicated by an arrow from the lone electron pair to the atom that accepts the electron pair
• Once formed, it acts like a standard covalent bond

Simple Covalent

• Substances with a simple molecular structure consist of covalently bonded molecules
• Molecules are held together by weak van der Waals forces
• These forces are intermolecular, acting between molecules

Shapes of Simple Molecules

• The shape of a simple molecule or ion is determined by the number of electron pairs around the central atom and the repulsion between them
• Lone pairs present around the central atom provide additional repulsive forces changing the bond angle
• A bond angle is the angle between two covalent bonds from the same atom
• Examples include linear, V-shaped, trigonal planar, trigonal pyramidal, tetrahedral, trigonal bipyramidal, and octahedral

Bond Polarity

• The negative charge around a covalent bond is not evenly spread
• Electronegativity is the atom's ability to attract the electron pair in a covalent bond
• Electronegativity increases across a period and decreases down a group
• A polar covalent bond forms when the difference in electronegativity between two atoms is between 0.4 and 1.7
• An ionic bond forms when the difference in electronegativity is greater than 1.7

Polar Molecules

• Polar molecules result when there's an overall difference in polarity due to the arrangement of polar bonds and the molecule's geometry
• A molecule with polar bonds may not necessarily be a polar molecule

Intermolecular Forces

• Van der Waals forces are the weakest type of intermolecular force
• They act as an induced dipole between molecules
• The strength of van der Waals forces is related to the Mr and shape of the molecule.
• The greater the Mr, the stronger the force. Straight chain is stronger than branched chain
• Permanent dipoles occur when two atoms in a bond have different electronegativities

Hydrogen Bonding

• Hydrogen bonding is the strongest type of intermolecular force
• It only occurs between hydrogen and highly electronegative atoms (nitrogen, oxygen, or fluorine)
• The lone pair on the highly electronegative atom forms a bond with the 8+ hydrogen atom
• Substances with hydrogen bonding have higher melting and boiling points compared to similar-sized molecules without this type of bonding
• Water and alcohols are examples

Metallic Bonding

• Metallic bonding consists of a lattice of positively charged ions surrounded by a 'sea' of delocalised electrons
• The electrostatic forces between the positive ions and delocalised electrons are very strong
• Metals generally have high melting points and are good conductors of electricity
• Metal layers are malleable and slide over one another

Giant Covalent Structures

• Macromolecular covalent substances are covalently bonded into a giant lattice structure
• Each atom has multiple covalent bonds which are very strong, making the substance very hard and having high melting points
• Examples:
  • Diamond (carbon atoms bonded to four others in a rigid tetrahedral structure)
  • Graphite (carbon atoms bonded to three others in flat hexagonal sheets with delocalised electrons, allowing it to conduct electricity and be a lubricant)
  • Graphene (a single layer of graphite, extremely strong and conducts electricity)

Physical Properties of Substances

• Physical properties include: boiling point, melting point, solubility, and conductivity
• These properties depend on the type of bonding, the type of particle and the crystal structure of the compound
• Substances with different bonding have dramatically different physical properties (e.g., boiling points)