IB Chemistry 4.1 - 4.2

IB Chemistry SL 4.1 - Ionic Bonding and Structure

4.1.1 - Ionic Bonding

• Ionic Bonding - Bonds where one atom donates electrons to another; the atom that loses an electron becomes a cation and the other becomes an anion

• Cation - An ion with a positive electric charge

• Anion - An ion with a negative electric charge

• Ionic Bonds are the result of the electrostatic attraction between ions

• Ionic Compounds - Usually form between a metal and a nonmetal (easy way to find them)

• Atoms use ionic bonding to reach full valence shells that satisfy the octet rule (the tendency of most atoms to want 8 electrons in their valence shells)

• Ionic compounds arrange themselves into a crystal/lattice structure (in other words, a regular pattern of repeating structures of atoms)

• The ratio of elements in a crystal structure is used to give the chemical formula (ex. In NaCl, there is one sodium atom for every chlorine, thus their ratio is 1:1 and the formula is NaCl)

• Ionic bonding is non-directional (force occurs on an atom from all sides)

• Coordination Number - gives the # of atoms another atom of a compound is in contact with

• Both coordination numbers define how a compound is coordinated (ex. In NaCl, each sodium atom is bonded to 6 chlorine atoms and the same goes for chlorine, so its coordination is 6:6)

• Note that many compounds contain bonds with both ionic and covalent properties

4.1.2 - Formulae of Ionic Bonds

• Groups 1, 2, and 13 have ionic charges of 1+, 2+, and 3+ respectively

• Groups 15, 16, and 17 have ionic charges of 3-, 2-, and 1- respectively

• Polyatomic Ions - ions made of multiple atoms bonded covalently 

• The charges of ionic compounds must cancel out (the sum of the charges of the atoms within it must be 0)

4.1.3 - The physical Properties of Ionic Compounds

• Strong electrostatic forces between ions lead to a high boiling/melting point

• Melting point depends on ionic charge and radius 

• Melting point tends to increase when ionic charge increases

• Melting point tends to decrease when ionic radius increases

• Ionic compounds have low volatility (they do not change state easily)

• Most ionic compounds are soluble in water

• The first step in dissolution in water is a breaking of the lattice structure

• Then water surround the ions that have broken off (hydration)

• Water is polar (more on that later)

• Non-polar solvents have limited solubility with ionic compounds while polar solvents tend to dissolve ionic compounds very effectively 

• Ionic compounds have a low electrical conductivity when they are solid because of the lack of free (delocalized) electrons

• Ionic compounds are good electric conductors when they are molten or dissolved

• Ionic compounds tend to be brittle as they fracture along an entire plane when ions are misaligned

IB Chemistry SL 4.2 - Covalent Bonding 

4.2.1 - Nature of the Covalent Bond

• Covalent bonding usually occurs between elements with a difference in electronegativity (EN) less than 1.8 units

• Non-metals form covalent structures

• Covalent Bonding - An electrostatic attraction between positive nuclei and shared pairs of electrons

• Each pair helps each atom fill its valence shell (octet rule)

• Single pairs contain 2 electrons, double pairs contain 4, triple pairs contain 6

• Bond Length - The distance between bonded atoms

• Bond Strength - The amount of energy needed to break a bond

• More bonds lead to a decrease in length but an increase in strength

• Coordinate covalent bond - A covalent bond where both electrons in a pair are given by one atom

• Electron Deficient (lacking electrons) atoms are able to form covalent bonds

• Dimer - Molecule formed by two identical smaller molecules linked by coordinate covalent bonds or hydrogen bonds

• Bond Order - # of bonds between a pair of atoms

• To calculate the bond order of a multi-atom molecule, sum the individual bond orders and divide by the number of bonds

4.2.2 - Polarity of Covalent Bonds

• The following table gives the polarity ranges of types of bonds:

• Notice that most bonds are a mixture of ionic and covalent bonds

• An increase in EN leads to an increase in polaitry and ioinc character

• In a pure covalent bond, electrons are shared equally

• Polar-covalent bonds have properties between ionic and covalent bonds

• Electrons are unequally shared and form a bond dipole (one side is partially positive and the other is partially negative, more on this later)

• The existence of polar bonds is a requisite for a polar molecule