S2.1 The Ionic Model

S2.1.1 and S2.1.2 – The Ionic Bond

📌 Ions :

• Metals are electropositive in nature, as they tend to lose electrons
• Metals have low effective nuclear charges and low ionization energies, making it easy for them to lose electrons
• When a metal atom loses electrons, it forms a positive ion called a cation
• Non metals are electronegative in nature, as they tend to gain electrons
• Non metals have high effective nuclear charges and can attract the transferred electrons strongly
• When a non metal atom gains electrons, it forms a negative ion called an anion
• The number of electrons lost/gained depend on the electronic configuration of the element
• Transition metals can form more than 1 ion and show a range of oxidation states because they do not usually have fully filled outer shells
• The most rigorous reactions occur between elements that are farthest apart on the periodic table

⭐️ Caesium Fluoride (CsF) is considered the most ionic compound.

• The attraction between ions increases with ionic charge but removing multiple electrons has an energy cost
• High ionization energies of metals prohibit electron loss from inner shells, preventing them from attaining higher ionic charges
• Addition of multiple ions makes negative ions more attractive, but addition becomes increasingly difficult due to electron-electron repulsions

⭐️ The formation of Si^4- is not feasible for this reason.

📌 The Ionic Bond :

• When an ionic compound is formed, electrons are transferred from the electropositive atom to the electronegative atom to form cations and anions

Eg. Lewis Dot and Cross Structure of NaCl

⭐️ Ionic bonding is the electrostatic attraction between oppositely charged ions

📌 Naming Ionic Compounds :

• Systematic names of compounds use oxidation numbers – only necessary when element shows more than one common oxidation state

• Named with cation followed by anion
• No net loss/gain of electrons : deducing formula involves balancing the total number of positive and negative charges
• Polyatomic ions are made up of more than one atom which together have lost or gained an electron

📌 Common polyatomic ions to know :

Polyatomic ion name	Formula
nitrate	NO3 –
sulfate	SO4 2-
phosphate	PO43-
hydroxide	OH–
hydrogencarbonate	HCO3–
carbonate	CO32-
ammonium	NH4+

🧠 While bonding between the polyatomic ion and the other ion is ionic, bonding within the polyatomic ion remains covalent.

S2.1.3 Ionic Structure and Properties :

📌 Physical Properties of Ionic Compounds :

• Many cations and anions arrange themselves in a characteristic lattice structure
  • no individual molecules
  • all positive ions attract negative ions
  • bonding is uniform
• Fixed arrangement of repeating units called a formula unit (ratios of ions present)

🧠 Make sure you avoid the term ‘molecule’ while describing an ionic compound and instead use the term ‘formula unit’

⭐️ Coordination number represents the number of ions that surround a given ion in a lattice

⭐️ Lattice enthalpy is the energy required to break 1 mol of ionic solid into its constituent gaseous ions

• Lattice enthalpy as a measure of the strength of ionic bonding

📌 Calculating lattice enthalpy using the ionic model :

• Increase in ionic charge increases ionic attraction and lattice enthalpy
• Increase in radius of one of the ions decreases ionic attraction and lattice enthalpy
• Lattice enthalpy is greater for smaller ions with a larger charge density

• Melting and Boiling points
  • High melting and boiling points
  • Crystalline solids at room temperature
  • Large amount of energy required to overcome the strong electrostatic forces
• Volatility
  • ⭐️ How readily a substance vaporizes
  • Ionic compounds have low volatility
• Solubility in water
  • Soluble in water as it is a polar solvent
  • Energy released when ions are surrounded by water (ion-dipole) pays back energy required to overcome lattice
• Solubility in non polar solvents
  • Not soluble in non polar solvents
  • Weak interactions (LDF) would not pay back lattice enthalpy
• Electrical conductivity
  • ⭐️ Conductivity is the flow of charged particles
  • Ionic compounds do not conduct when solid as the ions are not free to move
  • Conduct electricity when molten
  • Conduct electricity when dissolved in water
• Ionic compounds are generally brittle – crystals tend to shatter when shear force is applied
• Ionic character can be calculated using difference in electronegativities

⭐️ CsF is considered 100% ionic with a difference of 3.2.

⭐️ Ionic compounds >1.8 on the Pauling scale