T2- Chemical Bonding_Part1 PDF

T2- Chemical Bonding_Part1 PART 1: IONIC BONDING 1. Cations Cations form when METALS loose electrons. The energy to form cations depend on the IONIZATION ENERGY Reminder: Ionization energy The energy required to release one mole of electrons from a gaseous (neutral) atom completely, this is called...

T2- Chemical Bonding_Part1 PART 1: IONIC BONDING 1. Cations Cations form when METALS loose electrons. The energy to form cations depend on the IONIZATION ENERGY Reminder: Ionization energy The energy required to release one mole of electrons from a gaseous (neutral) atom completely, this is called the first ionisation energy, or Ei1 for short A(g) → A+(g) + e- Withdrawing an electron costs energy, Ei has a positive value. Ei>0 The ionization energy depends on the attraction of the nucleus to the electron. The smaller that force, the easier the electron can leave the atom can leave, so the smaller the ionization energy. Rule of thumb (no law) The ionization energy increases at: - greater nuclear charge - smaller main quantum number To form X2+ ions, the required energy is Ei1 + Ei2 and so on. 1/10  T2- Chemical Bonding_Part1 2. Anions Anions form when NON-METALS gain electrons. To form a negative ion, an atom must absorb an electron. This energy is called ELECTRON AFFINITY Ea Reminder: Electron affinity Energy exchanged when an electron is absorbed by an atom in gaseous state. A(g) + e- → A-(g) Rule of thumb (no law) The Electron affinity increases at: -greater nuclear charge -smaller main quantum number 2/10  T2- Chemical Bonding_Part1 3. Ionic bond An ionic bond is generated by the transfer of one or more electrons from one atom to another. This creates two ions of opposite charge, which are attracted to each other by forces: these STRONG electrotstatic attraction forces between cations and anions represent the ionic bond. As ionic bonds are non-directional, ions are arranged in giant, three- dimensional close-packed structures called ionic lattices. Salts form ionic crystalline structures Salts and more general, about 90% of all minerals, are essentially ionic compounds forming crystalline ionic compounds. Sodium chloride as an example For example, in sodium chloride (NaCl) there is a positively charged sodium cation, Na+, and a negatively charged chlorine anion, Cl−. The visible structure of a salt is called a crystal. This is how you see small cubes if you take table salt ( NaCl ) up close. The crystal structure formed is called ionic lattice. The repeating structure in a crystal lattice is called a formula unit. Ionic bonds tend to be stronger than metallic bonds, so crystals containing ionic bonds tend to be unmalleable and much more brittle than metal crystals. 3/10  T2- Chemical Bonding_Part1 4. How to find the charge of ions - Ionic charges of elements in the main groups can be deduced from their position in the PTE: Group nr 1 2 3 4 5 6 7 8 Ionic + 2+ 3+ / 3- 2- - / charge - Ionic charges from transition metals CANNOT be deduced from the PTE. The following ionic charges must be memorized. Zinc Zn2+ zinc ion Silver Ag+ silver ion Some metals can form different ions. The charge of the ion is indicated in brackets after the name of the metal with roman numbers) Iron Fe2+ iron (II) ion Fe3+ iron (III) ion Copper Cu+ copper (I) ion Cu2+ copper (II) ion - Ion charges from atom groups (polyatomic ions) cannot be deduced and must hence be memorized. ANIONS OH- Hydroxide ion NO3- Nitrate ion CO32- Carbonate ion SO42- Sulfate ion PO43- Phosphate ion CATIONS NH4+ Ammonium ion 4/10  T2- Chemical Bonding_Part1 5. Chemical formula of ionic compounds Chemical compounds always contain cations and anions. - Ions can be monoatomic (i.e. sodium ions, halide ions etc) or polyatomic (i.e. carbonate/ammonium etc) - Cations can be metallic (i.e. Na+) or polyatomic (i.e. NH4+) - Anions can be non-metallic (i.e. Cl-) or polyatomic (i.e. NO3-) Because salts contain ions, but are neutral substances, the total positive charges form the cations compensate and are equal to the negative charges of the anions. The chemical formula of ionic substances do not show any electrical charges. The subscript numbers in ionic substances help to deduce the ratio between anions and cations. Example: calcium chloride CaCl2 The formula means that in the lattice, there are double as many chloride ions Cl- than calcium cations Ca2+. More examples: Aluminium oxide Al2O3 Ratio: Al : O 2 :3 For every 2 aluminium cations Al3+, there are 3 oxide anions O2-.In other words, in average, there are 1.5x more O2- anions than Al3+ cations. 5/10  T2- Chemical Bonding_Part1 6. Naming of ionic compounds ❖ CATIONS Metal cations use the names of their elements. Example: Mg2+ magnesium cation Cations with more than one type of ion indicate the charge by a roman number in brackets behind the name of the cation: Example: FeO iron (II) oxide Fe2O3 iron (III) oxide ❖ ANIONS Non-metals of elements change their name in compounds. They end on -IDE Sulfur → sulfide Nitrogen → nitride Oxygen → oxide Halogen → Hal-IDE Examples: Fluorine → fluoride Chlorine → chloride Bromine → bromide Iodine → iodide Careful with the name of polyatomic ions. Some of them end on -ATE Examples: nitrate / sulfate / carbonate / phosphate 6/10  T2- Chemical Bonding_Part1 PRACTICE 7/10  T2- Chemical Bonding_Part1 7. Properties of ionic compounds The strong attraction between ions creates a number of specific properties for salts (ionic compounds), High melting point Hard Brittle (easily breakable) Soluble in water (due to the interaction between ion charges and the water molecules having a dipole) Electrical conductivity in the liquid or dissolved state (this makes it an electrolyte) No conduction in the solid state (no electrolyte, as ions are not mobile) Dissolution equation of ionic compounds Ionic compounds easily dissolve in water: most of them are soluble in water. 8/10  T2- Chemical Bonding_Part1 When ionic compounds dissolve in water, the ions physically separate from each other. We can use a chemical equation to represent this process For example, with NaCl: When NaCl dissolves in water, the ions separate and go their own way in solution; the ions are now written with their respective charges, and the (aq) phase label emphasizes that they are dissolved in water. PRACTICE Write the name and the dissolution equation for the following salts: 1. KBr 2. Na2SO4 3. (NH4)PO4 9/10  T2- Chemical Bonding_Part1 Studentbook questions 10/10