Periodicity

• The periodic table is arranged in order of increasing atomic number
• a period is a horizontal row
• a group is a vertical column with elements having similar chemical properties (because same number of electrons in outer shell)
• periodicity is repeating patterns across a period
• electron confuguration increases across a period
• first ionisation energy is the energy needed to remove one mole of electrons from one mole of atoms in the gaseous state
• first ionisation energy increases across a period because same shells, similar shielding but more protons so a greater attraction between the nucleus and the electron being removed.
• atomic radius decreases across the period because same shells, similar shielding but more protons so attraction between nucleus and outer electrons is greater and the electrons are held more tightly
• Ionic radius: cations are smaller than their atoms because they have more protons than electrons so electrons held more tightly
• Ionic radius: anions are larger than their atoms because there are fewer protons than electrons so electrons are held less tightly
• electronegativity is the power of an atom to attract the pair of electrons in a covalent bond
• electronegativity increases across the period because same shell, simolar shielding but more protons so nuclear attraction for the electrons in a covalent bond is stronger
• Melting point for groups 1 to 3 (Na to Al) increases because they all have a giant metallic lattice with metallic bonding so strong electrostatic forces of attraction between positively charged ions and delocalised electrons but as the charge on the metal ions increases as you go across the period the number of delocalised electrons increases so more energy is needed to break the metallic bonds as they are stronger
• Si has a giant covalent lattice with covalent bonding so the MP and BP are high because many strong covalent bonds between atoms must be broken
• P4, S8 and Cl2: all have simple molecular structures with weak Van der Waal’s forces between molecules which must be overcome but do not require much energy. As the size of the molecule increases there are more electrons so stronger Van der Waal’s forces between molecules. S8>P4>Cl2
• Argon has a simple atomic structure with weak Van der Waal’s forces between atoms which don’t take much energy to break