First Ionisation Energy () is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions.
Generally, increases across a period because the increasing nuclear charge and decreasing atomic radius make it harder for the nucleus to lose its grip on valence electrons.
Subshell Dips: A slight decrease occurs between Group 2 and Group 3 because the outer electron in Group 3 is in a p-subshell, which is higher in energy and further from the nucleus than the s-subshell.
Spin-Pair Repulsion Dips: A decrease occurs between Group 5 and Group 6 because the fourth electron in the p-subshell must pair up in an orbital, creating repulsion that makes it easier to remove.
Melting points across a period reflect the bonding and structure of the elements, transitioning from metallic to giant covalent to simple molecular.
Metals (Groups 1-3): Melting points increase from Na to Al as the number of delocalized electrons increases and the ionic radius decreases, strengthening the metallic bond.
Giant Covalent (Group 4): Elements like Silicon have the highest melting points because every atom is linked by strong covalent bonds in a massive 3D lattice that requires significant energy to break.
Simple Molecular (Groups 5-7): Melting points drop significantly as these substances are held only by weak intermolecular forces (London forces). Larger molecules like have higher melting points than due to more electrons.
| Property | Metallic (Na, Mg, Al) | Giant Covalent (Si) | Simple Molecular (P, S, Cl) |
|---|---|---|---|
| Bonding | Electrostatic (ions/electrons) | Covalent (shared pairs) | Intermolecular (London forces) |
| Trend | Increases with charge density | Peak of the period | Low, depends on |
| Structure | Giant Metallic Lattice | Giant Molecular Lattice | Discrete Molecules |
Always mention three factors: When explaining trends, always discuss nuclear charge, atomic radius (distance), and electron shielding.
Shielding is the 'Constant': In a period-based question, explicitly state that shielding remains constant because electrons are added to the same principal shell.
Identify the 'Dips': If a graph shows a sudden drop in , check if it is the transition from to or the start of electron pairing in the p-orbital.
Molecular Size Matters: For non-metals, don't just say 'weak forces'; specify that larger molecules have more electrons, leading to stronger instantaneous dipole-induced dipole forces.
