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Periodicity Notes

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Unit 1: 4 Periodicity

PERIODICITY Periodicity relates to how the properties of elements and their compounds change in trends across and down the periodic table. Atomic Radius The atomic radius decreases across the period because the outer electrons are in the same main energy level and have the same shielding, however the nuclear charge across the atom is increasing across the period so the electrons are more strongly attracted and pulled closer to the nucleus. The atomic radius generally increases down a group

• A positive ion has a smaller ionic radius than the original atom and a negative ion has a larger ionic radius than the original atom. First ionisation energy The first ionisation energy generally increases across the period because the nuclear charge increases whilst the shielding remains the same, causing a greater attraction of the electrons to the nucleus. Melting and boiling points In metals; The melting point and boiling point increase across a period due to stronger metallic bonds.
>As the nuclear charge increases, that atoms gets smaller and the metallic elements produce positive ions with a larger charge, increasing from +1 to +3, so there are more delocalised electrons holding the giant structure together, so greater metallic bonds are formed meaning a higher melting point and boiling point. Silicon is a metalloid and has a macromolecular (giant covalent) structure with strong covalent bonds in a tetrahedral shape (like diamond) - it has v high mp and bp because a lot of energy is needed to overcome giant covalent structures. Simple molecular structures (non-metals) have very low melting and boiling points
-the larger the molecule, the more van der Waals forces (type on intermolecular forces) and so the higher the melting point. Therefore, for group 3 non-metals
- molecular structures are P4, S8, Cl2 and then Ar.
- S8 has the highest melting point, (largest molecule, most Van der Waals forces) then P4 the Cl2 and finally Ar, despite Ar individual atoms having more electrons than the others; we are talking about MOLECULES here.
- melting a simple molecular structure involves breaking VDW forces, separating adjacent molecules)

CHEMISTRY AS NOTES

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