Summary
Highlights
Periodicity is the study of repeating patterns or trends in physical or chemical properties across the periodic table. Elements are categorized into blocks (s, p, d) based on the subshell where their outer electrons are found. Group 1 and 2 elements are in the s-block, groups 13-18 are in the p-block, and transition elements are in the d-block.
Atomic radius is the distance from the nucleus to the outermost electron shell. Across Period 3, atomic radius decreases. This is because the nuclear charge increases as more protons are added, while electron shielding remains constant due to electrons being in the same energy level. The increased nuclear attraction pulls the outer electrons closer to the nucleus.
Electronegativity is an atom's ability to attract electron density in a covalent bond. Across Period 3, electronegativity increases because the increasing nuclear charge and decreasing atomic radius lead to a stronger attraction for bonding electrons. Noble gases are typically excluded as they don't readily form covalent bonds. Electronegativity increases up a group due to smaller atomic size and closer nucleus to bonding electrons.
Ionization energy is the energy required to remove a mole of electrons from a mole of gaseous atoms. Generally, ionization energy increases across a period because electrons are closer to the nucleus and nuclear charge increases. However, there are two dips in the trend for Period 3: between magnesium and aluminium (due to p-subshell electrons being easier to remove) and between phosphorus and sulfur (due to electron repulsion in a paired p-orbital).
Melting and boiling points depend on the strength of forces between particles. Stronger forces require more energy to overcome, leading to higher melting/boiling points. For Period 3 elements, there are three distinct regions: metals (Na, Mg, Al) with increasing melting points due to increasing charge density, silicon (Si) with the highest melting point as a giant covalent structure, and non-metals (P, S, Cl, Ar) with lower melting points due to weak intermolecular van der Waals forces.
Phosphorus (P4), sulfur (S8), and chlorine (Cl2) are simple molecular substances with van der Waals forces. Sulfur has the highest melting point among them due to being the largest molecule, followed by phosphorus and then chlorine. Argon (Ar) is monoatomic, with the weakest van der Waals forces, resulting in the lowest melting point.