Property

Going across Period:

Why?

Atomic Radius

Decreases

• Atomic charge increases
• Number of electrons in same principal quantum shell increases
• Shielding effect remains constant
• Greater attractive force
• Valence electrons are pulled closer to nucleus

Ionic Radius

• Decreases between Na⁺ & Si⁴⁺
• Increases between Si⁴⁺ & P^3-
• Decreases between P^3- & Cl^–
• Ar does not form ion

Between Na⁺ & Si⁴⁺:

• Metals lose electrons to form cations
• They lose valence shell
• Ionic radius is much smaller than atomic radius
• Going across: higher nuclear charge pulls outer electrons closer

Between P^3- & Cl^–:

• Non-metals gain electrons to form anions
• Valence shell contains more electrons than their atoms
• Larger repulsion between each valence electron
• Ionic radius is larger than atomic radius
• Going across: higher nuclear charge pulls outer electrons closer

Melting Point

• Increases between Na & Al
• Increases largely between Al & Si
• Decreases largely between Si & P
• Decreases between P & Ar

Between Na & Al:

• Bonding is metallic
• Structure is giant metallic
• Going across: more electrons are donated into sea of delocalized electrons
• Stronger forces of attraction
• More energy needed to overcome

Si:

• Bonding is covalent
• Structure is giant covalent
• High energy is needed to break covalent bonds

Between P & Cl:

• Bonding is covalent
• Structure is simple molecular
• Weak van der Waals’ forces require less energy to break compared to metallic bonds & covalent bonds
• Going across: structure of molecules changes
• Smaller the size, weaker VDW forces, lower melting point
  • P: tetrahedral (large)
  • S: puckered 8-membered ring (largest)
  • Cl: diatomic (small)
  • Ar: monoatomic (smallest)

Electrical Conductivity

• Increases between Na & Al
  Decreases largely between Al & Si
• Decreases between Si & S
• Cl & Ar do not conduct

Between Na & Al:

• Bonding is metallic
• Structure is giant metallic
• Going across: more electrons are donated into sea of delocalized electrons
• Higher conductivity

Between Si & S:

• Bonding is covalent
• No delocalised free electrons

1^st Ionisation Energy

Increases, with small ‘dips’

(See here for more in-depth explanations)

• Positive nuclear charge increases
• Shielding effect is similar
• Effective nuclear charge increases

Dips are due to:

• Electronic configuration (p-orbitals vs s-orbitals)
• Spin-pair repulsion (p¹ vs p²)