Chemistry Chapter on Alkali and Alkaline Earth Metals

s-block Elements

• Outer electronic configuration: ns1 for alkali metals and ns2 for alkaline earth metals.
• Characteristics: Soft metals, low melting and boiling points, high reactivity due to low ionization enthalpy, and strong reducing agents.
• Common oxidation states: +1 for alkali metals and +2 for alkaline earth metals.
• Reactivity increases down the group; most form ionic compounds, except lithium and beryllium.

p-block Elements

• Consist of elements in groups 13 to 18, with the outermost electronic configuration ranging from ns2np1 to ns2np6.
• Includes metals (e.g., tin, lead) and nonmetals (e.g., carbon, nitrogen).
• Nonmetallic character increases left to right across periods; metallic character increases top to bottom within groups.
• Higher ionization enthalpies than s-block elements.
• Group 18 elements (noble gases) have stable configurations (ns2np6) and exhibit very low reactivity.
• Halogens (group 17, ns2np5) and chalcogens (group 16, ns2np4) have highly negative electron gain enthalpies, gaining one or two electrons respectively to achieve stability.
• Some elements exhibit multiple oxidation states.

d-block Elements

• Comprised of groups 3 to 12 with outer electronic configuration (n-1)d1–10ns0–2.
• Mostly metallic, high melting and boiling points, form colored complexes, variable valency, and are often paramagnetic.
• Notable elements acting as catalysts include Mn, Ni, Co, Cr, V, and Pt.
• Classified as transition elements, bridging highly reactive s-block elements to less reactive p-block elements.

f-block Elements

• Positioned below the main periodic table and characterized by the entry of electrons into f-orbitals, denoted as (n-2)f1–14(n-1)d0or1ns2.
• Includes lanthanoids (Ce to Lu, Z=58 to 71) termed rare earth elements, and actinides which start from Actinium (Z=89) to Lawrencium (Z=103).

Cation and Anion Size

• Cation size decreases as effective nuclear charge increases, leading to reduced electron-electron repulsion.
• Example: Formation of Li+ reduces the size from 123 pm to 60 pm.
• Anion size increases with more electrons due to increased inter-electronic repulsion.

Ionization Enthalpy

• Ionization enthalpy is larger than ionization energy by a small margin (≈ 6 kJ/mol) and often considered negligible.
• Alkali metals have low ionization enthalpies due to single valence electrons easily lost for stability.
• Noble gases have high ionization enthalpies due to their stable electronic configurations.

Electron Gain Enthalpy

• Defined as the energy change when an electron is added to a gaseous atom forming an anion.
• NEG (electron gain enthalpy): if energy is released during electron addition, it is negative; if energy is absorbed, it is positive.
• Electron gain enthalpies are expressed in kJ/mol or eV.
• Positive electron affinity means the resulting anion has lower energy than the neutral atom.

Summary of Key Concepts

• Electron gain enthalpy (DEG) is often used interchangeably with electron affinity, but they follow opposite sign conventions (DEG = -Ea).