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Classification based on electron differentiation:
We know that the electrons in an atom are arranged in shells around the nucleus. Each shell comprises one or more sub-shells in which the electrons are shared in a particular manner. These sub-shells are named s, p, d, and f.
Different classes of elements
This classification divides the elements into four groups. i.e., s-, p-, d-, and f-block elements, depending on which atomic shell the last electron enters.
s-Block elements or alkali and alkaline earth metals:
The elements in the periodic table that fill the last electron in the s-orbit are known as s-block elements.
  • An s-orbital can hold a maximum of two electrons. Their general formulae are \(ns^1\) and \(ns^2\), respectively, where n = (\(1\) to \(7\)).
  • It contains groups 1A (alkali) and 2A (alkaline earth) elements.
  • These (1A and 2A) group metal oxides react with water to form alkali found in the soil or earth. That's why these are called alkali/alkaline (earth) metals.
  • s-Block elements are soft and have low boiling and melting points. They often form ionic compounds.
  • The total number of s-Block elements is \(14\). Hydrogen (H) and Helium (He) are gaseous elements, while Francium (Fr) and Radium (Ra) are radioactive elements. The liquid elements Caesium (Cs) and Francium (Fr) belong to the s-block.
p-Block elements:
The elements in the periodic table that fill the last electron in the p-orbit are known as p-block elements.
  • A p-orbital can hold up to six electrons. Hence, p-block elements are split into six groups, namely, 3A, 4A, 5A, 6A, 7A, and 8A.
  • This is the only block that includes all three types of elements: Metals, Non-metals, and Metalloids. The general formulae is \(ns^2np^1{^-}{^6}\), where n = (\(2\) to \(6\)).
  • The 7A group elements having the general formula \(ns^2np^6\) are inert because their energy levels are completely filled. They mostly form covalent compounds.
  • The periodic table p-block contains \(30\) elements (excluding He).
  • The top block contains nine gaseous elements; (\(Ne\), \(Ar\), \(Kr\), \(Xe\), \(Rn\), \(F_2\), \(Cl_2\), \(O_2\), and \(N_2\)). Bromine (Br) and gallium (Ga) are both liquids.
d-Block or transition elements:
The elements in the periodic table that fill the last electron in the d-orbit are known as d-block elements.
  • The d-block elements are placed (middle of the table) in 3B, 4B, 5B, 6B, 7B, 8B, 1B, and 2B.
  • Although the periodic table lists \(39\) d-block elements, there are only \(36\) transition elements because only those elements have a partially filled d-orbital.
  • These elements have the general formula \((n-1)d^1{^-}{^1}{^0}ns^1{^-}{^2}\), where n = (\(4\) to \(7\)).
  • These block elements' properties are intermediate to that of s- and p-block elements. Hence, they are called transition elements.
  • They are high melting and boiling metals. Most of them form colour compounds and express many anti-oxidant states.
f-Block or inner transition elements:
The elements in the periodic table that fill the last electron in the f-orbit are known as f-block elements.
  • The periodic table contains \(28\) f-block elements. These elements are placed at the bottom of the periodic table.
  • Elements with atomic numbers \(58\) to \(71\) are called lanthanides because they come after lanthanum (\(57\)).
  • The elements \(90\) to \(103\) are known as actinides because they come after actinium (\(89\)).
  • The standard formula of these elements is \((n-2)f^({^1}{^-}{^1}{^4}{^)}(n-1)d^0{^-}{^1}ns^2\), where n = (\(6\) and \(7\)).
  • s-Block elements are heavy metals with high boiling and melting points. They produce coloured ions.
  • All the actinide elements are radioactive.
  • All the elements after atomic number \(92\), (Uranium (U) \(92\)) are transuranic.
  • The elements that do not occur naturally and are artificially produced in the laboratories are called transuranic or synthetic elements.