Periodic Classification of Elements — Maharashtra (SSC) Class 10 Science Solutions (Free)

The modern periodic table is an arrangement of elements based on their atomic numbers in increasing order. It contains 18 groups (vertical columns) and 7 periods (horizontal rows). Basis: Elements are arranged according to their atomic number (number of protons), not atomic mass. Elements with similar properties are placed in the same group. Periodic law states: Properties of elements are periodic functions of their atomic numbers.

Periodic properties are those chemical and physical properties of elements that show a regular pattern when elements are arranged in order of increasing atomic number. Examples: (1) Atomic radius decreases across a period and increases down a group, (2) Ionization energy increases across a period and decreases down a group, (3) Electronegativity increases across a period and decreases down a group, (4) Metallic character increases down a group and decreases across a period.

Atomic radius decreases in order: Na > Mg > Al. Reason: All three are in period 3. As we move from left to right across a period, atomic number increases, protons increase, nuclear charge increases. Increased nuclear charge pulls electrons closer, reducing atomic radius. The trend is due to increased effective nuclear charge with constant number of electron shells.

Ionization energy is the energy required to remove an electron from an atom. Across a period: (1) Atomic radius decreases, so electrons are closer to nucleus, (2) Nuclear charge increases as atomic number increases, (3) Increased nuclear attraction makes electron removal harder, (4) Electrons are in the same shell, so shielding effect remains nearly constant, (5) Therefore, more energy is needed to remove electrons as we move left to right.

Order: Cs < K < Na (increasing ionization energy). Explanation: All are in Group 1 (alkali metals). As we go down the group: (1) Atomic radius increases, (2) Outermost electrons are farther from nucleus, (3) Shielding effect increases due to more inner electron shells, (4) Nuclear attraction on valence electrons decreases, (5) Less energy needed to remove electrons, so ionization energy decreases down the group.