Classification of Elements and Periodicity in Properties: JEE Main Mock Test 2025-26 Preparation

The periodic law states that the physical and chemical properties of elements are a periodic function of their atomic numbers. This law led to the formation of the modern periodic table, where elements are arranged by increasing atomic number in such a way that those with similar characteristics recur at regular intervals (periodicity).

In the modern periodic table:

• Groups are the vertical columns numbered from 1 to 18, where elements have similar valence electron configurations and thus, similar chemical properties.
• Periods are the horizontal rows numbered from 1 to 7, where the atomic number increases progressively and properties gradually change across a period.

Periodicity in properties refers to the repeated trends observed in the physical and chemical properties of elements when they are arranged according to increasing atomic number. This happens due to the recurring pattern in the electronic configuration of elements.

The atomic radius is the distance from the nucleus to the outermost electron shell of an atom. Across a period (left to right), the atomic radius decreases due to increasing nuclear charge that pulls electrons closer. Down a group, atomic radius increases because new electron shells are added, making the atom larger.

Ionization enthalpy is the energy required to remove the outermost electron from a gaseous atom to form a positive ion. Across a period, ionization enthalpy increases due to higher nuclear charge and smaller atomic size. Down a group, it decreases as atomic size increases and the outer electrons are farther from the nucleus, making them easier to remove.

Electronegativity is the tendency of an atom to attract a shared pair of electrons in a chemical bond. Across a period, electronegativity increases due to increasing nuclear charge. Down a group, it decreases as atomic size increases and the nucleus is farther away from the bonding electrons.

Noble gases are placed in group 18 because they have completely filled outermost electron shells, making them chemically inert or very stable. This group reflects unique properties such as high ionization enthalpy and zero or negligible electronegativity.

The periodic table is significant because it organizes all known chemical elements based on their increasing atomic number and recurring chemical properties, helps in predicting element trends, valency, reactivity, and chemical behavior, and is essential for studying and understanding chemistry at all levels.

Valency is the combining capacity of an atom, determined by the number of electrons present in the outermost shell or by the number of electrons an atom can gain, lose, or share to achieve a stable configuration. In the periodic table, valency generally increases from 1 to 4 in a period and then decreases from 4 to 0.

The electronic configuration determines an element's period (number of electron shells) and group (number of valence electrons). Elements in the same group have similar valence shell configurations, leading to analogous chemical properties. Thus, electronic configuration is essential for classifying elements and predicting periodic properties.

Elements in the same group of the periodic table have similar valence electron configurations. This means they have the same number of electrons in their outermost shell, which is responsible for their similar chemical properties and reactivity patterns.