MP Board · Class 10 · Science · Periodic Classification of ElementsExplain the Modern Periodic Law and discuss the main features of the Modern Periodic Table, including periods and groups, along with the trends in atomic size down a group and across a period.

Modern Periodic Law

• Statement: The Modern Periodic Law states that "Properties of elements are a periodic function of their atomic numbers." This means when elements are arranged in order of increasing atomic numbers, elements with similar properties recur at regular intervals.

Main Features of the Modern Periodic Table

• Periods: There are 7 horizontal rows in the Modern Periodic Table called periods. As we move from left to right across a period, the number of valence electrons increases by one, and the shell number remains the same.
• Groups: There are 18 vertical columns in the Modern Periodic Table called groups. Elements in the same group have the same number of valence electrons and hence exhibit similar chemical properties.

Trends in Atomic Size

• Across a Period: The atomic radius generally decreases from left to right across a period. This is because the nuclear charge increases with each successive element, pulling the electrons closer to the nucleus while the number of shells remains the same.
• Down a Group: The atomic radius increases down a group. As we move down a group, a new shell is added at each step, which increases the distance between the outermost electrons and the nucleus, thereby increasing the overall atomic size despite the increasing nuclear charge.