Question

Name the following:

The smallest element in period 3.

Answer 1

• Modern periodic table is the arrangement or organization of elements on the basis of increasing atomic number in 7 horizontal rows and 18 vertical columns. Horizontal rows are termed as Periods while the vertical columns are termed as Groups.
• In the periodic table, there are eight elements in the third Period.
• Period 3 includes Sodium (Na), Magnesium (Mg), Aluminum (Al), Silicon (Si), Phosphorus (P), Sulfur (S), Chlorine (Cl), and Argon (Ar) elements.
• The atomic size of an atom is defined as the distance from the nucleus of that atom to its outermost shell. It is generally measured in angstroms (A°).
• In a Period, the effective nuclear charge increases on moving from left to right due to the successive increase in the atomic number by one unit while the number of shells remains the same. Consequently the force of attraction between the outermost electrons and the nucleus increase which result in a decrease in atomic size. Therefore, it can be concluded that the atomic size decreases from left to right in any Period.

Exception: Argon is bigger than Chlorine in spite of the fact that atomic radius decreases from left to right across a period.

• The electron configuration of Argon is 2,8,8 while that of Chlorine is 2,8,7.
• Since Argon has a stable octet configuration, the interelectronic repulsion between the electrons in Argon is more than that in Chlorine. As a result, the effective nuclear charge is less in Argon compared to that in Chlorine. Therefore, the Argon atom is larger than the Chlorine atom.
• Hence, among the Period 3 elements, Chlorine is the smallest element.