Why do atomic radii decrease across one period?
The Correct Answer and Explanation is:
Correct Answer:
Atomic radii decrease across a period because the number of protons increases, which pulls the electrons closer to the nucleus.
Explanation:
Atomic radius refers to the distance from the nucleus of an atom to the outermost shell of electrons. When moving from left to right across a period in the periodic table, the atomic number increases. This means that each successive element has one more proton in its nucleus and one more electron in its outer shell compared to the element before it.
Even though more electrons are added, they are being placed in the same energy level or shell. Since no new shell is added, the size does not increase due to electron shielding. Electron shielding occurs when inner electrons block the attraction between the nucleus and the outer electrons. However, in a single period, the inner electron configuration remains the same, so shielding does not increase significantly.
The increase in the number of protons makes the nucleus more positively charged. This stronger positive charge creates a greater attraction to the negatively charged electrons. As a result, the entire electron cloud is drawn slightly closer to the nucleus. This leads to a reduction in atomic size, or radius, across the period.
For example, compare sodium (Na) and chlorine (Cl), which are in the same period. Sodium has 11 protons and chlorine has 17 protons. Even though both elements have electrons in the third shell, the stronger nuclear charge in chlorine pulls the electrons in more tightly, making chlorine’s atomic radius smaller than that of sodium.
In summary, atomic radii decrease across a period because the increasing number of protons strengthens the nuclear attraction, pulling the electrons closer to the nucleus. Since electrons are added to the same energy level and shielding remains almost unchanged, the net effect is a decrease in atomic size.
