Which of the following is one of the main reasons for the observed periodic trends (such as atomic radius, ionization energy, and electronegativity) across the periodic table?

• A. The random arrangement of elements in the periodic table
• B. The decrease in atomic number down a group
• C. The constant number of electron shells across a period
• D. The increase in effective nuclear charge across a period

Answer: (D)

Verified step by step guidance

1. Understand that periodic trends such as atomic radius, ionization energy, and electronegativity change predictably across periods and groups in the periodic table.
2. Recognize that the atomic number increases as you move from left to right across a period, meaning more protons are added to the nucleus.
3. Learn that the effective nuclear charge (\(Z_{\text{eff}}\)) is the net positive charge experienced by valence electrons, calculated roughly as \(Z_{\text{eff}} = Z - S\), where \(Z\) is the atomic number and \(S\) is the shielding constant representing inner electron repulsion.
4. Note that across a period, the number of electron shells remains constant, so shielding does not increase significantly, but the nuclear charge increases, leading to a higher \(Z_{\text{eff}}\).
5. Conclude that the increase in effective nuclear charge pulls the valence electrons closer to the nucleus, causing atomic radius to decrease, ionization energy to increase, and electronegativity to increase across a period.