In the modern periodic table, the atomic radius ______ in moving from left to right along a period. 

The correct answer is decreases.

Key Points

• The atomic radius generally decreases from left to right across a period in the modern periodic table.
• This decrease is due to the increase in the effective nuclear charge as protons are added to the nucleus.
• As the nuclear charge increases, the electrons are pulled closer to the nucleus, reducing the atomic radius.
• Despite the addition of electrons to the same energy level, the increased attraction from the nucleus causes the atomic size to shrink.

Additional Information

• Effective Nuclear Charge (Z_eff)
  • The effective nuclear charge is the net positive charge experienced by an electron in a multi-electron atom.
  • It is calculated as Z_eff = Z - S, where Z is the atomic number and S is the shielding or screening constant.
  • Higher Z_eff results in a stronger pull on electrons, reducing atomic size.
• Shielding Effect
  • The shielding effect refers to the reduction in the effective nuclear charge on the electron cloud, due to a difference in the attraction forces of electrons in different shells.
  • Inner electrons shield the outer electrons from the full charge of the nucleus, but this effect is relatively constant across a period.
• Periodic Trends
  • Along a period, atomic size decreases from left to right due to increasing nuclear charge.
  • Down a group, atomic size increases due to the addition of electron shells.
• Atomic Radius Measurement
  • The atomic radius is typically measured in picometers (pm) or angstroms (Å).
  • Different types of atomic radii include covalent radius, van der Waals radius, and metallic radius.