* Increased Nuclear Charge: The number of protons in the nucleus increases as you move across a period. This leads to a stronger positive charge in the nucleus, pulling the electrons closer.
* Similar Electron Shielding: While the number of electrons increases, they are all in the same energy level (same shell). The inner electrons (core electrons) shield the outer electrons from the full nuclear charge, and this shielding effect remains relatively constant across a period.
* Stronger Attraction: The stronger nuclear charge overcomes the shielding effect, leading to a stronger attraction between the nucleus and the electrons. This pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.
Example:
* Lithium (Li): 3 protons, 2 core electrons, and 1 outer electron.
* Beryllium (Be): 4 protons, 2 core electrons, and 2 outer electrons.
Beryllium has a smaller atomic radius than lithium because its stronger nuclear charge pulls the electrons closer, even though both elements have their outer electrons in the same energy level.
Important Note: This trend applies to neutral atoms. When atoms gain or lose electrons to form ions, their size can change. Cations (positively charged ions) are smaller than their neutral atoms, while anions (negatively charged ions) are larger.
