1. Increasing Nuclear Charge:
- As you move across a period, the number of protons in the nucleus increases, leading to a stronger positive charge.
- This stronger attraction pulls the electrons closer to the nucleus, effectively shrinking the atom's size.
2. Same Number of Electron Shells:
- All elements in the same period have the same number of electron shells. This means that the outermost electrons are in the same energy level.
- However, as the nuclear charge increases, the electrons experience a stronger attraction, causing them to be pulled closer to the nucleus.
3. Increased Effective Nuclear Charge:
- The effective nuclear charge refers to the net positive charge experienced by an electron in an atom.
- As you move across a period, the number of core electrons (electrons in inner shells) remains constant.
- However, the number of protons increases, so the effective nuclear charge experienced by the valence electrons (outermost electrons) becomes stronger. This stronger attraction pulls the valence electrons closer to the nucleus, reducing the atomic radius.
4. Shielding Effect:
- The core electrons shield the valence electrons from the full attraction of the nucleus.
- As you move across a period, the core electrons remain the same, providing the same level of shielding.
- However, the increasing nuclear charge overcomes the shielding effect, resulting in a stronger attraction between the nucleus and valence electrons, leading to a smaller atomic radius.
In summary, the increase in nuclear charge and effective nuclear charge, coupled with the constant number of electron shells and the shielding effect of core electrons, are the primary reasons for the decrease in atomic radius across a period.
