Short Answer

Nuclear charge is the total positive charge of protons in an atom’s nucleus, and it strongly affects periodic trends.

• Across a period, as nuclear charge increases, electrons are pulled closer → atomic radius decreases, ionization energy and electronegativity increase.
• Down a group, shielding reduces the effect of increased nuclear charge → atomic size increases, ionization energy decreases, and electronegativity decreases.

Thus, nuclear charge is a key factor in determining chemical and physical properties of elements.

Detailed Explanation :

Definition of Nuclear Charge

Nuclear charge refers to the total positive charge from protons in an atom’s nucleus. It is denoted by Z, the atomic number.

• Higher Z → stronger attraction for electrons.
• The effective nuclear charge (Z_eff) felt by valence electrons is less than the actual nuclear charge due to shielding by inner electrons.
• Nuclear charge influences electron distribution, atomic size, ionization energy, electronegativity, and chemical reactivity.

Effect Across a Period

1. Increase in Nuclear Charge:
   • Moving left to right in a period, proton number increases → more positive charge in the nucleus.
   • Electrons are added to the same principal energy level, so shielding effect does not increase significantly.
2. Impact on Atomic Properties:
   • Atomic Radius: Electrons are pulled closer → radius decreases.
   • Ionization Energy: Stronger nuclear pull → more energy required to remove an electron → increases.
   • Electronegativity: Greater nuclear attraction → atom attracts bonding electrons more strongly → increases.
3. Example:
   • Period 2: Lithium (Z=3, low nuclear charge) → large atomic radius, low ionization energy, low electronegativity.
   • Fluorine (Z=9, high nuclear charge) → small atomic radius, high ionization energy, high electronegativity.
4. Conclusion Across Period:
   • Increasing nuclear charge explains the gradual changes in periodic trends across a period.

Effect Down a Group

1. Increase in Nuclear Charge:
   • Moving down a group, the number of protons increases → nuclear charge increases.
   • However, new electron shells are added → valence electrons are farther from the nucleus.
2. Effect of Shielding:
   • Inner electrons shield outer electrons, reducing the effective nuclear charge felt by valence electrons.
   • As a result, the influence of increased nuclear charge is weakened.
3. Impact on Atomic Properties:
   • Atomic Radius: Despite higher nuclear charge, radius increases because electrons are in higher shells.
   • Ionization Energy: Lower because valence electrons are farther → easier to remove.
   • Electronegativity: Decreases because nucleus cannot attract bonding electrons strongly.
4. Example:
   • Group 1: Lithium (Z=3) → small radius, high ionization energy relative to group.
   • Cesium (Z=55) → large radius, low ionization energy, low electronegativity.
5. Conclusion Down a Group:
   • Shielding dominates over increasing nuclear charge → explains trend in periodic properties.

Significance of Nuclear Charge in Periodic Trends

1. Atomic Size:
   • Higher nuclear charge → smaller atoms (across period).
   • Shielding reduces effect → larger atoms (down group).
2. Ionization Energy:
   • Directly related to nuclear pull on valence electrons → increases across period, decreases down group.
3. Electronegativity:
   • Atoms with higher nuclear charge attract bonding electrons more strongly → increases across period, decreases down group.
4. Reactivity:
   • Metals with low nuclear charge and large size → highly reactive.
   • Non-metals with high nuclear charge and small size → highly reactive.
5. Chemical Bonding:
   • Nuclear charge influences ionic and covalent bond formation, bond polarity, and stability.

Conclusion

Nuclear charge, the total positive charge of the nucleus, is a fundamental factor in periodic trends. Across a period, increasing nuclear charge decreases atomic size and increases ionization energy and electronegativity. Down a group, added shells and shielding reduce the effect of higher nuclear charge, increasing atomic size and lowering ionization energy and electronegativity. Understanding nuclear charge helps explain variation in chemical and physical properties of elements across the periodic table.