Chemistry · Periodic Classification Of Properties · NEET
A nucleus has many protons that pull the outer electrons toward it. But the inner electrons sit between the nucleus and the outer electrons and push them away a little. Effective nuclear charge (Zeff) is the leftover, real pull that an outer electron actually feels after this inner-electron push. It is always less than the full nuclear charge Z.
Zeff = Z − S. Here Z is the atomic number (total protons in the nucleus) and S is the shielding constant (the amount of pull blocked by inner electrons). For example, in sodium (Z = 11) the 10 inner electrons block most of the charge, so the single outer 3s electron feels a Zeff of only about +1, not +11. This is why sodium loses that electron easily.
Nuclear charge (Z) is the full positive charge from all the protons. Effective nuclear charge (Zeff) is only the part of that charge an outer electron actually feels, after inner electrons screen the rest. Z is fixed for an element. Zeff is smaller than Z and is what really controls size, ionization energy, and electronegativity. NEET questions test Zeff, not just Z.
Across a period (left to right), Zeff increases. You add protons one by one, but the new electrons go into the SAME shell, so they do not shield each other well. More pull, almost the same shielding, so Zeff rises. This is why atoms get smaller across a period.
Down a group, each new period adds a full inner shell of electrons. These new inner electrons shield the outer electron strongly. So even though Z (protons) rises a lot, the extra shielding cancels most of it, and Zeff on the outer electron stays roughly similar. Size still grows down a group mainly because a new, farther shell is added.
Higher Zeff means the nucleus pulls the outer electrons in more tightly, so the atom is smaller. Lower Zeff means a looser pull and a bigger atom. That is why across a period (Zeff up) size decreases, and why gallium is even smaller than aluminium — poor shielding by 3d electrons makes Ga feel a higher Zeff (this is a real NEET 2018 answer).
No. Inner electrons feel a Zeff close to the full Z because little screens them. Outer (valence) electrons feel a much smaller Zeff because all the inner shells screen them. When NEET talks about Zeff affecting reactivity or size, it means the Zeff felt by the valence electrons.
The correct order of atomic radii in group 13 elements is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes, for any real atom Zeff is always less than Z, because there is always at least some shielding by electrons. Only a bare nucleus with no electrons would feel the full Z. The more inner electrons there are, the bigger the gap between Z and Zeff.
For most NEET questions you only need the idea Zeff = Z − S and the trends (rises across a period, nearly constant down a group). Full Slater's-rules calculation is rarely asked directly, but knowing that inner electrons shield more than same-shell electrons helps you compare Zeff between elements.
Zeff is the hidden reason behind almost every periodic trend: atomic radius, ionization enthalpy, electron gain enthalpy, and electronegativity. If you understand Zeff, you can reason out these trends instead of memorising them, which helps with tricky NTA anomaly questions like the Ga/Al radius comparison.
A 2s electron feels a slightly stronger Zeff than a 2p electron of the same atom. This is because 2s electrons penetrate closer to the nucleus, so they are shielded less. Higher penetration means less screening and a bigger Zeff, which is why 2s is lower in energy than 2p.
Yes. When an atom loses an electron to form a cation, the number of protons stays the same but there are fewer electrons to share the shielding. So the remaining electrons each feel a higher Zeff, and the cation is smaller than the neutral atom.
