Chemistry · Periodic Classification Of Properties · NEET
A positive electron and a positive nucleus attract each other. This attraction holds the electron inside the atom. To pull the electron out, you must fight this attraction, and fighting an attraction always needs energy from outside. Because energy is absorbed, the sign is positive. It can never be negative, because the atom will never release energy just to lose an electron on its own.
Ionization is always endothermic. Endothermic means the reaction absorbs energy from the surroundings. The reaction M(g) to M+(g) + e- needs energy input to happen, so it absorbs heat. This is true for every element, even very reactive metals like sodium. Removing the electron never gives out heat.
This confuses many students. A sodium atom losing its electron (ionization) still needs energy. Sodium looks reactive because later steps release a lot of energy, for example when Na+ and Cl- come together to form a crystal lattice. So the FULL reaction gives out energy, but the single ionization step by itself always costs energy.
They are opposite processes. Ionization = pulling an electron AWAY from the pull of the nucleus, so you spend energy (positive). Electron gain = an electron falling INTO the pull of the nucleus, so energy is released (usually negative). One breaks attraction, the other lets attraction do work.
After the first electron leaves, the atom becomes a positive ion (M+). Now you are removing an electron from a positive charge, not a neutral atom. The positive ion holds its electrons more tightly, so more energy is needed. That is why the second ionization enthalpy is always larger than the first, and it is still positive.
Yes. NCERT states this directly. For ionic solids, even if ionization plus electron gain is positive overall, the crystal still forms because a large amount of energy is released when the ions arrange into a lattice (lattice enthalpy). The single ionization step stays positive; it is the extra lattice energy that makes the whole process favourable.
For the second period elements the correct increasing order of first ionisation enthalpy is:
Assertion A: The first ionization enthalpy of O is lower than that of N and F. Reason R: The loss of an electron from O leads to a stable half-filled p orbital. Choose the most appropriate answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Every atom's electron is held by the nucleus, so some energy is always needed to remove it. The value is always greater than zero, meaning always positive.
It is measured in kilojoules per mole (kJ/mol), or sometimes in electron volts (eV) per atom. NEET usually uses kJ/mol.
Noble gases have completely filled, very stable shells. The nucleus holds these electrons very tightly, so a large positive energy is needed to remove one.
It decreases down a group. The outer electron gets farther from the nucleus and is shielded by inner electrons, so it is easier to remove. The value stays positive but becomes smaller.
It generally increases across a period because effective nuclear charge rises and the atom gets smaller, holding electrons tighter. Small dips occur at group 13 and group 16 due to stable configurations.
