Chemistry · Structure Of Atom · NEET
In the Bohr model the electron sits at the lowest level n=1 with energy E1 = -13.6 eV. The electron becomes free (fully removed) only when its energy reaches 0, which happens at n = infinity. So the energy you must ADD is 0 - (-13.6) = +13.6 eV. That added energy is the ionisation energy. In joules it is 2.18x10^-18 J per atom, which NCERT uses directly.
Ionisation energy is always POSITIVE. It is energy you give TO the atom. Do not confuse it with the electron's energy E1 = -13.6 eV, which is negative because the electron is bound. Rule: the electron's orbit energy is negative, but the ionisation energy (the amount you must supply) is the positive magnitude of that energy.
For any single-electron (hydrogen-like) species, IE from the ground state = 13.6 x Z^2 eV, where Z is the nuclear charge. So He+ (Z=2): IE = 13.6 x 4 = 54.4 eV. Li2+ (Z=3): IE = 13.6 x 9 = 122.4 eV. It grows as Z^2 because the electron is held more tightly by a bigger nuclear charge.
Per atom: 13.6 eV x 1.6x10^-19 J/eV = 2.18x10^-18 J. Per mole: multiply by Avogadro's number 6.022x10^23 to get about 1312 kJ/mol. NEET usually gives the joule value 2.18x10^-18 J, so learn that all three numbers describe the SAME ionisation energy of hydrogen.
n=infinity is not a real far orbit you jump to - it just means the electron has left the atom completely and now has zero binding energy. Ionisation is the transition n=1 to n=infinity. Any transition that stops at a smaller n (like n=1 to n=3) is only excitation, not ionisation, and needs LESS energy.
Yes. Ionisation energy is measured from the level the electron is IN. From n=1 it is 13.6 eV. From n=2 it is only 13.6/4 = 3.4 eV, because E2 = -3.4 eV is already closer to zero. The standard ionisation energy of hydrogen (13.6 eV) always means removal from the ground state n=1.
The energy of an electron in the ground state (n=1) for the He+ ion is -x J. Then the energy of an electron in the n=2 state for the Be3+ ion, in J, is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
13.6 eV, which equals 2.18x10^-18 J per atom, or about 1312 kJ/mol. All three are the same quantity in different units.
Because the electron is free at E = 0. Starting at E1 = -13.6 eV, you must add exactly 13.6 eV to reach zero, so IE = -E1 = +13.6 eV.
IE = 13.6 x Z^2 = 13.6 x 4 = 54.4 eV, because He+ has Z=2 and only one electron, making it hydrogen-like.
No. Excitation moves the electron to a higher but still bound level (n=2, 3...). Ionisation removes it completely (n=1 to n=infinity) and needs the full 13.6 eV for hydrogen.
It rises as Z^2. So He+ needs 54.4 eV and Li2+ needs 122.4 eV, because a larger nuclear charge holds the single electron more tightly.
