Physics · Atoms · NEET
The ionisation energy is +13.6 eV. The number -13.6 eV is the total ENERGY of the electron in the ground state (it is negative because the electron is bound to the nucleus). Ionisation energy is how much energy you must ADD to reach E = 0, so it is 0 - (-13.6) = +13.6 eV. Rule: energy of a level is negative, but ionisation energy is always positive.
They describe the same event but use different units. Ionisation ENERGY is measured in electron-volts (eV) or joules: for hydrogen it is 13.6 eV. Ionisation POTENTIAL is measured in volts (V): for hydrogen it is 13.6 V. The link is E = eV, so 13.6 eV of energy corresponds to a potential of 13.6 V. The numbers match in these units; only the label changes.
Use E(ionise) = 0 - E(n), where E(n) = -13.6/n^2 eV. From the first excited state (n = 2): 0 - (-13.6/4) = 3.4 eV. From the second excited state (n = 3): 0 - (-13.6/9) = 1.51 eV. The electron is already partly up the ladder, so less energy is needed than the full 13.6 eV. Only from the ground state (n = 1) is the answer 13.6 eV.
A bound electron sits in an energy well below zero, so E(n) is negative. To free the electron you must supply energy to lift it up to E = 0 (the free level). Supplying energy is a positive quantity. So even though the level is at -13.6 eV, the energy you give the atom is +13.6 eV.
For the ground state, yes in size. Binding energy is how tightly the electron is held = +13.6 eV. Ionisation energy is the energy needed to remove it = +13.6 eV. They are equal for the ground state of hydrogen. In general, the ionisation energy from level n equals the binding energy of that level, both = 13.6/n^2 eV.
The ground state energy of hydrogen atom is -13.6 eV. The energy needed to ionize hydrogen atom from its second excited state will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
13.6 eV = 13.6 x 1.6 x 10^-19 J = 2.18 x 10^-18 J. Multiply eV by the electron charge 1.6 x 10^-19 to convert to joules.
13.6 volts. Since energy = charge x potential (E = eV), an ionisation energy of 13.6 eV gives an ionisation potential of 13.6 V.
For hydrogen-like ions, ionisation energy from the ground state = 13.6 x Z^2 eV. For He+, Z = 2, so it is 13.6 x 4 = 54.4 eV.
Yes. Ionisation energy from level n = 13.6/n^2 eV. It is largest (13.6 eV) from the ground state and gets smaller for higher orbits (3.4 eV from n=2, 1.51 eV from n=3), because the electron is already closer to being free.
Zero energy is taken as a free, motionless electron far from the nucleus. A bound electron has lower energy than this, so its total energy is negative (-13.6/n^2 eV). The negative sign shows the electron is trapped and needs energy to escape.
