Ionisation potential Ionisation energy of an element is defined as the amount of energy required to remove the most loosely bound electron from isolated neutral gaseous atom in its lowest energy state. The process is represented as ⎯⎯→M e − I M Energy supplied (g) (g) 1 Ionisation energy is measured in electron volts per atom (eV/atom), kilo calories per mole (kcal/mole) or kilo joules per mole (kJ/mole). Successive ionisation potentials In addition to first ionisation potential (I1) defined above, second, third. etc. ionisation potentials are also known. Second ionisation potential (I2) is the energy required to remove one more electron from the gaseous cation, M+ (g) to get the doubly positively charged gaseous cation, M2+(g), i.e., MI →M e−(g) 2 (g) 2 Similarly, third ionisation potential (I3) is the energy required to remove still one more electron from M2+ (g) cation to get M3+ (g) cation, i.e. M I →M e−(g) 3 (g) 3 2 Similarly ionisation potentials of higher and higher grades are also known. Each successive ionization potential or energy is greater than the previous one, since the electron must be removed against the net positive charge on the ion. 47 Factors governing ionization energy. The ionization energy depends upon the following factors: (a) Size of atom or ion. The ionization energy decreases with the increasing size of atom. The larger the size of atom, lesser is the ionization energy. This is due to the fact that electrons are tightly held in smaller atoms whereas in large atoms, electrons are held quite loose, i.e., lesser energy is required for removal of electrons from larger atoms than the smaller one. Hence ionization energy is lower for larger atoms and higher for smaller atoms. Example 1 The I.E of Be (At. No.4) is greater than that of Li (At. No.3) because the nuclear charge of Be (Z=4) is greater than Li(Z=3). Higher the nuclear charge, greater would be the force of attraction between nucleus and outermost electron. Hence, the first I.E. of Be is than that of Li. Example 2 The I.E. of Be is more than that of B though the nuclear charge of boron atom (Z=5) is greater than that of beryllium atom (Z = 4). This can be explained as follows: Boron atom (Z = 5; 1s2 2s2 2px 1 2py 02pz 0) is having one unpaired electron in the 2p-subshell. Be-atom (Z = 4; 1s2 2s2) is having paired electrons in the 2ssubshell. As the fully filled 2s-subshell in Be-atom is more stable than B-atom due to symmetry, more energy would be needed to remove an electron from Be-atom. Hence, Be has high I. E.
Posted on: Thu, 13 Jun 2013 16:40:24 +0000