Determination of electron affinity energy

For atoms, the energy released when a neutral atom acquires an electron and becomes a negative ion is the affinity energy of the atom.

Definition: expressed by reaction formula:

Variation law of affinity energy of the first electron

The first electron affinity energy of internal elements decreases with the increase of atomic number (that is, less and less energy is released when negative ions are formed). Because fluorine does not conform to this law, we will explain it separately in the following example.

Electron affinity energy is an index to measure the attraction between nucleus and foreign electrons. The stronger the attraction between the nucleus and foreign electrons, the more energy is released.

What factors can affect the attraction between nucleus and foreign electrons? These factors are the same as those affecting ionization energy-atomic charge, the distance between electron and nucleus, and shielding.

With the increase of the atomic number of elements in the group, the nuclear charge of elements increases, but their electrons also increase, and the shielding produced by the increased electrons will offset the increase of nuclear charge. No matter which group 7 element it is, its outer electrons feel the attraction of 7+ charge from the nucleus.

For example, the electronic configuration of fluorine (f) is/kloc-0 1s2s2px2py2pz. Its nucleus has nine protons.

The foreign electrons enter the second energy level and are shielded by two 1 electrons. The net attractive force felt by foreign electrons is 7+ charge (9 protons minus 2 shielded electrons).

In contrast, the electronic configuration of chlorine (Cl) is/kloc-0 1s2s2p3s3px3py3pz. Its nucleus has 17 protons.

However, foreign electrons still feel the net attraction of 7+ charge (the first and second levels 17 proton minus 10 electron shielding).

In this way, the distance between the nucleus and foreign electrons becomes the decisive factor. With the increase of the number of atoms in the group, the distance between the nucleus and the foreign electrons is getting farther and farther (the atomic radius is getting bigger and bigger), the attraction between them is getting smaller and smaller, and the energy released (electron affinity energy) is getting less and less.

The atomic radius of (1) main group elements decreases gradually from left to right in the same period, so the electron affinity energy increases gradually.

(2) In the same main group element, the atomic radius gradually increases from top to bottom, so the electron affinity energy gradually decreases.

(3) There are few data of electron affinity energy of atoms of sub-group elements, and the change law is not obvious. Electron affinity energy is related to atomic radius and electronic configuration.

The size of electron affinity energy indicates the difficulty of atom binding electrons, and the ability of atom binding electrons indicates the strength of nonmetallic elements. Generally speaking, the electron affinity energy of atoms (the greater the negative value), the stronger the nonmetallic nature of elements. But we can't judge the nonmetal of elements only by electron affinity. In addition, due to the lack of electron affinity data, the reliability is poor, and it is generally not used alone.