Why is First EA Exothermic?

When an atom acquires an electron to convert into an anion, it is known as the Electron Affinity (EA). In the arrangement of the periodic table, generally, the compounds formed on the left side are metallic in nature and tend to lose electrons easily. On the other hand, non-metals on the right side gain electrons to attain a stable configuration.

Trend in First Electron Affinity

The first electron affinity (EA1) generally increases across a period and decreases down a group. There are several factors which govern the trend of EA1 in the periodic table. They are:

Let us look into each of these in detail:

Atomic Size

As we move across a period from left to right, the atomic size decreases. This means that the valence electrons are held more tightly by the nucleus. Therefore, it takes more energy to remove an electron, and the first electron affinity (EA1) increases.

Nuclear Charge

The nuclear charge is the number of protons in the nucleus. As we move across a period, the nuclear charge increases. This means that the nucleus has a stronger attraction for the electrons, and it takes more energy to remove an electron. Therefore, the first electron affinity (EA1) increases.

Slater’s Rules

Slater's rules are a set of rules that predict the order of energy levels in atoms. According to Slater's rules, the energy of an electron in an orbital depends on three factors:

• The principal quantum number (n)


• The angular momentum quantum number (l)


• The spin quantum number (ms)

Slater's rules predict that the energy of an electron increases as the value of n increases. This means that it takes more energy to remove an electron from an orbital with a higher value of n. Therefore, the first electron affinity (EA1) increases as we move across a period.

Exchange Energy

Exchange energy is the energy associated with the exchange of electrons between orbitals. When two electrons occupy the same orbital, they must have opposite spins. This exchange of electrons leads to a decrease in the overall energy of the atom. Therefore, it is easier to remove an electron from an orbital that is already occupied by another electron. This is known as the Hund's rule. The Hund's rule states that the ground state of an atom is the state with the maximum number of unpaired electrons. Therefore, the first electron affinity (EA1) decreases as we move down a group.

First EA is Exothermic

The First Electron Affinity (EA1) is exothermic for most elements. This means that when an atom acquires an electron to form an anion, the energy of the system is released as heat. First, Electron Affinity is a negative value and it is the enthalpy change accompanying the addition of an electron to an atom in the gas phase, provided that the atom and the anion are in their ground states. Hence a negative EA value means energy released.

The exothermic nature of the first electron affinity can be explained by the fact that the addition of an electron to an atom results in the formation of a more stable electronic configuration. This is because the added electron occupies an empty orbital, which lowers the overall energy of the atom.

Conclusion

The first electron affinity (EA1) is a measure of the energy released or absorbed when an atom acquires an electron. The EA1 is generally exothermic for most elements, which means that energy is released when an electron is added to an atom. The trend of EA1 in the periodic table can be explained by several factors, including atomic size, nuclear charge, Slater's rules, and exchange energy.

FAQs

1. What is the trend of EA1 in the periodic table?
   
   The EA1 generally increases across a period and decreases down a group.
2. What factors affect the EA1 of an element?
   
   The EA1 of an element is affected by atomic size, nuclear charge, Slater’s rules, and exchange energy.
3. Why is EA1 exothermic for most elements?
   
   The EA1 is exothermic for most elements because the addition of an electron to an atom results in the formation of a more stable electronic configuration.
4. What are some exceptions to the exothermic nature of EA1?
   
   There are a few exceptions to the exothermic nature of EA1. For example, the EA1 of helium is endothermic.
5. How can the EA1 of an element be measured?
   
   The EA1 of an element can be measured using a variety of techniques, including photoelectron spectroscopy and thermochemistry.