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The concept of electron affinity, a chemical property that measures the energy change when an electron is added to a neutral atom to form a negative ion. It discusses the factors affecting electron affinity, including atomic size, effective nuclear charge, shielding effect, and stability of half-filled and fully filled orbitals. The document also covers periodic trends in electron affinity and provides exceptions and important points.
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Definition: Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion. In other words, the neutral atom's likelihood of gaining an electron. A(g) + e
A chemical reaction that releases energy is called an exothermic reaction and a chemical reaction that absorbs energy is called an endothermic reaction. Energy from an exothermic reaction is negative, thus energy is given a negative sign; whereas, energy from an endothermic reaction is positive and energy is given a positive sign. When an electron is added to a neutral atom (i.e., first electron affinity) energy is released; thus, the first electron affinities are negative. However, more energy is required to add an electron to a negative ion (i.e., second electron affinity) which overwhelms any the release of energy from the electron attachment process and hence, second electron affinities are positive. The higher an element’s electron affinity, the more thermal energy is given off when an electron is added to an atom of the element. First Electron Affinity (negative energy because energy released):
−
−
Second Electron Affinity (positive energy because energy needed is more than gained):
−
−
2−
Factors affecting E.A
Atomic size decreases with increase in effective nuclear charge because, higher the effective nuclear charge stronger will be the attraction of the nucleus orbit and higher will be the E.A. E.A Zeff
Why is energy needed to do this? You are forcing an electron into an already negative ion. It's not going to go in willingly! O(g) + e −→ O − (g) 1st EA = -142 kJ mol − O − (g) + e −→ O 2− (g) 2nd EA = +844 kJ mol − The positive sign shows that you have to put in energy to perform this change. The second electron affinity of oxygen is particularly high because the electron is being forced into a small, very electron-dense space. Lanthanide contraction: The decrease in size (as of radii of atoms or ions or of atomic volumes) with increasing atomic number of the metals of the lanthanide series is termed as Lanthanide contraction. Each succeeding lanthanides differs from its immediate predecessor in having one or more electron in the 4f (though there are some exceptions) and an extra proton in the nucleus of the atom. The 4f electrons constitute inner shells and are rather ineffective in screening the nucleus. This leads to a gradual increase in the attraction of the nucleus to the electrons in the outermost shell as the nuclear charge increases, and a consequent contraction in the atomic radius. In lanthanide contraction, the ionic radii of the lanthanides decrease progressively with increase atomic number from lanthanum (La
= 0.115nm) to Lutenum (Lu
= 0.093nm) (see Table 2). As the ionic radii contract along the lanthanide series, the ability to form complex ions increases and this is the basis of their separation on an ion exchange column.