Docsity
Docsity

Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity


Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan


Guidelines and tips
Guidelines and tips

Electron Affinity: Definition, Factors, and Trends, Exercises of Chemistry

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.

Typology: Exercises

2021/2022

Uploaded on 09/12/2022

shanthi_48
shanthi_48 🇺🇸

4.8

(36)

901 documents

1 / 5

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
Electron Affinity
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-(g) + thermal energy
Unit: KJ mol-1
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):
X(g) + e→ X(g)
Second Electron Affinity (positive energy because energy needed is more than gained):
X(g) + e → X2− (g)
Factors affecting E.A
1. Atomic Size: The electron affinity decreases with increase in atomic size and increases with
decrease in atomic size.
E.A Size
2. Eff ective Nuclear Charge : (Zeff)
pf3
pf4
pf5

Partial preview of the text

Download Electron Affinity: Definition, Factors, and Trends and more Exercises Chemistry in PDF only on Docsity!

Electron Affinity

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 - (g) + thermal energy Unit: KJ mol

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):

X(g) + e

→ X

(g)

Second Electron Affinity (positive energy because energy needed is more than gained):

X

(g) + e

→ X

2−

(g)

Factors affecting E.A

  1. Atomic Size: The electron affinity decreases with increase in atomic size and increases with decrease in atomic size. E.A Size
  2. Eff ective Nuclear Charge : (Zeff)

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

  1. Shielding Effect : An increase in shielding effect decrease the decreased E.A. E.A 1/Shielding effect
  2. Stability of half filled and fully filled orbitals : Elements having half filled and fully filled electronic configuration have almost zero or negligible E.A. Periodicity in electron affinity : In Period : In general E.A value increases on moving from left to right in a period effective nuclear charge increases. Atomic size decreases with increase in effective nuclear charge because, higher the effective nuclear charge stronger will be the attraction of the nucleus towards the electron of the outermost An increase in shielding effect decrease the nuclear charge. This result in lly filled orbitals : Elements having half filled and fully filled electronic configuration have almost zero or Periodicity in electron affinity : In general E.A value increases on moving from left to right in a period effective nuclear charge increases. Atomic size decreases with increase in effective nuclear charge because, higher the effective towards the electron of the outermost nuclear charge. This result in Elements having half filled and fully filled electronic configuration have almost zero or In general E.A value increases on moving from left to right in a period because
  1. The electron affinity of inert gas elements is zero due to fully filled configuration.
  2. Electron affinity of second period elements is smaller than the electron affinity of third period elements. This unexpected behaviour can be explained by the very much high value of charge densities of second period elements due to much smaller size. The electron being experiences comparatively more repulsion and the electron affinity value decreases. Important Points :
  3. Elements having highest EA = Cl
  4. Elements having zero EA = noble gases
  5. The periodic trends of IE and EA are same in periodic table i.e both increase across the period both decrease down the group.
  6. IE is an absorption of energy when e. is removed from an atom while EA is an emission of energy when e. is added to an atom. Why is Fluorine an Anomaly? The incoming electron is going to be closer to the nucleus in fluorine than in any other of these elements, so you would expect a high value of electron affinity. However, because fluorine is such a small atom, you are putting the new electron into a region of space already crowded with electrons and there is a significant amount of repulsion. This repulsion lessens the attraction the incoming electron feels and so lessens the electron affinity. A similar reversal of the expected trend happens between oxygen and sulfur in Group 16. The first electron affinity of oxygen (- kJ mol
    • ) is smaller than that of sulfur (-200 kJ mol-1) for exactly the same reason that fluorine's EA is smaller than chlorine's. Second Electron Affinity You are only ever likely to meet this with respect to the group 16 elements oxygen and sulfur which both form -2 ions. The second electron affinity is the energy required to add an electron to each ion in 1 mole of gaseous 1
    • ions to produce 1 mole of gaseous 2 - ions. This is more easily seen in symbol terms. X − (g)+e −→ X − (g) It is the energy needed to carry out this change per mole of X − .

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.