In this explainer, we will learn how to define electron affinity and describe and explain trends throughout the periodic table.
If we have an atom, we can measure the energy required to remove each electron one by one until we have a bare nucleus. We call the process of removing these electrons ionization. Here are the first and second ionizations of a helium atom.
Electron affinities relate to the reverse process: adding electrons successively to an atom and its cations. Atoms of different elements often bind together differently, so electron affinities are always considered with respect to separated atoms (in the gas state). This way, we are not comparing atoms of hydrogen in molecules and atoms of lithium in solid lithium, allowing us to make more meaningful comparisons.
When a hydrogen atom is close enough to an electron, they will attract each other a little and they can stick together. The energy released in this process is known as the electron affinity of hydrogen:
Definition: Electron Affinity of an Atom
The electron affinity of an atom is the energy released when an electron is added to a neutral atom in the gas state to form a negative ion, per mole of atoms.
Electron affinities are usually expressed in kilojoules per mole (kJ/mol). We also use the word electron affinity to refer to the overall process of adding electrons to atoms or ions.
Example 1: Identifying the Equation That Shows the First Electron Affinity of an Atom
Which of the following equations correctly shows the first electron affinity of an atom?
The first electron affinity of an atom is technically the energy released when an atom accepts an electron ( per mole of atoms). However, we often consider the process overall as being “the electron affinity.”
Electron affinities are defined with the atom in the gas state; since elements have very different bonding characteristics, it is easier to compare them this way. The electron is introduced, and a gaseous ion is produced. The energy given out is the electron affinity of the atom (this can have a negative value too).
This is the process we have just described:
This corresponds with answer E.
Because of the way electron affinity is defined, we need to be careful to not equate electron affinities with enthalpy changes. The electron affinity of hydrogen is not the same as the enthalpy change when we add an electron to a hydrogen atom. The electron affinity is the release of the energy, the amount of energy coming out, while the enthalpy change is the change in energy of the system.
If the electron affinity, , is positive, the enthalpy change for the same process is negative, which corresponds to an exothermic process.
Meanwhile, a negative electron affinity indicates that the enthalpy change for the process is positive. This corresponds to an endothermic process.
If an element has a positive electron affinity, the ion of the element is more stable than the atom of the element and a separate electron.
If an element has a negative electron affinity, the ion of the element is less stable than the atom of the element and a separate electron:
|If is ,
|energy is ,
|and enthalpy change is ,
|and the process is .
The electron affinity of hydrogen is about 73 kilojoules per mole:
This means that if we have one mole of hydrogen atoms in the gas phase and we add an electron to each of them, we will convert 73 kilojoules of chemical potential energy to other forms of energy like heat. So, for each mole of hydrogen atoms we turn into ions, we will get 73 kilojoules of energy out to the surroundings:
What this means is that an ion is more stable than a hydrogen atom and a free electron that are completely separated from each other.
This is not the case for atoms of all elements. When we try to add an electron to helium, the repulsion from the electrons is greater than the attraction from the nucleus:
This means that adding an electron to a helium atom requires the input of energy. It is nearly impossible to measure this directly since the ion is not stable, but we can do some calculations.
If we could add an electron to a helium atom, it would take about 48 kilojoules per mole of helium atoms. This means that the electron affinity of helium is about kJ/mol.
It is clear so far that some elements have positive first electron affinities and some elements have negative first electron affinities. For some elements, adding an electron to their gaseous atoms is an exothermic process, and for others, it is an endothermic process.
If we look at the first electron affinities of all the elements on the periodic table below, we can see a few interesting trends.
Empty white squares correspond to elements whose first electron affinities have yet to be determined or predicted.
There is no consistent global trend, but we do see a few patterns here and there:
- All the noble gases have negative first electron affinities.
- Overall, first electron affinities become more positive moving left to right and bottom to top.
- Overall, first electron affinities increase across a single period, moving left to right.
- For a few of the groups, we can see an increase in first electron affinities as we go from the bottom to the top, with maybe one exception per group; this applies to groups 1, 14, 16, and 17.
- Nitrogen atoms have a half-ﬁlled 2p sublevel of three electrons. The half-ﬁlled orbitals give the atom some extra stability, resulting in a small electron affinity value close to zero:
- When an electron is forced to occupy a new shell by the Pauli principle, as with noble gas atoms, electron affinities are typically small and may be negative as stability is lost. This explains the negative value of noble gases such as neon with a full 2p shell and the group 2 metal beryllium, which has a full 2s shell:
As we go down a group on the periodic table, atoms of the elements get bigger, as they have more occupied electron shells. As the size of the atoms increases, the attraction to an extra electron will decrease. If we try to add an electron to a small atom, it will be able to get closer to the nucleus, while for atoms with more electrons, the electron will not be able to get as close.
Fluorine has an unusually high electron affinity at 328 kJ/mol, but it is even higher for chlorine at 349 kJ/mol. As the fluorine atom is smaller in size, it should have a higher electron affinity than chlorine. However, the smaller value for the fluorine atom is due to its atom’s small size, and any arriving electrons will be strongly repelled by the high electron density around the nucleus. Meanwhile, the electron affinities of bromine, iodine, and the remaining group 17 elements continue as expected, following the downward trend. While it is hard to know exactly why this happens, we can come up with a decent theory.
A fluorine atom has a tiny atomic radius of only 42 pm (); chlorine’s is almost double that, at 79 pm, while those of bromine and iodine increase incrementally.
A fluorine atom is even smaller than a hydrogen atom that has an atomic radius of 53 pm. We can imagine that the negative charge of the 9 electrons in a fluorine atom will be very, very dense. This is going to reduce the electron affinity of fluorine relative to chlorine because the incoming electron is experiencing significantly greater repulsion.
Next is the trend from left to right across a period. Let’s look at period two.
As we go left to right, the atomic number increases. Since the number of electron shells in atoms of these elements is not increasing and the nuclear charge is increasing, the atomic radius goes down.
In an atom of lithium, the nucleus has a charge of , so we have three electrons in the electron cloud. In total, these electrons have a charge. Meanwhile, a fluorine atom has a nucleus and nine electrons.
If we imagine an extra electron being introduced next to a lithium or a fluorine atom, the electron approaching the fluorine atom can get much closer to the nucleus before being repelled by the electrons.
This is a basic explanation of why, in general, we see an increase in electron affinity going left to right across a period of the periodic table.
There is such a thing as a second electron affinity, where the electron is added to a one minus ion instead of a neutral atom. Second electron affinities are associated with this process:
Here, we are introducing an electron (which is negatively charged) to a negative ion. The negative ion will repel the electron, raising the energy required to add them together. Because of this extra repulsion compared with the first electron affinity process, second electron affinities are always negative (they are endothermic).
Example 2: Identifying the Sign of the Energy Change for a Second Electron Affinity
shows the second electron affinity of an element.
Will this process result in a positive or a negative change in energy?
The second electron affinity of an element is the energy released when a ion of that element gains another electron.
The equation in the question shows the overall process involved.
For many of the elements, the equivalent process with a neutral atom releases energy. These first electron affinities are positive. However, for some elements, they are negative.
The process for second electron affinities starts with a negative ion, . The repulsion between an atom and an electron will always be less than the repulsion between a negative ion and an electron. This tips the scales such that all second electron affinities are negative (energy is required to force an electron onto a ion).
However, the question is not asking about the energy released (which would be negative), but the change in energy.
In circumstances like this, “change in energy” means “change in energy of the system.” Since energy is required to push the electron onto the ion, energy must be added to the system. This is an endothermic process, with a positive change in the energy of the system.
The answer is “Positive.”
We can see this with oxygen. The first electron affinity of oxygen is 141 kJ/mol. The enthalpy change for this process is negative (it is an exothermic process), but the second electron affinity of oxygen is predicted to be about kJ/mol. So, the enthalpy change for this process is positive (it is strongly endothermic).
However, ions are formed regularly, for instance, when metals react with oxygen. The energy cost is paid for when ions of different charges come together. The energetics of reactions are often complex, and electron affinities will form a small part of the overall process.
Example 3: Calculating the Energy Change for the Addition of 3 Electrons to a Phosphorus Atom
The estimated electron affinities for successive additions of electrons in an atom of phosphorus are listed below:
Consider the reaction
What is the total energy change for the formation of the ion?
There are a few ways of approaching this question. The key point is that the question is asking for the “total energy change”: this is the change in energy of the system when 3 electrons are added to a phosphorus atom, forming . An energy change will be for the system we are describing, not the surroundings. If energy comes in (an endothermic process), the energy of the system will increase (positive ), but if energy goes out (an exothermic process), the energy of the system will decrease (negative ).
What we have are the first, second, and third electron affinities of phosphorus; electron affinities are labelled .
Electron affinities are the energies released when electrons are added to an atom or ion, so they have the opposite sign to energy change. In other words, a positive electron affinity means energy is released, which is exothermic, which means is negative.
Using this, we can turn each into a , and then sum the for each electron addition. Or we can sum the electron affinities, and then turn the result into a . Either is valid.
To solve this, we are going with the second option.
The energy released when three electrons are added to a phosphorus atom is the same as the sum of the energies released when those three electrons are added in sequence ():
This means that adding three electrons to a phosphorus atom is an overall endothermic process (energy is required, since the energy released is negative).
The total energy change is simply the opposite of the energy released:
- The electron affinity of an element is the energy released when an electron is added to a neutral atom of that element in the gas state, to form a singly negative ion (it is also called the first electron affinity).
- A positive electron affinity indicates that the process releases energy (it is exothermic), lowering the energy of the system.
- A negative electron affinity indicates that the process absorbs energy (it is endothermic), increasing the energy of the system.
- Electron affinities are normally expressed in kJ/mol.
- “Electron affinity” is often used to refer to the process itself (adding an electron to a gaseous atom):
- Second electron affinities are the energy released by this process:
- On the periodic table, broadly speaking, we see increases in the first
electron affinities of the elements
- when moving left to right and bottom to top,
- within a group, moving bottom to top,
- across a period, moving left to right.