The electron affinity trend describes the trend across the periodic table and describes how much energy in an atom is released or spent when an electron is added to a neutral atom or the energy change that occurs when an electron is added to a neutral atom.
The electron affinity trend describes how as one follows the periodic table left to right electron affinity increases and how it usually decreases as one moves down a group of elements, top to bottom. While that’s a short description of the electron affinity trend, it would be helpful to dig deeper into the relationship between electron affinity and the periodic table.
A Refresher On Atoms
Atoms are made out of three different parts: protons, neutrons, and electrons. The protons and neutrons are found within the center of the atom, the atom’s nucleus. The nucleus of the atom contains almost all of the atom’s mass, and both the neutrons and protons that make up the atom have essentially the same mass (though the mass of the proton is slightly less).
The protons in the atom are positively charged, and the number of protons found within the nucleus basically defines what element an atom is. The number of protons within an atom is the element’s atomic number. The neutrons within the atom have no charge, hence their neutrality. The neutrons are used as a point of comparison to find the mass of electrons and protons.
Electrons are about 1800 times smaller than either neutrons or protons, and they have a negative charge. The electrons orbit the nucleus of the atom, and they orbit in multiple layers known as shells. The outermost layer of the electron shells is known as the valence shell, and it’s usually the only layer that matters in chemistry. The electrons in the valence shell are known as valence electrons, and they are the electrons most capable of bonding with other atoms to create chemicals bonds and molecules.
Elements that have complete valence shells, like the noble gases, are stable and chemically non-reactive. Elements that have only one electron in their valence shell, like alkali metals, or are missing a single electron in the shell (like halogens) are the most reactive elements. Reactivity and electron affinity are tightly correlated, with the reactivity of an element increasing as the electron affinity increases. In other words, the greater an element’s tendency to gain electrons, the more reactive the element is.
Ions of atoms may have a net positive charge or a net negative charge. Positively charged atoms are called cations while negatively charged ions are called anions. The energy of an atom has can be gained or lost through chemical reactions, so these chemical reactions form either anions or cations. Ionization energies deal with the formation of positive ions while electron affinities deal with the formation of negative ions. It’s important to remember that, so you’ll know that electron affinities deal exclusively with negative ions of atoms and that their use is almost always relegated to the elements found within groups 16 and 17 of the element table.
The electron affinity of an atom depends upon when it is added to the atom. The initial addition of an electron to a neutral atom, the first electron affinity, will always have negative energy. This is because energy is released when an electron is added to a neutral atom. The ion is now negative, and more energy is necessary when an electron is being added to a negative ion. This means that the energy required overwhelms the energy released by the electron attachment process, and so second electron affinity will be positive.
The addition of an electron to a metal element requires energy. This is because metals don’t exert a very strong pull on their valence electrons and are therefore lose electrons in the valence shell rather easily, becoming cations. For this reason, many metals have very low electron affinities.
Electron affinities are given in kj/mol (joules per mole), a measurement of given energy per amount of material. As an example of the fact that metals have low electron affinity, look at the following electron affinity values for the metals found in Group 1 of the periodic table:
Lithium (Li) Electron Affinity: 60 KJ mol-1
Sodium (Na) Electron Affinity: 53 KJ mol-1
Rubidium (Rb) Electron Affinity: 47 KJ mol-1
Cesium (Cs) Electron Affinity: 46 KJ mol-1
Unlike metals, when a nonmetal gains an electron, the amount of energy change is typically negative. This is because nonmetals have enough energy to form negatively charged ions, anions. This means that the electron affinity value of nonmetals is typically negative. Nonmetals have more electron affinity than metals do because of their atomic structure. Nonmetals have more valence electrons, which makes it easier for them to gain electrons and complete a set. The valence shell also tends to be closer to the nucleus than in metals, meaning that it’s more difficult to remove electrons from nonmetals and easier for nonmetals to attract electrons to them. As an example of the higher electron affinity that nonmetals have, look at the electron affinity for the halogens in group 17:
Fluorine (F) Electron Affinity: -328 kJ mol-1
Chlorine (Cl) Electron Affinity: -349 kJ mol-1
Bromine (Br) Electron Affinity: -324 kJ mol-1
Iodine (I) Electron Affinity: -295 kJ mol-1
The Electron Affinity Trend
The Electron Affinity trend, like other trends in the periodic table, reflects the fact that electron affinity follows a predictable trend as one reads the periodic table. In this case, electron affinity increases from top to bottom and left to right. As one moves from the bottom of the periodic table upwards through groups (columns) of elements, electron affinity tends to increase. Electron affinity also tends to increase as one follows the periodic table from left to right across periods (rows) of the table.
The greater the distance between the nucleus and the shells of the electrons, the less attraction there is and the less energy released when an electron is introduced to the outside shell. The more valence electrons an element has, the more likely it is to gain electrons so that a complete octet of electrons will be formed. The opposite trend holds true as well, electron affinity decreases from right to left and down the groups because the electrons are located farther away from the nucleus and therefore have less attraction. The reason that elements lower in groups don’t have higher electron affinities despite their higher number of valence electrons is the shielding effect. The shielding effect increases as one moves down a group, making electrons repel each other more.