The legend said:
Homework Statement
By a general rule the electron affinity of elements increases from left to right along the period and decreases down a group. Then why does chlorine have more electron affinity than fluorine even though it is below fluorine in the periodic table?
Homework Equations
none
The Attempt at a Solution
No idea actually, i thought it maybe so because of more electrons in Cl but then that reasoning should hold for the rest of the elements too.which actually doesnt... so i have no idea..
The electron affinity of an element is the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion. A fluorine atom in the gas phase, for example, gives off energy when it gains an electron to form a fluoride ion.
F(g) + e- -----> F-(g) deltaHo = -328.0 kJ/mol
Several patterns can be found in these data.
* Electron affinities generally become smaller as we go down a column of the periodic table for two reasons. First, the electron being added to the atom is placed in larger orbitals, where it spends less time near the nucleus of the atom. Second, the number of electrons on an atom increases as we go down a column, so the force of repulsion between the electron being added and the electrons already present on a neutral atom becomes larger.
* Electron affinity data are complicated by the fact that the repulsion between the electron being added to the atom and the electrons already present on the atom depends on the volume of the atom. Among the nonmetals in Groups VIA and VIIA, this force of repulsion is largest for the very smallest atoms in these columns: oxygen and fluorine. As a result, these elements have a smaller electron affinity than the elements below them in these columns as shown in the figure below. From that point on, however, the electron affinities decrease as we continue down these columns.
- As we go across the table, the electron affinity increases. Therefore, I'm thinking there must be an exception to the electron affinity. It's true that as you go down the periodic table the electron affinity should decrease but I"m thinking about the number of electrons maybe that's why Cl has more electron affinity than F.
There are enough exceptions to the periodic trends in electron affinity that it is worthwhile to consider electron affinity of specific groups in the periodic table.
Halogens (group 7A, F to At) � Most negative EA values, addition of an e- leads to noble gas configuration, very favorable.
Group 5A (N to Bi) � � filled shell discourages addition of an electron, EA values less negative than neighbors (groups 4A & 6A).
Alkaline Earths (group 2A, Be to Ba) � Filled s subshell discourages addition of an electron, EA values nearly zero.
Noble Gases (Group 8A, He to Rn) � Completely filled shell strongly discourages addition of an electron, EA values are positive.
