Trend of Electronegativity in periodic table
Electronegativity is a chemical property that describes an atom's ability of attraction on bonded electrons. The concept of electronegativity was developed by Linus Pauling to describe the attraction of electrons by individual atoms. Since electronegativity describes a qualitative property, there is no standardized method of calculating electronegativity. However, the scale that most chemists use in quantifying electronegativity is the Pauling scale, named after the chemist Linus Pauling. The numbers assigned by the Pauling scale are dimensionless due to electronegativity being largely qualitative. Electronegativity is a combination of ionization energy, electron affinity and other factors.
Electro negativities show the same diagonal trend as do ionization energies and electron affinities. Fluorine has the highest electronegativity of 3.98 Pauling units and francium the lowest. The electronegativity concept is used in determining how electrons are distributed in molecules. In a periodic table, the electronegativity of elements shows an increasing trend from the lower left corner to the upper right corner. This is because, the elements on the left side of the periodic table have less than a half–filled valence shell, the energy required to gain electrons is significantly higher compared to the energy required to lose electrons. As a result, the elements on the left side of the periodic table generally lose electrons in forming bonds. Conversely, elements on the right side of the periodic table are more energy–efficient in gaining electrons to create a complete valence shell of 8 electrons. Down the group, electronegativity decreases. This is because the atomic number increases down the group and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.
HCl showing partial positive and partial negative charges.
Dipole moment is towards chlorine because of higher electronegativity. So, the molecule is polar. In
hydrogen there is no difference in electronegativity. So, the molecule is non polar.
Exceptions: Important exceptions include the noble gases, lanthanides, and actinides. The noble gases possess a complete valence shell and do not usually attract electrons. The lanthanides and actinides possess a more complicated chemistry that does not generally follow any trends. Therefore, noble gases, lanthanides, and actinides do not have standard electronegativity values. As for the transition metals, while they have values, there is little variance across the period and up and down a group. This is because of their metallic properties that affect their ability to attract electrons as easily as the other elements.