Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 118

The electronegativity of an element is the power of an atom in a molecule to pull electrons towards itself when the electrons are in a bond. Linus Pauling developed a scale that compares the relative electronegativities of elements with each other. Pauling's scale has values from 0.7 to 4.0. The symbol for electronegativity is the Greek letter chi: χ. The more electronegative an element, the higher the value of χ. Fluorine is the most electronegative element with a value of 4.0; and francium, at the bottom of Group 1, is the least electronegative with a value of 0.7. Note that electronegativities are relative values, which means they don't have units.

Electronegativity varies across the periodic table, as seen in Figure 2.20. Moving across a period, electronegativity increases from Group 1 to Group 7 (17). From left to right across a period (row) of the periodic table, nuclei have an increasing number of protons; therefore, the force of attraction between the nucleus and outer electrons increases. This increased force of attraction means that the outer electrons are pulled in more strongly and the atomic radii decrease; so the nucleus exerts an increasing attractive force on any bonded electrons, and hence the electronegativity, or power of attracting electrons, increases. Atomic radii will be discussed in more detail in Chapter 10.

Figure 2.20 Periodic table showing Pauling electronegativity of most of the elements. You are unlikely to be expected to memorise these values. Source: By Anne Helmenstine. Retrieved from https://sciencenotes.org/electronegativity-definition-and-trend/

On going down a group in the periodic table, the nucleus is becoming increasingly distant from the electrons in the outer shell. Although there is an increase in nuclear charge, the nucleus is screened by more and more layers of filled electron shells, so the outer electrons experience less and less of the nuclear charge. Thus, the elements become less electronegative and more electropositive. The least electronegative element is francium at the bottom of Group 1. Notice that no values are given for the electronegativities of some noble gas elements as they rarely form bonds.