Electronegativity Trend | What Is Electronegativity Trend?
Electronegativity Trend, symbol χ, is a chemical property that describes the tendency of an atom to attract a shared pair of electrons (electron density) towards itself. An atom’s electronegativity is affected by both its atomic number and the distance at which its valence electrons reside from the charged nucleus. The higher the associated electronegativity number, the more an atom or a substituent group attracts electrons towards itself.
On the most basic level, electronegativity is determined by factors like the nuclear charge (the more protons an atom has, the more “pull” it will have on electrons) and the number/location of other electrons present in the atomic shells (the more electrons an atom has, the farther from the nucleus the valence electrons will be, and as a result the less positive charge they will experience—both because of their increased distance from the nucleus, and because the other electrons in the lower energy core orbitals will act to shield the valence electrons from the positively charged nucleus).
The term “electronegativity” was introduced by Jöns Jacob Berzelius in 1811, though the concept was known even before that and was studied by many chemists including Avogadro. In spite of its long history, an accurate scale of electronegativity was not developed until 1932, when Linus Pauling proposed an electronegativity scale, which depends on bond energies, as a development of valence bond theory. It has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed, and although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements.
Trend Periodic Table
Just to review, groups on the periodic table are just columns. Let’s compare atoms in Group 2: the alkaline earth metals. As you move down from beryllium to radium, what happens to the size of an atom? Well, the atomic radius increases as you move down a group because of the increase in the number and size of the energy levels, so the valence electrons in each atom are getting farther and farther away from the nucleus. The nucleus has quite an effect on pulling those negative electrons in with its positive charge.
So, which atoms do you think will more easily pull in electrons? Atoms that are tiny and have valence shells close to the nucleus or atoms that are large and have valence shells a great distance from the nucleus? Well, the closer to the nucleus electrons can get, the more pull that nucleus will have, so the smaller atoms are going to pull in electrons a lot more easily than the larger atoms.
- From left to right across a period of elements, electronegativity increases. If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.
- From top to bottom down a group, electronegativity decreases. This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.
- Important exceptions of the above rules 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 more complicated chemistry that does not generally follow any trends. Therefore, noble gases, lanthanides, and actinides do not have electronegativity values.
- As for the transition metals, although they have electronegativity values, there is little variance among them across the period and up and down a group. This is because their metallic properties affect their ability to attract electrons as easily as the other elements.
Specific Examples of Electronegativity
Strontium – Strontium is an alkaline earth metal with atomic number 38 and symbol Sr. It is found in Group 2 on the periodic table. Strontium was frequently used to made glass for cathode ray tube television, though as CRTs fall out of favor use of strontium is declining. It burns red when added to fireworks. Strontium has an electronegativity value of 0.95.
Beryllium – Beryllium is a fairly rare element which occurs when cosmic rays collide with atomic nuclei. It has the atomic number 4 and its symbol is Be. Beryllium is also part of group 2 on the periodic table and as it is higher up the chart than Strontium it has an electronegativity value of 1.57. Beryllium is used to make stable but lightweight structural components for aircraft and satellites.
Cobalt – Cobalt is a transition metal found in group 9 of the periodic table. Its atomic number is 27 and its symbol is Co. Cobalt is frequently used in the construction of lithium-ion batteries as well as a pigment for its striking blue color. It has an electronegativity value of 1.88.
Ionization Energy Trends
Ionization energy is the energy required to remove an electron from a neutral atom in its gaseous phase. Conceptually, ionization energy is the opposite of electronegativity. The lower this energy is, the more readily the atom becomes a cation.
Therefore, the higher this energy is, the more unlikely it is the atom becomes a cation. Generally, elements on the right side of the periodic table have a higher ionization energy because their valence shell is nearly filled. Elements on the left side of the periodic table have low ionization energies because of their willingness to lose electrons and become cations. Thus, ionization energy increases from left to right on the periodic table.