Question

In each of the following groups, which element is the most electronegative? Which is the least electronegative? a. \(\mathrm{Cs}, \mathrm{Ba}, \mathrm{At}\) b. Ba , Sr, Ra c. \(\mathrm{O}, \mathrm{Rb}, \mathrm{Mg}\)

Short answer

For the given groups, the most and least electronegative elements are as follows: a. Most electronegative: \(\mathrm{At}\), Least electronegative: \(\mathrm{Cs}\) b. Most electronegative: \(\mathrm{Ba}\), Least electronegative: \(\mathrm{Ra}\) c. Most electronegative: \(\mathrm{O}\), Least electronegative: \(\mathrm{Rb}\)

Step-by-step solution

Group (a) Elements

Cs, Ba, and At are our given elements. Referring to the periodic table, we see that Cs is in Group 1 (alkali metals), Ba is in Group 2 (alkaline earth metals), and At is in Group 17 (halogens). According to the electronegativity trends, At being in the far right of these elements and higher in the periodic table, is the most electronegative, while Cs, which is farthest to the left and lower in the periodic table, is the least electronegative.

Group (b) Elements

In this group, we have Ba, Sr, and Ra. All of these elements belong to Group 2 (alkaline earth metals). Ba is the element that comes first in the periodic table, followed by Sr and then Ra. Electronegativity decreases from top to bottom within a group, so Ba is the most electronegative, and Ra, being the lowest in the group, is the least electronegative.

Group (c) Elements

The given elements for this group are O, Rb, and Mg. Using the periodic table, we can see that O is in Group 16 (chalcogens), Rb in Group 1 (alkali metals), and Mg in Group 2 (alkaline earth metals). Since electronegativity increases across a period from left to right and decreases down a group, O is the most electronegative element as it is furthest to the right and highest in the table. Rb, being farther to the left and lower in the table, is the least electronegative element.

Key concepts

Periodic Table Trends

Understanding periodic table trends is fundamental to grasping the concepts of electronegativity. Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it forms a chemical bond. One key trend is that electronegativity increases as you move from left to right across a period. This is due to the number of charges on the nucleus increasing, which attracts the bonding electrons more strongly.

Another trend is that electronegativity decreases as you go down a group or column. As the atomic number increases, more electron shells are added, which means there is more distance and shielding between the nucleus and the bonding electrons, thus weakening the pull.

These trends are extremely useful for predicting which elements will be more electronegative in a comparison. Remember that the most electronegative elements are found towards the top right of the periodic table, excluding the noble gases, which generally do not form bonds and thus their electronegativity is not discussed in the same context.

Alkali Metals

Alkali metals make up Group 1 of the periodic table and include elements like lithium (Li), sodium (Na), and potassium (K). They are known for being highly reactive, especially with water, and have low electronegativity values.

Since electronegativity decreases as you move down a group, the element at the top of the alkali metals group, lithium, is the most electronegative, while the element at the bottom, francium (Fr), is the least. Despite being highly reactive, alkali metals have only one electron in their outer shell which they readily lose to form positive ions (cations) and achieve a stable electronic configuration.

Halogens

Halogens occupy Group 17 of the periodic table and are one element short of having a full valence shell, which makes them highly electronegative. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

Electronegativity in halogens decreases down the group; thus, fluorine is the most electronegative element in the entire periodic table, while astatine, which is the lowest in the group, has the lowest electronegativity among halogens but is still highly electronegative relative to most other elements. Halogens are reactive and commonly gain an electron to form anions (negative ions) in chemical reactions.

Alkaline Earth Metals

The alkaline earth metals, found in Group 2 of the periodic table, include beryllium (Be), magnesium (Mg), calcium (Ca), and others down the line. Similar to alkali metals, they are not as reactive but still have low electronegativity compared to elements on the right side of the table.

Electronegativity among alkaline earth metals also decreases down the group. Magnesium, for example, is more electronegative than calcium. They characteristically have two electrons in their outer shell and tend to lose them to form divalent cations. While they are less electronegative than nonmetals, they are more electronegative than alkali metals in the corresponding periods.