An atom can attract shared electrons in a covalent bond. It is seen as the higher the electronegativity value, the more strongly the elements attract the shared electron. Electronegativity predictably varies in the periodic table. It increases from bottom to top in groups and increases from left to right across periods. Hence, fluorine becomes the most electronegative element, and francium becomes one of the least electronegative element.
Talking about the electronegative trends, they are not so smooth among the transition and inner transition metals. But the electronegativity trend is fairly regular for main group elements.
It is defined as when atoms with an electronegativity difference of greater than 2 units are joined together, the bond between them is named as an ionic bond. Here, the less electronegative elements have a positive charge and the more electronegative element has a negative charge.
For example, We will include Sodium and Chlorine for further understandability of ionic bonds.
Sodium has an electronegativity of .93 whereas Chlorine has an electronegativity of 3.16. When Sodium and Chlorine form an ionic bond, where Chlorine takes away the electron from Sodium cation Na+ and Chloride anion Cl-. Chloride and Sodium ions attract each other very strongly due to the opposite charges, therefore form a crystal lattice.
When atoms with an electronegativity difference of fewer than two units are joined, the bond is said to be a covalent bond. Here, electrons are shared by both atoms. When the same atom shares electrons in a covalent bond, this results in no electronegativity difference between them. Hence, there is a symmetrical distribution of electrons between bonded atoms. These bonds are nonpolar covalent bonds. For this let us assume as an example of when two chlorine atoms are joined together, the electron spends as much time close to one chlorine as it spends with another. This results in a nonpolar molecule.
When the electronegativity difference is between 0 and 2, the more electronegative element attracts the shared more strongly. They are not strong enough to remove the electrons completely to form an ionic compound. The electrons are shared in an unsymmetrical distribution of electrons between the bonded atoms. These bonds are called polar covalent bonds. There are two things included further in a covalent bond. These are discussed further:
Partial Negative Charge, D- : The more electronegative atom has a partial negative charge. It is because the electrons spend time closer to the atom.
Partial Positive Charge, D+ : The less electronegative atom has a partial positive charge. It is because the electrons are not completely pulled away from the atom.
Let us take an example of a hydrogen chloride molecule to understand it even further. In the hydrogen chloride molecule, chlorine is electronegative than hydrogen by 0.96 units. Hence, the shared electron spends more time close to the chlorine atom making the chlorine molecule slightly negative, whereas the hydrogen end of the molecule is slightly positive. This results in a polar molecule.
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The distance of electrons from the nucleus remains constant in a periodic table row but talking about a periodic table column. The distance is not relatively constant. Coulomb’s law is the force decided between charges. It is given as below:
F = k ( Q1Q1) / x2
In this expression, Q represents a charge, k represents constant and r is the distance between the charges. The statement concluded from the equation is that the distance between the charges increases, the force decrease. This is also called quadratic change. This change results in an increase in electronegativity from bottom to top in a column in the periodic table. Elements at the topmost of the column have higher electronegativities than elements at the bottom of a given column.
The overall electromagnetic trend in the periodic table is diagonal from the lower-left corner to the upper right corner. Since there are exceptions in every concept, here as well we find this trend not applicable to all elements. We need to memorize some of the elements and their trend. The sequence is followed by F, O, Cl, N, Br, I, S, C, H, and lastly metals.
The positively charged protons attract the negatively charged electrons. As the number of protons increases, the electronegativity will increase. Looking at this explanation, the electronegativity increase from left to right in a row in the periodic table.
As we go down a group, electronegativity decreases. The attraction that a pair feels for a particular nucleus depends upon the following factors:
Number of protons in the nucleus
Amount of screening by inner electrons
Distance from the nucleus
1. What do we mean by a diagonal relationship concerning the electronegativity trend?
To explain the answer, we will take the example of beryllium and boron. Electronegativity falls as we go down the periodic table. The electronegativity of boron and aluminium are 2.0 and 1.5 respectively and the electronegativity of beryllium and boron is 1.5 and 2.0 respectively. The increase from group 2 to group 3 is formed by the fall as we go down Group 3 from Boron to aluminium.
Similar electronegativities between the diagonal pairs mean that they are likely to form similar types of bonds.
2. Why does electronegativity fall as we go down a Group?
The electronegativity decreases because the bonding pair of electrons is increasingly at a distance from the attractive core or nucleus. Let us consider the example of hydrogen fluoride and hydrogen chloride molecules. Fluorine has the bonding pair in the 2 levels rather than the 3 levels. As it is closer to the nucleus, the attraction force is greater. The bonding pair is shielded from fluorine’s nucleus by 1s2 electron. In chlorine, it is shielded by all the 1s2 2s2 2p6 electrons. In both the case, there is a pull of fluorine and chlorine of +7.