Question

A covalent bond is a bond in which electrons are shared between atoms of elements. A covalent bond can be polar or nonpolar. In a nonpolar covalent bond, the bond is between two identical atoms and the electrons are evenly shared between the atoms.

In contrast, in a polar covalent bond, the bond is between two nonidentical atoms and the electrons are unevenly shared between the atoms. The uneven sharing of electrons takes place because of the difference in the electronegativity of the two atoms.

Electronegativity is the ability of an atom of an element to attract electrons toward it. A separation of charges, called a dipole, arises due to the difference in the electronegativity of the two atoms. The electronegativities of the elements are related to where they are placed in the periodic table. The larger the distance between the two atoms in the periodic table, the larger the difference in their electronegativities, and the greater the polarity of the bond between them.

As you go across a period, the electronegativity increases. Observe how the electronegativity increases from sodium, Na, to chlorine, Cl, in the periodic table. As you go down a group, electronegativity decreases. Observe how the electronegativity decreases from lithium, Li, to cesium, Cs, in the periodic table. Fluorine, the most electronegative element, has a value of 3.98, and francium, the least electronegative element, has a value of 0.7.

For example, in hydrogen iodide (H−I), the position of hydrogen is to the left of the periodic table when compared to the position of iodine. That is, the bonding electrons spend more time around the iodine atom. So the iodine side of the bond is slightly negative and the hydrogen side of the bond is slightly positive.

Part A

Part complete

Use the periodic table in the introduction to label the atoms of the following bonds with partial positive (δ+), partial negative (δ-), or no charge (NC), and indicate the dipole by placing the vector (crossed arrow) below each molecule.

Drag the appropriate labels to their respective targets.

If a molecule is diatomic and the atoms are different, they have different electronegativities. This electronegativity difference leads to the formation of dipole and a polar bond.

The electronegativity difference of a bond

The electronegativity difference of a bond can be determined by first finding the electronegativity values of the respective elements in the periodic table given in the introduction. Then, subtract the smaller electronegativity value from the larger one.

For example, in HCl, the electronegativity values of H and Cl are 2.2 and 3.16, respectively. Next, subtract the smaller electronegativity value 2.2 from the larger value 3.16 to obtain the electronegativity difference of HCl, which is 0.96.

Part B

Part complete

Rank the bonds in order of their decreasing electronegativity difference using the periodic table of electronegativity values given in the introduction.

Rank from highest to lowest polarity. To rank items as equivalent, overlap them.

A molecule with an electronegativity difference greater than 1.7 has an ionic bond. An electronegativity difference of a molecule in the range of 0.4–1.7 is an indication of polar covalent bond between the atoms. A molecule with an electronegativity difference less than 0.4 has a nonpolar covalent bond.

The criteria for molecular polarity

A molecule will bepolar if there is an uneven distribution of electrons.

For example, the hydrogen atom of a water molecule has a partial positive charge, and the oxygen atom has a partial negative charge. Since water has a bent shape, the net dipole will be roughly between the two hydrogen atoms pointing towards the oxygen atom, and the molecule is polar.

In carbon dioxide, oxygen also has a partial negative charge. Since the molecule is linear, the dipoles exactly cancel; there is no net dipole on the molecule, and the molecule is nonpolar.

In general, you can look for symmetry to determine the polarity of molecules. Start by drawing each molecule including lone pairs on the central atom. You can think of the lone pair as another group when determining symmetry. For example, ammonia (NH3) has the same tetrahedral geometry as methane (CH4) when bonding and nonbonding electrons are counted.

After drawing the molecule and the dipoles, look for symmetry that will allow the dipoles to cancel. For example, in the carbon tetrachloride (CCl4) molecule, the four dipoles are arranged in a symmetrical tetrahedral manner around the central carbon. The dipoles are all equal and opposite, and therefore cancel each other. Hence, carbon tetrachloride is a nonpolar molecule. However, the four dipoles in the chloroform or trichloromethane (CHCl3) molecule, namely, one C−H and three C−Cl, have different strengths. If dipoles have different strengths, then the dipoles do not cancel each other, and they add up to give a net dipole. Hence, chloroform (CHCl3) is a polar molecule.

Carbon tetrachloride dipoles are equal and opposite; the molecule is nonpolar.		Carbon trichloride dipoles do not cancel; the molecule is polar.

Part C

Part complete

The first step when determining if a molecule is polar is to draw the Lewis structure and assign dipoles. Complete the diagrams showing dipole placement for each bond.

Drag the appropriate labels to their respective targets.

Polarity

The table shows the molecules, their molecular geometry, and the nature of charge distribution on the central atom.

Molecule	Geometry	Charge Distribution	Molecule	Geometry	Charge Distribution
	Bent	Asymmetric		Triangular planar	Symmetric
	Trigonal pyramidal	Asymmetric		Tetrahedral	Asymmetric
	Tetrahedral	Symmetric		Triangular planar	Asymmetric

Part D

Using your answer from Part C and the above table, sort the molecules based on whether they are polar or nonpolar.

Drag the appropriate items to their respective bins.

Answer 1