Question

which of the following molecules are polar? Why? ch4, ch3br, ch2br2, cbr4

Answer 1

CH4 - non polar

The molecule methane has four Carbon-Hydrogen single covalent bonds. These covalent bonds are called nonpolar covalent bonds because the consequence of this equal sharing of electrons is that there is no charge separation (dipole moment).

In CH3Br, the C-Br bonds are morepolar than the C-H bond as the electronegativity of the Br is greater than the electronegativity of the C and H. Although the bonds are arranged symmetrically around the carbon atom, the lower polarity of the C-H bond means that the bond dipoles do not cancel so the molecule is polar.

the shape of CH2Br2 is tetrahedral when it's saturated (has 4 single bonds).

Polarity depends on the structure -- see below:

....Br.................Br

H--C--Br........H--C--H

.....H.................Br

POLAR...........NOT POLAR

The polar molecule has 2 dipoles (Br) on the same side of the molecule, making the net dipole on the Br side. The NOT polar molecule has 4 polarities evenly distributed, which cancel each other out.

When molecules are polar, they have a net dipole moment. The dipole moment of CBr4is 0 from the atoms cancelling out. In order to determine the dipole moment, one must consider the geometry of the chemical bond of the atoms. Where each molecule is placed determines if there is a net dipole moment. Because the CBr4 molecule has a tetrahedral shape, with the carbon atom in the middle with four bromine atoms attached to the sides, the net charges of the bromine molecules cancel each other out.