Chemical Bonding

How to know the shape of the molecule is symmetrical or asymmrtrical ?
To identify whether is polar or non polar molecules

Which spec are you doing?

For example, a molecule of carbon dioxide has two carbon—oxygen bonds that are polar due to the electronegativity difference between the carbon and oxygen atoms.
But it has no net dipole moment because its symmetrical
Dipoles cancel out , why and how ? How to know which one is symmetrical

For example, a molecule of carbon dioxide has two carbon—oxygen bonds that are polar due to the electronegativity difference between the carbon and oxygen atoms.
But it has no net dipole moment because its symmetrical
Dipoles cancel out , why and how ? How to know which one is symmetrical

Determining whether something is symmetrical or not is quite simple, and I assume you can do this

Electronegativity is the ability of an atom to attract electrons in a covalent bond; let’s just call this the ‘pull’ of electrons
If you consider that in a carbon dioxide molecule both C=O bonds are of the same electronegativity, then the ‘pull’ of electrons on one side of the molecule is the same as the ‘pull’ of electrons on the other side of the molecule
Therefore the dipoles cancel out because on both sides of the molecule, there’s no overall ‘pull’ of electrons, and so there’s no net dipole
This is due to the molecule being symmetrical, i.e. quite simply, one side of the molecule is identical to the other side, and thus both sides will counteract each other

If you were to take an asymmetrical molecule, you’d see that it does have a net dipole, for reasons opposite to above
Consider the following simple molecules: methane, chloromethane, dichloromethane, trichloromethane and tetrachloromethane
If you have a quick search on Wikipedia for these molecules, you’ll see that the dipole moments of them change with how symmetrical they are; this is such that methane and tetrachloromethane have no dipole moment, as they are completely symmetrical

Depening on which exam board you do, there are shortcuts.

A level

OCR A/B? Edexcel? AQA? etc.

CIE

According to the examples on your spec, if a molecule has no lone pairs, then it will be symmetrical, the dipoles will cancel out and it will be non-polar.

If it has lone pairs, they won't and it won't.

For clarity, I assume you mean that the central atom has no lone pairs, not the molecule itself?

Good point, that's exactly what I meant - CO2 has no lp on the central C, but the lp on each of the O atoms have no effect on the shape around the C.