how did we know one molecule is symmetric or asymmetris

ou look for a plane or an axis of symmetry.

Many students find it difficult to visualize molecules in three dimensions. It often helps to make simple models from coloured sticks and putty or Styrofoam balls.

Planes of Symmetry

Aplane of symmetryis an imaginary plane that bisects a molecule into halves that are mirror images of each other.This is helpful
This is confusingIn 2-chloropropane, (a), CH₃CHClCH₃, the vertical plane bisects the H atom, the C atom, and the Cl atom.

The CH₃ group (brown) on the right hand side of the mirror is a mirror image of the CH₃ group (brown) left hand side. So are the left and right halves of the bisected atoms.
So the vertical plane is a plane of symmetry, and the molecule issymmetric.

2-Chlorobutane, (b), CH₃CHClC₂H₅, has a plane that bisects the C, H, and Cl atoms into mirror image halves.

But the C₂H₅ group (yellow) on the right hand side is not a mirror image of the CH₃ group (brown) on the left hand side.

So 2-chlorobutane doesnothave a plane of symmetry. It has anasymmetricgeometry.

Axes of SymmetryThis is helpful
This is confusingAnaxis of symmetrycorresponds to a rotation by360°n, wherenis an integer.

A water molecule has one axis and two planes of symmetry. The water molecule issymmetric.
To be symmetric, a molecule must have either an axis of symmetry or an imaginary plane that bisects it into mirror-image halves.This is helpful
This is confusingA molecule that has neither of these isasymmetric. CHFClBr has no axes or planes of symmetry, so it is asymmetric.