Lewis Structures and VSEPR Theory: Electron Domains and Molecular Shapes

What is the first step in drawing a Lewis Structure?

Calculate the total number of valence electrons for the molecule, adding or subtracting for polyatomic ions based on their charge.

What is the second step in drawing a Lewis Structure?

Select the central atom, which is usually the first in the formula and has the lowest subscript; hydrogen is never the central atom.

What is the third step in drawing a Lewis Structure?

Draw the skeletal structure by attaching other atoms to the central atom with dashes and distribute remaining electrons to give each atom an octet, except for hydrogen, boron, and beryllium.

What should you do if there are extra electrons left after drawing a Lewis Structure?

Subtract the used electrons from the total; if extra electrons remain, place a lone pair on the central atom. If the central atom lacks an octet and has no extra electrons, take a lone pair from a terminal atom to form a double bond.

What does VSEPR stand for?

Valence Shell Electron Pair Repulsion.

What is the geometry of a molecule with 2 electron clouds and 2 bonds with no lone pairs?

Linear, with a bond angle of 180 degrees.

What is the geometry of a molecule with 3 electron clouds and 3 bonds with no lone pairs?

Trigonal planar, with a bond angle of 120 degrees.

What is the geometry of a molecule with 3 electron clouds, 2 bonds, and one lone pair?

Bent, with a bond angle of less than 120 degrees.

What is the geometry of a molecule with 4 electron clouds and 4 bonds with no lone pairs?

Tetrahedral, with a bond angle of 109.5 degrees.

What is the geometry of a molecule with 4 electron clouds, 3 bonds, and one lone pair?

Trigonal pyramidal, with a bond angle of less than 109.5 degrees.

What is the geometry of a molecule with 4 electron clouds, 2 bonds, and 2 lone pairs?

Bent, with a bond angle of less than 109.5 degrees.

What is the first step for figuring out the 3D shape of a molecule?

Draw the Lewis structure.

What is the second step for determining the 3D shape of a molecule?

Count the electron domains/clouds around the central atom, including single, double, triple bonds, and lone pairs.

What is the third step for determining the 3D shape of a molecule?

Orient all electron pairs as far away from each other as possible in three-dimensional space.

What is the fourth step for determining the 3D shape of a molecule?

Look only at the arrangement of atoms to name the shape.