Lewis Structures & Polarity

What is the first step in drawing a Lewis Structure?

Find the total number of valence electrons. (For ions, add electrons for negative charges, subtract for positive charges).

After counting electrons, what do you do?

Choose the central atom (least electronegative) and connect all other atoms to it with single bonds.

Where do you place the remaining electrons?

On the outer atoms first to satisfy the octet rule (duet for Hydrogen). Any leftovers go on the central atom.

What if the central atom doesn't have an octet after step 3?

Form double or triple bonds by converting lone pairs from outer atoms into bonding pairs.

What does the VSEPR (Valence Shell Electron Pair Repulsion) theory state?

Electron groups (bonds and lone pairs) repel each other and arrange themselves as far apart as possible.

What counts as an "electron group"?

A single, double, or triple bond (each counts as ONE group), or a lone pair.

If an atom has 2 electron groups and 0 lone pairs, what is the molecular shape?

Linear

If an atom has 3 electron groups and 0 lone pairs, what is the molecular shape?

Trigonal Planar

if an atom has 3 electron groups and 1 lone pair, what is the molecular shape?

Bent

If an atom has 4 electron groups and 0 lone pairs, what is the molecular shape?

Tetrahedral

If an atom has 4 electron groups and 1 lone pair, what is the molecular shape?

Trigonal Pyramidal

If an atom has 4 electron groups and 2 lone pairs, what is the molecular shape?

Bent

What makes a bond polar?

A difference in electronegativity between the two bonded atoms.

What two conditions are required for a molecule to be polar?

1. It must have polar bonds. 2. It must have an asymmetrical shape (dipoles don't cancel).

Which molecular shapes are typically symmetrical (and non-polar if all outer atoms are the same)?

Linear, Trigonal Planar, and Tetrahedral.

Which molecular shapes are always asymmetrical (and polar if they have polar bonds)?

Bent and Trigonal Pyramidal.

A molecule has polar bonds and a Bent shape. Is it polar or non-polar?

Polar. (Bent shape is asymmetrical, so dipoles don't cancel).

A molecule has polar bonds and a Tetrahedral shape with identical outer atoms (e.g., CCl₄). Is it polar?

Non-polar. (The symmetrical shape causes the bond dipoles to cancel out).

What two things are needed for a molecule to be polar?

1. Polar bonds AND 2. Asymmetrical shape (dipoles don't cancel).

When is a molecule instantly nonpolar?

When it's made of two identical atoms (e.g., H₂, O₂, N₂, Cl₂).

What symmetrical shapes are nonpolar (if all outer atoms are the same)?

Linear, Trigonal Planar, and Tetrahedral.

What asymmetrical shapes are almost always polar?

Bent and Trigonal Pyramidal shapes.

When is a symmetric shape actually polar?

When the outer atoms are NOT all the same (e.g., CH₃Cl - tetrahedral but polar).

What shape is this and is it polar? O=C=O

Linear - NONPOLAR (bond dipoles cancel).

What shape is this and is it polar? (Flat triangle with identical atoms)

Trigonal Planar - NONPOLAR (bond dipoles cancel).

What shape is this and is it polar? CH₄ or CCl₄

Tetrahedral - NONPOLAR (bond dipoles cancel).

What shape is this and is it polar? H₂O

Bent - POLAR (bond dipoles don't cancel).

What shape is this and is it polar? NH₃

Trigonal Pyramidal - POLAR (bond dipoles don't cancel).

How can you quickly spot a polar molecule?

Look for lone pairs on the central atom (usually makes shape bent or pyramidal).

A molecule has polar bonds and a tetrahedral shape. Is it polar?

Only if the outer atoms are different (e.g., CH₃Cl). If all identical (CCl₄), it's nonpolar.