Topic 2 – Lewis Structures, Formal Charge & VSEPR

30 Question-and-Answer flashcards covering valence electrons, Lewis structure steps, formal charges, electronegativity, significant figures, VSEPR geometries, shapes and example molecules.

What are valence electrons?

The outer-most electrons of an atom that participate in chemical bonding.

Only the valence (outer-shell) electrons.

Which electrons are shown in a Lewis structure?

1) Count total valence electrons. 2) Build the bonding framework with single bonds (least electronegative atom in the centre). 3) Place three lone pairs on each outer atom except H. 4) Assign any remaining electrons to the central atom. 5) Minimise formal charges.

List the five steps for drawing a Lewis structure.

Formal charge = valence electrons of free atom − lone-pair electrons − ½(bonding electrons).

What is the formula for formal charge?

Because the arrangement with the smallest (or zero) formal charges is generally the most stable and representative of the real molecule.

Why do chemists minimise formal charges in Lewis structures?

The least electronegative atom (never hydrogen).

In a Lewis structure, which atom is usually placed at the centre?

All atoms have a formal charge of 0 (C=O double bond present).

After minimising formal charges, what are the charges on C, O and Cl in COCl₂?

24 valence electrons (B = 3, 3×Cl = 21).

How many valence electrons must be distributed in BCl₃?

Boron is electron-deficient; an octet is favourable but not required for elements in period 2 with fewer than four valence electrons.

Why can boron in BCl₃ have only six electrons?

A numerical measure of an atom’s ability to attract shared bonding electrons.

Define electronegativity.

It increases left-to-right across a period and decreases down a group.

What is the general electronegativity trend in the periodic table?

Lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.

State the VSEPR repulsion order.

Geometry considers all electron sets (bonds + lone pairs); shape describes only the positions of the atoms, ignoring lone pairs.

Differentiate between molecular geometry and molecular shape.

Four sets of electron pairs (AX₄, AX₃E, AX₂E₂).

How many electron sets produce a tetrahedral geometry?

Tetrahedral (no lone pairs), trigonal pyramidal (1 lone pair), bent (2 lone pairs).

Name the possible shapes for a tetrahedral geometry.

Trigonal bipyramidal electron geometry; seesaw molecular shape (one lone pair).

What geometry and shape does SF₄ have?

Octahedral geometry; square pyramidal shape (one lone pair).

What geometry and shape does BrF₅ possess?

Three electron sets; shapes are trigonal planar (0 lone pairs) or bent (1 lone pair).

How many electron sets and possible shapes arise from trigonal planar geometry?

16 valence electrons (3 × 5 + 1 extra for the negative charge).

How many total valence electrons are in the azide ion N₃⁻?

Trigonal pyramidal with ~107° bond angles.

What VSEPR shape is predicted for NH₃?

To prevent rounding errors so the final answer has the correct significant figures.

Why are extra digits kept during intermediate calculations?

An amount of substance containing 6.022 × 10²³ entities (Avogadro’s number).

What is a mole?

A mole of elephants—mass depends on the individual entity’s mass even though the number of entities is the same.

Which weighs more: a mole of chickens or a mole of elephants?

The student counted bonds instead of electron sets; four electron sets give tetrahedral geometry and shape.

What mistake is made when PO₄³⁻ is labelled trigonal bipyramidal?

As lone pairs replace bonding pairs, greater repulsion compresses the bond angle.

Explain the bond-angle trend CH₄ (109.5°), NH₃ (≈107°), H₂O (≈104.5°).

Linear.

What shape results from five electron sets with three lone pairs (AX₂E₃)?

Square planar.

What shape results from six electron sets with two lone pairs in trans positions (AX₄E₂)?

In equatorial positions to minimise 90° repulsions.

Where do lone pairs prefer to sit in a trigonal bipyramidal geometry?

As ONE electron set because the electron density is localised between the same two atoms.

In VSEPR, how is a double or triple bond counted?

Shape controls properties such as polarity, reactivity and biological function (e.g., bent water vs hypothetical linear H-O-H).

Why is molecular shape important in chemistry?