GENERAL CHEMISTRY · MOLECULAR STRUCTURE

Count the electrons.
Then let shape decide.

Connect Lewis-dot Diagrams, Bond Types, and Molecular Geometry through one repeatable sequence, then retrieve all nine study objectives with twelve randomized five-choice questions.

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3guided lessons

12practice questions

9study objectives

5choices per item

A dependable molecular routine

Budget. Connect. Shape. Sum.

01Budget
Count valence electrons and apply ionic charge before drawing.
02Connect
Check bonds, lone pairs, octets, and formal-charge totals.
03Shape
Count each bonded atom and lone pair as one VSEPR region.
04Sum
Add bond dipoles as vectors instead of assuming polar bonds mean a polar molecule.

Three prerequisite-linked lessons

The ledger comes before the label.

Each lesson carries one invariant forward: electron totals constrain the graph, the graph constrains shape, and shape constrains polarity.

LESSON 1 · 18 MIN

Study + retrieve

Lewis structures as an electron ledger

Build valence-electron budgets, audit bonds and lone pairs, assign formal charges, and recognize resonance without moving atoms.

ESSENTIAL QUESTIONDoes the proposed structure preserve both the electron budget and the atomic framework?

Lewis electron and charge ledger

Electron-budget checks for representative species
Species	Neutral-atom sum	Charge adjustment	Total
NH₄⁺	5 + 4(1) = 9	Subtract 1	8 e⁻
NO₃⁻	5 + 3(6) = 23	Add 1	24 e⁻
SO₄²⁻	6 + 4(6) = 30	Add 2	32 e⁻

[ O=N(–O⁻)–O⁻ ]↔[ O⁻–N(–O⁻)=O ]

Graph invariantSame atoms · same connectivity · same charge · different electron placement

STUDY MODEL · TEXT EQUIVALENT INCLUDED

Budget before drawing

Add every atom’s valence electrons, add electrons for negative charge, and subtract electrons for positive charge. Every bond and lone pair must come from that total.

Negative charge adds electrons
A bond line represents two electrons

Audit each atom

Formal charge equals valence electrons minus nonbonding electrons minus bond-order sum. The formal charges must add to the species charge.

FC = V − N − B
Check the net charge

Hold the sigma framework

Resonance contributors keep atom identities and connectivity fixed while π bonds, lone pairs, and formal charges change. Standard deficient- and expanded-octet cases must be explicit.

Move electrons, not nuclei
A contributor is not a separate switching molecule

Worked example

Audit one nitrate contributor, NO₃⁻, with one N=O bond and two N–O bonds.

1
Budget 5 + 3(6) + 1 = 24 valence electrons.
2
Use eight electrons in the four bond orders, then place sixteen electrons as oxygen lone pairs.
3
Formal charges are +1 on N, 0 on the double-bonded O, and −1 on each single-bonded O; the sum is −1.

ConclusionMoving the N=O bond to either other oxygen gives an equivalent contributor while preserving connectivity and the 24-electron budget.

Close the notes first

Retrieve the model.

01How does a 2− charge change the valence-electron budget?

Add two electrons.

Negative charge represents additional electron density.

02What must the sum of all formal charges equal?

The overall molecular or ionic charge.

Formal charge is a complete electron-bookkeeping ledger.

03May atoms move between resonance contributors?

No; only electron placement changes.

Resonance preserves the sigma framework and atom connectivity.

LESSON 2 · 17 MIN

Study + retrieve

Bonding models and multiplicity

Classify dominant bonding models, count sigma and pi bonds, compare bond order, and connect electron groups to introductory hybridization.

ESSENTIAL QUESTIONWhat electron behavior and overlap model best explains the structure?

Bond model and multiplicity ledger

IONICOppositely charged ionsExtended electrostatic lattice

COVALENTShared electron densityDirectional orbital overlap

METALLICDelocalized valence electronsMobile electrons across a lattice

Sigma, pi, and introductory hybridization relationships
Local pattern	Bond components	Electron groups	Model
C–C	1σ	Count this connection once	Depends on all groups at C
C=C	1σ + 1π	Count this connection once	Often sp² at each alkene C
C≡C	1σ + 2π	Count this connection once	sp at each alkyne C

Trend boundaryFor the same atom pair: greater bond order is generally shorter and stronger.

STUDY MODEL · TEXT EQUIVALENT INCLUDED

Classify the dominant model

Ionic solids are extended attractions among ions, covalent bonds share electron density, and metallic solids use delocalized valence electrons. Real bonding lies on a continuum.

Ionic lattice—not molecule pairs
Metallic electrons are delocalized

Count the framework once

Every connected atom pair contributes one sigma bond. A double bond adds one pi bond; a triple bond adds two pi bonds.

Single = 1σ
Triple = 1σ + 2π

Use electron groups for hybridization

In the standard valence-bond model, two, three, and four electron groups map to sp, sp², and sp³. Multiple bonds count as one group, leaving unhybridized p orbitals for π overlap.

Count regions, not lines
π overlap follows the σ framework

Worked example

Count sigma and pi bonds in acetylene, H–C≡C–H.

1
Each of the two C–H connections contributes one sigma bond.
2
The C≡C connection contains one sigma bond and two pi bonds.
3
Add by type: three sigma bonds and two pi bonds.

ConclusionH–C≡C–H contains 3σ and 2π; each carbon has two electron groups and is sp hybridized in the introductory model.

Close the notes first

Retrieve the model.

01What holds an ionic solid together?

Collective electrostatic attraction among oppositely charged ions.

An ionic crystal is an extended lattice, not a collection of isolated two-ion molecules.

02How many pi bonds are in one double bond?

One.

The other component is the sigma bond connecting the nuclei.

03What hybridization follows from three electron groups?

sp² in the standard introductory model.

Three hybrid orbitals arrange in a trigonal-planar electron-group geometry.

LESSON 3 · 19 MIN

Study + retrieve

VSEPR geometry and molecular polarity

Convert a Lewis structure into electron-group and molecular geometry, place lone pairs, and decide whether bond-dipole vectors cancel.

ESSENTIAL QUESTIONHow many electron groups surround the center, and what remains after lone-pair positions are hidden?

VSEPR and polarity ledger

Common electron-group and molecular geometries
AXE pattern	Electron groups	Electron geometry	Molecular geometry
AX₂	2	Linear	Linear
AX₃	3	Trigonal planar	Trigonal planar
AX₂E	3	Trigonal planar	Bent
AX₄	4	Tetrahedral	Tetrahedral
AX₃E	4	Tetrahedral	Trigonal pyramidal
AX₂E₂	4	Tetrahedral	Bent
AX₄E	5	Trigonal bipyramidal	Seesaw
AX₄E₂	6	Octahedral	Square planar

CO₂O ← C → Olinear · vectors cancel

SO₂O ↖ S ↗ Obent · net dipole remains

Decision orderValid Lewis structure → electron groups → molecular shape → vector sum

STUDY MODEL · TEXT EQUIVALENT INCLUDED

Count regions once

Every single, double, or triple bond to the same atom counts as one electron group; each lone pair is another group.

Multiple bond = one VSEPR group
Steric number = bonds + lone pairs

Name the requested geometry

Electron-group geometry includes bonds and lone pairs. Molecular geometry names only atom positions, so lone pairs can turn tetrahedral into trigonal pyramidal or bent.

NH₃: tetrahedral groups
NH₃: trigonal-pyramidal atoms

Add dipoles as vectors

Polar bonds do not guarantee a polar molecule. Equivalent vectors cancel in a sufficiently symmetric geometry; bent or asymmetric arrangements often leave a nonzero net dipole.

CO₂ dipoles cancel
SO₂ dipoles do not cancel

Worked example

Predict the electron geometry, molecular geometry, and polarity of NH₃.

1
The nitrogen has three N–H bonding domains and one lone pair, for four electron groups.
2
Four groups give tetrahedral electron-group geometry; hiding one lone-pair vertex leaves trigonal-pyramidal molecular geometry.
3
The three N–H bond dipoles do not cancel in that geometry.

ConclusionNH₃ has tetrahedral electron-group geometry, trigonal-pyramidal molecular geometry, and a net molecular dipole.

Close the notes first

Retrieve the model.

01How many electron groups does one double bond count as?

One.

VSEPR counts regions of electron density around the central atom, not bond lines.

02Why do NH₃ and CH₄ have different molecular shapes?

NH₃ has one central lone pair; CH₄ has none.

They share four electron groups, but molecular geometry omits lone-pair positions.

03Can a molecule with polar bonds be nonpolar?

Yes, when the bond-dipole vectors cancel.

Molecular polarity depends on both bond polarity and three-dimensional arrangement.

All twelve Molecular Structure problems

Choose the answer that preserves every constraint.

Question order and all five answer options shuffle each time. Reports identify the exact question version and randomized session.

12 PRACTICE QUESTIONS

Retrieve before you review.

Question order and all five answer options are shuffled when you begin. The correct answer stays attached to the same underlying choice.

Complete the domain

All eight Atomic and Molecular Structure topics now have guided coverage.

Use the mixed-practice route for an eight-topic breadth set or a 24-objective mastery set.

These original practice questions support learning and do not estimate an official DAT score.

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