Chemistry VSEPR, Molecular Polarity, Bonding & Resonance

What determines overall molecular polarity?

Bond dipoles and 3D shape; dipoles may cancel or add.

Why is H2O polar?

It is bent, so its O-H bond dipoles do not cancel.

Why is CH2Cl2 polar?

Its tetrahedral shape has different surrounding atoms, so dipoles do not cancel.

Why is PCl5 nonpolar?

Its symmetric trigonal-bipyramidal shape makes bond dipoles cancel.

Why is PF3 polar?

Its trigonal-pyramidal shape makes bond dipoles fail to cancel.

What is a covalent bond?

A bond formed when atoms share electron pairs.

How many electrons are shared in a single bond?

2 electrons.

How many electrons are shared in a double bond?

4 electrons: two shared pairs.

How many electrons are shared in a triple bond?

6 electrons: three shared pairs.

What makes a covalent bond more polar?

A larger electronegativity difference between the bonded atoms.

What is the most electronegative element?

Fluorine.

What partial charges occur in H2O?

O is δ−; each H is δ+. These are partial, not full ionic charges.

What is an electron domain in VSEPR?

A bond or lone pair around the central atom; multiple bonds count as one domain.

What is the difference between electron geometry and molecular geometry?

Electron geometry counts bonds and lone pairs; molecular geometry describes atom positions only.

What is the ideal geometry for 2 electron domains?

Linear; 180°.

What is the geometry for 3 domains and 0 lone pairs?

Trigonal planar; 120°.

What is AX2E molecular geometry?

Bent; trigonal-planar electron geometry; angle less than 120°.

What is the geometry for 4 domains and 0 lone pairs?

Tetrahedral; 109.5°.

What is AX3E molecular geometry?

Trigonal pyramidal; tetrahedral electron geometry; angle less than 109.5°.

What is AX2E2 molecular geometry?

Bent; tetrahedral electron geometry; angle less than 109.5°.

What is the geometry for 5 domains and 0 lone pairs?

Trigonal bipyramidal; 90°, 120°, and 180°.

What is AX4E molecular geometry?

Seesaw; trigonal-bipyramidal electron geometry.

What is AX3E2 molecular geometry?

T-shaped; trigonal-bipyramidal electron geometry.

What is AX2E3 molecular geometry?

Linear; trigonal-bipyramidal electron geometry.

What is the geometry for 6 domains and 0 lone pairs?

Octahedral; 90° and 180°.

What is AX5E molecular geometry?

Square pyramidal; octahedral electron geometry.

What is AX4E2 molecular geometry?

Square planar; octahedral electron geometry.

What is the molecular geometry of TeCl4?

Seesaw; Te has four bonds and one lone pair (five domains).

What is the molecular geometry of XeF3+?

T-shaped; Xe has three bonds and two lone pairs (five domains).

Which repels more strongly: lone pairs or bonding pairs?

Lone pairs generally repel more strongly than bonding pairs.

What is the usual VSEPR repulsion order?

LP-LP > LP-bonding pair > bonding pair-bonding pair.

How do you find VSEPR geometry?

Draw Lewis structure, count central-atom domains, then identify the AXE pattern.

What hybridization corresponds to 2 electron domains?

sp.

What hybridization corresponds to 3 electron domains?

sp2.

What hybridization corresponds to 4 electron domains?

sp3.

What hybridization is commonly assigned to 5 domains in introductory chemistry?

sp3d.

What hybridization is commonly assigned to 6 domains in introductory chemistry?

sp3d2.

What is a sigma (σ) bond?

Head-on orbital overlap along the axis between nuclei.

What is a pi (π) bond?

Side-by-side overlap of parallel p orbitals.

How many sigma and pi bonds are in a single bond?

One sigma, zero pi.

How many sigma and pi bonds are in a double bond?

One sigma and one pi.

How many sigma and pi bonds are in a triple bond?

One sigma and two pi.

How do you count sigma bonds?

Each connection between two atoms contributes one sigma bond, even in double/triple bonds.

How do you count pi bonds?

Each double bond has one pi; each triple bond has two pi.

What is the bond-length order for the same two atoms?

Single > double > triple (longest to shortest).

What is a resonance structure?

A valid Lewis structure differing from another only in electron placement.

What changes between resonance structures?

Electron placement changes; atom positions stay the same.

How many equivalent resonance structures for PO4^3− in the quiz's model?

Four; the P=O double bond can be placed with any of four oxygen atoms.

How many double bonds in the preferred Lewis structure of POCl3?

One P=O double bond in the formal-charge-minimized structure.

What is formal charge?

Valence electrons − nonbonding electrons − half of bonding electrons.

Which common central atoms must obey the octet rule?

Period 2 atoms such as C, N, O, and F cannot expand their octets.

Can P and S have expanded octets in the course's Lewis-structure model?

Yes; Period 3 elements such as P and S may be drawn with expanded octets.

What is the Lewis structure count for SF2?

Two S-F single bonds and two lone pairs on sulfur.

What values can ml have when l = 1?

−1, 0, +1; ml = −2 is impossible.

What l value identifies an s subshell?

l = 0.

What l value identifies a p subshell?

l = 1.

What l value identifies a d subshell?

l = 2.

What is the only ml value when l = 0?

ml = 0.

What shape is an s orbital?

Spherical.

How many electrons can a d subshell hold?

10 electrons: five orbitals, two electrons per orbital.

What is Hund's rule?

Equal-energy orbitals fill singly with parallel spins before electrons pair.

What is the ground-state valence configuration of Group 14 atoms?

ns2 np2; the two p electrons occupy separate orbitals before pairing.

How many unpaired electrons does ground-state Sn have?

Two; its valence configuration is 5s2 5p2.

What values can l have for a given n?

Integers from 0 to n−1.

What does ml describe?

Orbital orientation; integer values from −l through +l.

What does isoelectronic mean?

Having the same number of electrons.

How many electrons does Cl− have?

18; neutral chlorine has 17 and gains one.

Which ion is isoelectronic with Cl−: F−, Ne, Na+, or S2−?

S2−; it has 16 + 2 = 18 electrons.

How do you count electrons in an ion?

Positive charge: subtract from atomic number; negative charge: add its magnitude.

How does atomic radius change across a period?

It generally decreases from left to right.

Which is smallest among K, Ca, As, and Br?

Br; it is farthest right in Period 4.

What is effective nuclear charge (Zeff)?

Net positive attraction experienced by an electron after shielding.

How does Zeff generally change across a period?

It increases from left to right.

What is first ionization energy?

Energy required to remove the first electron from a gaseous atom.

What is the general ionization-energy trend?

Generally increases across a period and decreases down a group.

Why can N have a higher first ionization energy than O?

N has a stable half-filled 2p3 subshell; O has a paired 2p electron that is easier to remove.

What is electron affinity?

Energy change when a gaseous atom gains an electron.

What does a large jump between successive ionization energies indicate?

The number of electrons removed before the jump equals the valence-electron count.

If the large IE jump occurs after IE2, which group is likely?

Group 2, with two valence electrons.

What is the electron configuration 1s2 2s2 2p6 3s2 3p6's electron count?

18 electrons; S2− has this configuration.

Why does NO3− have equal N-O bonds and equal bond angles?

Resonance makes the three N-O bonds equivalent; trigonal planar geometry gives 120° angles.

What is the molecular geometry of NO3−?

Trigonal planar; all O-N-O angles are about 120°.

Can a molecule have polar bonds but be nonpolar overall?

Yes; symmetric geometry can cancel bond dipoles, as in PCl5.

What is the difference between bond polarity and molecular polarity?

Bond polarity depends on electronegativity difference; molecular polarity depends on dipoles and shape.

For a C-C=O angle at trigonal-planar carbon, what happens to the angle involving C=O?

It can be slightly greater than 120° because the double-bond domain repels more strongly.

Why is a C-O-C angle at ether oxygen less than 109.5°?

Two lone pairs on oxygen repel bonding pairs and compress the angle.

What is the general bond-angle effect of lone pairs?

They usually compress angles between bonded atoms because lone pairs repel more strongly.

What is the electron-group geometry of a central atom with 4 bonds and 1 lone pair?

Trigonal bipyramidal; molecular geometry is seesaw.

What is the electron-group geometry of a central atom with 3 bonds and 2 lone pairs?

Trigonal bipyramidal; molecular geometry is T-shaped.

What is the electron-group geometry of a central atom with 2 bonds and 2 lone pairs?

Tetrahedral; molecular geometry is bent.