Periodic Trends, Bonding & VSEPR

Dmitri Mendeleev

created 1st version of PT

realized periodicity occurred when elements were arranged according to atomic weight

Henry Mosely

developed concept of atomic #

Z_eff

effective nuclear charge

• net effect of attraction from the nucleus along with the repulsion from other e-’s

Z_eff formula

Z_eff = Z - S

where Z = atomic #

S = screening constant (# of core e-)

Why is E_3s < E_3p < E_3d?

increased repulsion

Why does 3_p have greater Z_eff than 3_d?

greater chance of e- being closer to the nucleus (more Z_eff)

Z_eff trend

L to R: increases

T to B: increases but not as dramatic

The closer an e- is to the nucleus/the stronger Z_eff is, the _____ the E of the e-

lower

When an e- is closer to the nucleus, its Z_eff _________

increases

Bonding Atomic Radius

½ the distance between the two nuclei of bonded atoms

Bonding radii are _______ than the radius of a non-bonded atom.

shorter (because the pull of the p+ on the e- is greater than the e- repulsions)

Bonding Atomic Radius trend

L to R: decreases

T to B: increases

A cation is _______ than its neutral atom.

An anion is ______ than its neutral atom.

smaller, larger

Ionic Radius trend

L to R: generally decreases (skipping transition metals)

T to B: increases

Isoelectronic

elements that have the same e- configuration (same # of e-)

Ionization E

minimum E required to remove an e- from an atom

I.E.₁

E required to remove the 1st e-

I.E.₂

E required to remove the 2nd e-

What would the trend be in successive I.E. values?

increases

I.E. trend L to R

increases

If there is more than one occupied subshell for a given value of n, e- are first removed from the orbital with the highest value of _.

(e.g. Tin loses its 5p e- before its 5s e-)

l

Electron Affinity

E change that occurs when an e- is added to a gaseous atom

E.A.₁

E change to add the 1st e-

E.A.₂

E change to add the 2nd e-

+ve E.A. indicates

E is required to add an e- (atom doesn’t want e-)

-ve E.A. indicates

E is released when an e- is added (atom wants e-)

Would an atom with a strong attraction for an e- have a +ve E.A. or -ve E.A?

-ve (wants the e- and releases E bc it becomes more stable)

Which elements would have the most +ve E.A. values?

noble gases, don’t want e-

Which elements would have the most -ve E.A. values?

halogens, really want e-

Chemical Bond

force of attraction holding atoms together in a cpd

Why do atoms/molecules form bonds?

to become more stable

bond formation ________ E (exothermic process)

releases

bond breaking ________ E (endothermic process)

requires

bond E is expressed in

kJ/mol

Chemical bonds determine the

• physical state of a substance

• shape of a substance

• properties of the substance

What 2 things determine bond type?

difference in EN

simultaneous attraction + repulsion in a molecule

Ionic Bond

transfer of e-’s between a metal and non-metal

Electrostatic Force of Attraction

attraction between opposite signs

Solids formed through ionic bonding form a

crystal lattice

Lattice E

E required to separate 1 mol of a solid ionic cpd into its gaseous ions

What does a large +ve value in lattice E mean?

strong attraction between ions

What determines lattice E?

size of ionic charge + size of ions

increased charge = increased lattice E

smaller ions = increased lattice E

Covalent Bonding

overlap of orbitals between 2 non-metals

The optimal distance between atoms results in the ______ E possible = greatest stability.

lowest

EN values for polarity

0-0.5 = non-polar

0.5-1.7 = polar

1.7+ = ionic

Dipole Moment

quantitative measure of the magnitude of a dipole (μ)

stereochemistry

study of how the shape of a molecule affects its chemistry

VSEPR stands for

valence shell electron pair repulsion

True or False: lone pairs repel more than bond pairs.

True

Bent AX₂E₂ Angle

104.5

Trigonal Pyramidal Angle

107.3

Tetrahedral Angle

109.5

Gillespie-Nyholm Theory

VSEPR Theory

Electron Domain

region where a bond pair of e- are most likely to be found

Steric #

total # of e- pairs

Electron Domain Geometry

all e- pairs (lone pairs + bond pairs)

(the “shape the e- pairs make”)

Molecular Geometry

only bond pairs (the “actual” shape of the molecule)

Bonding Domains

bond pairs

Non-Bonding Domains

lone pairs

When talking about the location of e- domains around the central atom, we can refer to the position or “_____” they are in

plane

axial plane/position

y-axis (vertical)

equatorial plane/position

x-axis (horizontal)

The position of domains around the central atom affects their repulsion and therefore their _____.

shape