Chemistry 20 IB - Bonding

Structural Formulas

Show the way in which atoms are bonded to each other

Why does a chemical bond happen

Results from the simultaneous attraction of electrons by 2 atomic nuclei

Why were the concept of orbitals created?

To describe where electrons exist in the atom

What is an orbital?

- region of space around an atom's nucleus where an electron may exist
-is not a definite race track, it is a 3-D space that defines
where an electron may be (like a rain drop in a cloud)

What orbitals are we concerned about for bonding study?

Valence Orbitals

What is a valence orbital?

the volume of space that can be occupied by electrons in an atom's highest energy level

For bonding study why are we only concerned about the valence orbital?

because lower energy levels are held so strongly by their positively charged nucleus

How are valence electrons classified?

- In terms of orbital occupancy.
- 0 \= empty, 1 \= half filled , 2 \= full

What is a bonding electron?

An atom with a valence orbital that has a single electron can
theoretically share that electron with another atom

What is a Lone Pair?

An atom with a full valence orbital (2 e-'s), repels nearby
orbitals and wants to be alone

What is the first rule for the 4 rules of bonding Theory?

The first energy level has room for only one orbit
- can only hold 2 e-'s max

What is the second rule for the 4 rules of bonding Theory?

Energy levels above the first have room for
four orbitals \= 8 electrons max

What is the third rule for the 4 rules of bonding Theory?

An orbital can be unoccupied, or it may contain one or two
electrons - but never more than two (Pauli Exclusion Principle)

What is the fourth rule for the 4 rules of bonding Theory?

Electrons "spread out" to occupy any empty valence orbitals
before forming electron pairs

What is the octet rule?

Atoms tend to gain, lose or share electrons in order to have a full set of valance electrons.

What is the Lewis Model?

valence electrons are represented as dots surrounding the symbol of an element

What is electronegativity?

A measure of the force that an atom exerts on electrons of other atoms; (the "pull" on bonding electrons)

For electronegativity the \____________ the number the \___________ the "pulling" force

larger, greater

What is the value range electronegativity?

0.0 - 4.0

How do we assign each atom an electronegativity number?

- The farther away from the nucleus that electrons are, the weaker their attraction to the nucleus
- Inner electrons shield valence electrons from the attraction of the positive nucleus
-The greater the number of protons in the nucleus, the greater the attraction for more electrons

Electronegativity rule

increases from left to right, decreases from top to bottom on the periodic table

How can we predict the 3 different types of bonds?

By comparing the electronegativities of the two atoms

What is covalent bonds?

- Both atoms have a high EN so neither atom "wins"
- The simultaneous attraction of two nuclei for a shared pair of
bonding electrons
-each atom approximating a complete outer shell because of the shared electrons.

What is a polar covalent bond?

- Unequal sharing of electrons
- bonded atoms with different Electronegativity
- Example H - Cl En \= 2.2 En \= 3.2

What is a non-polar covalent bond?

- equal sharing of electrons
- bonded atoms with the same electronegativity

What is Ionic Bonding

- The EN of the two atoms are quite different
- Transfer of electrons from a metal to a non-metal

What is a crystal lattice structure?

- arrangement of atoms ions or molecules in a crystalline material
- each positive ion is surrounded by a negative ion and vice versa

What is metallic bonding?

- A sea of electrons
- Both atoms have a relatively low EN so atoms share valence
electrons, but no actual chemical reaction takes place

In metallic bonding...

- e-'s are not held very strongly by their atoms
- the atoms have vacant valence orbitals

Metallic bonding properties

- flexible, malleable and ductile \= useful alloys (Brass, Stainless Steel, etc.)

Max of electrons in level 1

2 electrons

Max of electrons in level 2

8 electrons

Max of electrons in level 3

8 electrons

Max of electrons in level 4

18 electrons

What is a molecular compound?

covalent bonds (shared electrons) between non-metals and non-metals

molecular compound properties

- May or may not be soluble in water
- Don't ever conduct electricity - even when (aq)
- Generally have lower melting and boiling points than ionic compounds

What are Emprical Formulas?

- show the simplest whole-number ratios of atoms in a compound
- Very useful for ionic compounds
- Not useful for molecular compounds

What is the Formula Unit?

the ratio of ions that repeats in a pattern within the
crystal; the chemical formula of ionic compounds represents the formula unit

What are molecular formulas?

shows the actual number of atoms that are covalently bonded to make-up each molecule

What is Bonding Capacity?

the maximum number of single covalent bonds that an atom can form

What types of covalent bonds are possible?

F - F : single \= sharing one e- pair
O \= O : double \= sharing two e- pairs
N Ξ N : triple \= sharing three e- pairs

Why do we care about bonding capacity?

If we know how many bonding e-'s an atom has, we can
predict what structure a molecular compound will have

So how do you know which is the central atom?

- Usually the one in lesser quantity
- The one with the higher bonding capacity

What electrons are important for molecular shape?

Valence electrons

What are valence electrons paired in?

molecule or polyatomic ion

How do valence electrons repel eachother?

electrostatically

How is the molecular shape determined?

by the positions of the electron pairs when they are a maximum distance apart

What is stereochemistry?

the study of the 3-D spatial configuration
of molecules and how this affects their reactions

How is the shape of molecules determined?

by the repulsion that happens between electron pairs

What is Valence Shell Electron Pair Repulsion (VESPR)?

The theory behind molecular shapes

What is the general rule of VESPR?

- Pairs of electrons in the valence shell of an atom stay as far
apart as possible because of the repulsion of their negative
charges
- The type, number and direction of bonds to the central atom of a molecule determine the shape of the resulting molecule.

VESPR Shape \#1 - Linear

- 2 bond pairs
- 0 lone pairs
- 2 total pairs
- General Formula : AX2
- VSPER theory suggests that the two bond pairs will be farthest apart by moving to opposite sides to a bond angle of 180°

VESPR Shape \#2 - Trigonal Planar

- 3 Bond pairs
- 0 Lone pairs
- 3 total pairs
- General Formula: AX3
- VSPER theory suggests that the three bond pairs will be farthest apart by moving to a bond angle of 120° to each other.

VESPR Shape \#3 - Tetrahedral

- 4 Bond pairs
- 0 Lone pairs
- 4 Total pairs
- General Formula: AX4
- VSPER theory suggests that the four bond pairs will be
farthest apart by arranging in three dimensions so that
every bond makes an angle of 109.5° with each other.

VESPR Shape \#4 - Trigonal Pyramidal

- 3 Bond Pairs
- 1 Lone pair
- 4 Total Pairs
- General Formula : AX3E
- Electron Pair Arrangement: Tetrahedral
- VSPER theory suggests that the four groups of e-'s should repel each other to form a tetrahedral shape (bond angle \= 109.5°)
- But the lone pair is very repulsive, thus pushes the atoms more
to a 107.3° bond angle

VESPR Shape \#5 - Angular (Bent)

- 2 Bond Pairs
- 2 Lone Pairs
- 4 Total Pairs
- General Formula: AX2E2
- Electron Pair Arrangement: Tetrahedral
-VSPER theory suggests that the four groups of e-'s should repel each other to form a tetrahedral shape (bond angle \= 109.5°)
- But the TWO lone pairs are very repulsive, thus pushes the atoms more to a 105° bond angle

VESPR Shape \#6 - Linear (Tetrahedral)

- Bond Pairs 1
- Lone Pairs 3
- Total Pairs 4
- General Formula: AXE3
- Electron Pair Arrangement: Linear Tetrahedral
- VSPER theory suggests that the four groups of e-'s should repel each other to form a tetrahedral shape (bond angle \= 109.5°)
- But since there are only two atoms with one covalent bond holding them together, by definition, the shape is linear, as is the shape of every other diatomic molecule.

What is Polarity?

a molecule in which the charges are unevenly distributed

What is a Polar Molecule?

- a molecule in which the negative (electron) charge is not distributed symmetrically among the atoms making up the molecule.
- Thus, it will have partial positive and negative charges on opposite sides of the molecule.

How does polar molecules work with liquids?

- will rotate so that their positive sides are closer to a
negatively charged material.
- Near a positively charged material they become oriented in
the opposite direction.

Empirical Rules: Type AB

- Polar Molecules
- diatomic with different atoms
- Ex: HCl(g), CO(g)

Empirical Rules: Type NxAy

- Polar Molecules
- containing nitrogen and
other atoms
- Ex: NH3(g), NF3(g)

Empirical Rules: Type OxAy

- Polar Molecules
- containing oxygen and other atoms
- Ex: H2O(l), OCl2(g)

Empirical Rules: Type CxAyBz

- Polar Molecules
- containing carbon and two other kinds of atoms
- Ex: CHCl3(l), C2H5OH(l)

Empirical Rules: Type Ax

- Non-Polar
-all elements
- Ex: Cl2(g), N2(g)

Empirical Rules: Type CxAy

- Non-Polar
- containing carbon and only one other kind of atom (except CO(g))
- Ex: CO2(g), CH4(g)

Electronegativity differences

- nonpolar (< 0.4)
- polar (0.4+)
- ionic (metal + non-metal)

DOES BOND POLARITY \= MOLECULAR POLARITY?

No

What is molecular polarity?

- the uneven distribution of molecular charge
- the distribution of partial charges among the atoms in a molecule

Like dissolves Like rule

Polar substances are soluble in polar substances; Non-polar
substances are soluble in non-polar substances

What happens when you mix non-polar and polar substances?

- results in them forming layers, with the least dense one on top.
- This occurs because polar molecules attract each other more
strongly; thus they stay close together excluding nonpolar
molecules

What is exothermic energy?

energy is released into the surroundings - the product's bonds have less energy than the reactant's bonds

What is endothermic energy?

energy is absorbed from the surroundings - the product's bonds have more energy than the reactant's bonds

What is bond energy?

the energy required to break a chemical bond or the energy released when a bond is formed

What is Intranuclear force?

bonds within the nucleus between protons and neutrons (very strong)

What is Intramolecular force?

bonds between atoms within the molecule or between ions within the crystal lattice (quite strong)

What is intermolecular force?

bonds between molecules (quite weak); are electrostatic (involve positive and negative charges)

What are the 3 types of intermolecular force?

- Dipole - Dipole
- London Force
- Hydrogen Bonding

What is Dipole - Dipole?

- The simultaneous attraction between oppositely charged ends of polar molecules.
- Simply put, the attraction between dipoles
- The strength of the force is dependent on the overall
polarity of the molecule

What are Dipoles?

a partial separation of positive and negative charges within a molecule, due to electronegativity differences

What is London Force/ Dispersion?

- Simultaneous attraction between a momentary dipole in a
molecule and the momentary dipoles in surrounding molecules
- They last for just the instant that the electrons are not distributed perfectly even
- The strength of the force is directly related to the number
of electrons in the molecule, and inversely related to the distance between the molecules.
- Increase electrons \= Increase force (directly related).
- Increase distance \= Decrease force (inversely related)

What is momentary dipole?

an uneven distribution of electrons around a
molecule, resulting in a temporary charge difference between its ends

Which intramolecular force is present in all molecules?

London Force

You cannot predict boiling points when...

- One molecule has a stronger dipole-dipole force and the other has a stronger London force
- The two molecules differ significantly in shape
- The central atom of either molecule has an incomplete octet

What is Hydrogen Bonding?

- Occurs when a hydrogen atom bonded to a strongly electronegative atom, (N, O and F) is attracted to a lone pair of electrons in an adjacent molecule.
- only act as continuous bonds between molecules in solids, where the molecules are moving slowly enough to be
locked into position.

For Hydrogen Bonding why do you need a strongly electronegative atom?

It pulls the hydrogen's electron away making it "unshielded", so the lone pair on the other side can come much closer

What is the weakest intermolecular force?

London Force/Dispersion

What is the strongest intermolecular force?

hydrogen bonding

Hydrogen bonding in ice

hydrogen bonds between the molecules result in a regular hexagonal crystal structure.

Why does ice have a lower density than water?

The hydrogen bonds hold water molecules in a hexagonal lattice with open space in the center

Hydrogen Bonding in DNA

- The red bonds are hydrogen bonds.
- If the helix were held together by covalent bonds, the DNA molecule would not be able to unravel and replicate and life could not continue!!

Why do we care about intermolecular forces?

Why do we care about intermolecular forces?

Intermolecular forces and surface tension

- Molecules within a liquid are attracted by other molecules in all directions equally, but right at the surface, molecules are only attracted downwards and sideways. This means the net pull is downward so the surface tends to stay intact
- The stronger the intermolecular force the stronger the surface tension.

What is adhesion?

attraction between unlike molecules

What is cohesion?

attraction of like molecules

What is capillary action?

- the attraction of the surface of a liquid to the surface of a solid
- Occurs when a liquid rises in a thin tube because the adhesive forces between the tube and the liquid are stronger than the cohesive forces between the liquid's molecules.

What are ionic Crystals?

- positive and negative ions arranged in a regular pattern; hard, brittle, high melting points, insulators
- Very stable arrangment

What are Metallic Crystals?

Valence electrons are not held strongly by individual atoms, but act as freely-moving, negative "glue" that holds the positive ions together.

Properties of metals

Shiny luster, high malleability, ductility, electrical and thermal conductivity.

What are Molecular Crystals?

- Most nonmetal elements and molecular compounds are not solid at room temperature.
- Individual molecules are held together by relatively weak intermolecular forces (London, dipole-dipole hydrogen bonding).