CHEM 4

What is an ionic bond

- metal cation bonded to a non-metal anion
- A chemical bond formed between two ions with opposite charges
- the uneven sharing of electrons

Why are the atoms charged in ionic bonds?

metal atoms lose electrons to non-metallic atoms so they become positively charged metal ions
non-metals gain electrons from the metal atoms and become negatively charged non-metalions

How does an ionic compound form?

a large number of positive and negative ions combine to form a 3D giant ionic lattice structure

How does the 3D lattice stay together?

held together strongly by electrostatic forces of attractions between the positive and negative ions (aka ionic bonding)

Why is the mpt and bpt high in ionic compounds?

due to the strong electrostatic forces of attractions

Standard conditions for ionic compounds

- Ionic substances are solid
- need to be dissolved in water to make a solution

3D Giant ionic lattice drawing

what are electrostatic attractions? (definition)

the attraction of oppositely charged ions

Ionic bonding diagram (Na & Cl)

Na wants to loose 1 electron and chlorine wants to gain one
Na will transfer its electron to the chlorine to become a sodium cation and a chlorine anion

What do the brackets mean by transition metals? eg: Iron(III)

Since transition metals have multiple oxidation states we must write the roman numerals in brackets to indicate which one it is

-IDE

the element is a monoatomic ion (by itself)

-ITE

contains oxygen, but less than ate

-ATE

contains oxygen, more than ite

What are the ionic properties

Volatility
Electrical conductivity
High MPT
Solubility

What is volatility?

the tendency of a substance to turn into gas (to vaporise)

Volatility in ionic compounds?

low volatility/non-volatile

Why do ionic compounds have low/none volatility?

SOLID: Because they have strong electrostatic attractions between ions in the ionic lattice
SOLUTION: When dissolved in water they ions have strong attractions with water molecules
= hard to break attractions

Electrical conductivity in ionic compounds?

SOLID: no
SOLUTION: yes

Why is the electrical conductivity trend in ionic compounds the way it is?

SOLID: cannot conduct electricity because ions are not moving freely (held by electrostatic attractions) so there is no flow of charge which is necessary for conducting electricity.
SOLUTION: ions are free to move and therefore they can conduct electricity (migration of ions = current)

What is needed to conduct electricity?

A flow of charge

Why is there a high MPT of ionic compounds?

strong force of electrostatic attractions in the lattice between ions = a large amount of heat (energy) needed to break those attractions
(PS: higher melting points when the charge on the ions are greater because there is an increased attraction between the ions)

What is solubility?

the ease at which a solute dissolves in a solvent to form a solution

Solubility in ionic compounds?

High solubility

Why are ionic compounds soluble? (eg NaCl)

when NaCl has dissolved in water the sodium and chloride ions are surrounded by water molecules
The ions are hydrates as new bonds form between ions and the water molecules (ion-dipole attractions)

Diagram of ionic substances dissolved in water (NaCl)

When dissolved positive and negative ions in the lattice separate because the solvent pulls the lattice apart

What determines how ionic a compound is?

electronegativity:
the greater the electronegativity difference of the atoms in a compound the more uneven the sharing of electrons is (more likely to be ionic)

What is a covalent bond?

the electrostatic force of attraction of one or more pairs of shared electrons to the 2 nuclei they are shared between

Covalent molecules: random facts

Share electrons to gain a full outer shell
Follow the octet rule
Covalent compounds have no free electrons and no ions so they don't conduct electricity

How does a covalent form?

Forms when outer electrons come close enough to each other to interact and rearrange themselves into a more stable arrangement

A double bond is ... than a single bond. finish the sentence and why?

shorter and stronger because there is a greater attraction between the nuclei and shared electrons pulling them closer

Bond length

a measure of the distance between the two bonded nuclei

Bond strength

described in terms of bond enthalpy (KJ/mol) - a measure of the energy needed to break a bond

Bond length trend

as the atomic radius increases as we move down a group atoms form molecules with longer bonds

non-polar covalent bond

when two atoms with the same electronegativity share an electron pair

polar covalent bond

when atoms with different electronegativity values share an electron pair
- the atom with the larger electronegativity draws the electrons pair closer to the nucleus making a polar covalent bond

When comparing covalent bond - what to mention?

talk about strength
talk about length
what type of bond
Bond order

what does the number of covalent bonds that an atom will form depend on?

number of electrons needed to fill its shell (look at periodic table)

What is the octet rule?

the tendency of atoms to gain a valance shell with a total of 8 electrons

Exceptions to the octet rule?

Period 3 and below can sometimes expand their octet
B & Be - don't need 8 electrons
S - can expand its octet

NH3

H20

CH4

CO2

HCN

BeCl2

(stable with 4 electrons in the outer shell)

BF2

(stable with 6 electrons in the outer shell)

What are electron domains?

refers to pairs of electrons on a central atom (both non-bonding and bonding)

What do the number of electron domains determine?

the shape of the molecule as a result of electron repulsion

How does electron repulsion work?

each pair of electrons is repelled as far as possible from each other

Dative/coordinate bond

a covalent bond in which both the shared electrons are provided by one of the atoms

NH4+ (polar?)

arrow pointing towards the top hydrogen atom

H3O+ (polar?)

Arrow pointing towards a hydrogen atom

What is molecular geometry?

Explains the 3D shape of the molecule

When is a molecule polar?

1) when theres a difference of electronegativity between atoms
2) Asymmetrical distribution of partial charge (one positive and one negative end, the charges do not cancel out)

What is a resonance structure?

A method of describing the delocalized electrons in some molecules where the bonding cannot be explicitly expressed by a single Lewis structure.

Why do resonance structures exist?

-when electrons can move between parts of the molecules
-since we dont know which position of the double bond is more common

Resonance structures examples

Ozone - 03
Carbonate - CO3 2-
Nitrate - NO3-

Carbon Molecules can be in the form of

- Allotropes - diamond, graphite, fullerene which contain carbon but vary in structure

Graphite (6 points)

- hard in one direction soft in the other
- layered structure, weak LDF between layers
- carbon atoms held together by very strong covalent bonds (each carbo has three bonds with 1 delocalised electron pair shared in layers)
- structure = covalent layer lattice
- MG = trigonal planar
- BA = 120

Dimond

- hardest naturally occurring substance and doesn't conduct electricity
- Carbon is covalently bonded to 4 others with very strong bonds
- structure = a covalent lattice
- MG = tetrahedral
- BA = 109.5

Fullerenes (eg: footballs)

- covalent bonding between 3 carbon atoms
- not in a lattice structure, penta/hexagon shape
- conducts electricity

About Silicone

- can form 4 covalent bonds
- can form covalent lattice with other silicone atoms

Si-Si compared to C-C

- longer and weaker
- more reactive
- easier to break

What are the different intermolecular forces?

London dispersion forces, dipole-dipole, hydrogen bonging

Relative strengths of the intermolecular forces?

(weaker, low mpt) LDR < diple-dipole < hydrogen bonging (stronger, high mpt)

Intramolecular forces include

(internal, between atoms in the molecule)
- polar covalent
- non-polar covalent
- ionic bonds
- metallic bonds

Intermolecular forces include

(bonding between 2 or more molecules)
- LDR
- dipole-dipole
- hydrogen bonding

What is a dipole?

formed when 2 non-metals form a polar covalent bond

What is a dipole-dipole attraction?

the attraction of 2 polar molecules to each other

Strength of Dipole-dipole attractions?

the strength depends on distance and orientation of the dipoles

Example of dipole-dipole attractions in HCl

London Dispersion Forces?

present in all molecules

Strength of LDF

as the number of electrons in the molecule increases the LDF become stronger and MPT increases

Instantaneous Dipoles?

Atoms with same electronegativities do not have permanenet dipoles; however they do have instantaneous dipoles due to the constant movement of the electrons

What is hydrogen bonding?

an electrostatic attraction between two polar groups that occurs when a hydrogen (H) atom, covalently bonded to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F),

Hydrogen bonding occurs when bonding with what three elements?

N-H
O-H
F-H
- because of their big electronegative difference there will be strong hydrogen bonds

What intermolecular forces do polar molecules have?

- LDF
- Dipole- Dipole
- SOMETIMES hydrogen bonding

What intermolecular forces do non-polar molecules have?

- only LDF
- instantaneous dipoles may occur

When are covalent molecules soluble?

If they can form bonds with the solvent

Non-polar solvents:

C6H12

Why is water such a good solvent?

Very polar, forms dipole-dipole/hydrogen bonds, pulls apart ionic compunds

what does metallic bonding consist of?

positively charged ions arranged in a regular lattice held together by electrostatic attractions of the cations and the delocalised electrons

Diagram for metallic bonding

About the delocalised electrons in metallic bonding

constant random motion
between the ions
came from outer shells of metals
free to move

Properties of metallic bonding?

Malleability
Electrical conductivity

Explain Malleability of metals

it enables metals to be pressed into any shape
- metallic bonding: the layers of ions can slide over each other while remaining connected to the sea of electrons and therefore it doesn't break but changes shape

Explain electrical conductivity of metals

There are free flowing delocalised electrons, a flow of electrons and they can therefore conduct electricity

How can properties of metals be altered by adding other substances?

other substances are added and (usually other metals or carbon) melted together with the metal and cool so the atoms mix (the resulted solid is called an alloy)

How is it possible to add other substances to metals?

because of the non directional nature of the delocalised electrons and that the lattice can accommodate ions of different sizes

Which is stronger; Ionic or covalent compounds?

Ionic because electrostatic attractions are stronger than the sharing of electrons

What are substitutional alloys?

made from addition of elements with similar chemical properties and atomic size
Still a strong lattice structure
The size difference between the atoms causes a restriction between the layers (harder and less malleable)

What are interstitial alloys?

Addition of a smaller atom to a metal
fits randomly between packed metal ions
causes more restriction = less malleable

Why is formal charge important?

allows us to determine the most stable and likely lewis structure

Equation for formal charge

FC= V-L-B/2
V- total number of valance electrons (P-table)
L- lone pair of electrons (the free ones around the atoms)
B- bonding electrons (in the bonds)

How do you know which lewis structure is the most stable?

The one with a formal charge closest to 0
The negative charges are located on the most electronegative atoms

How to calculate bond order of resonance structures?

# of bonds / # of possible locations of bonds

What is meant by a bond order of 1.5?

Bond is stronger than a single bond but weaker than a double bond

What are the three basic concepts of orbital overlap theory?

1) the orbitals occupy the same space
2) larger overlap = stronger bond
3) a pair of electrons is located at an overlap

What are the two types of covalent bonds?

sigma and pi bonds

When do sigma bonds occur?

axial - between the atomic nuclei