Chemical Bonding Flashcards
How do atoms get the same number of electrons as the noble gases
Gain, lose or share electrons
The octet rule
When atoms bond, they lose,share or gain electrons so that they have eight electrons in their outer shell
What’s different about elements near the noble gas helium?
Do not obey octet rule, seek 2 electrons on outer shell eg.hydrogen or lithium
Ammonium ion
NH4+
Carbonate ion
CO3 ^-2
Hydrogen carbonate
HCO3-
Chromate
CrO4-
Dichromate ion
Cr2 O7 -2
Cyanide ion
CN -
Thiosulphate
S2 O3 -2
Hydroxide ion
OH-
Manganate
MnO4-
Nitrate
NO3-
Nitrite
NO2-
Phosphate
PO4-3
Sulphate
SO4 -2
Sulphite
SO3 -2
Ite has…
Less oxygen than ate
Ide…
Has no oxygen except OH-
What’s the formula for NO3
Mg(No3)2
Derive the formula for Aluminium sulphate
Al2(SO4)3
Why is the octet rule not perfect
Many exceptions
2 types of bonding
Ironic and covalent
Ironic bond
The attraction between positively charged and negatively charged ions after these ions wer formed by their low or gain of electrons
Whee on the PTE do ionic bomds usually form and why
Usually in groups 1,2,6 and 7
Because these are elements which lose or gain electrons easily
Why do ionic compounds arrange themselves into?
Into regular rigid crystalline lattices
What is the simplest crystalline lattice?
Sodium chloride (NaCl)
Electronegativity difference of an ionic compound
Greater than 1.7
Show the electronic structure of magnesium fluoride and then give its structure
Ionic
MgF2
Covalent bonding
A bond which is formed by the sharing of electrons to obey the octet rule
Electronegativity difference of covalent bond
1.7 or less
Generally, compounds containing non-metals only…
Take part in covalent bomd8;g
Show the electronic structure of the simplest compound between phosphorus and chlorine and give it’s formula
Covalent
PCl3
Show the electronic structure and give the formula of the simplest compound between nitrogen and hydrogen
Covalent
NH3
Show the electronic structure and give the formula of the simplest molecule of 2 chlorine atoms
Covalent
Cl2
Show the electronic structure and give the formula of 2 oxygen atoms
O2
Show the electronic structure and give the formula of 2 nitrogen atoms
N2
Valency
The number or bond that an atom of an element forms when it reacts
Valency of 1,2,3,4
1,2,3,4
Valency of 5,6,7,8
8-group Number
3,2,1,0
Rule used to find the valencies of compound
The totals of the valencies of each element in the formula must be equal
Must be written in simplest ratio
Find the formula of calcium oxide using valencies
Ratio of 1:1
CaO
Find the formula for the compound between germanium and hydrogen using valencies
Ratio 1:4
GeH4
The valency of a complex ion is…
The charge of the complex ion
Use valencies to write the formula for calcium nitrate
Ratio 2:1
Ca(NO3-)2
Transition elements, referring to valencies
Can have a number of valencies (variable valencies)
How do we know the valency of the transition metal in a question
Given by Roman numbers eg. Cu (II)
Use valencies to form the formula of manganese (IV) oxide
Ratio 2:1
MnO2
4 properties of ionic compounds
- Usually hard and brittle solids
- High m.p and b.p
- Majority are soluble in water with ions moving apart (disassociation)
- Conduct electricity in molten state or aqueous solution (ions are free to move and carry charges with them
4 properties of covalent compounds
Usually gases, liquids or soft solids
Low m.p and b.p
Most are insoluble in water
Don’t conduct electricity (no ions present to carry charge)
Non polar covalent bond
Where pairs of electrons are shared equally between the atoms in a bond
Example of non polar covalent compound
O2
Polar covalent compound
Pairs of electrons are not shared equally between atoms in a bond
Example of polar covalent bond
HCl
In HCl, a polar covalent bond, the pairs of electrons are pulled closer to Cl than H, giving Cl a…
Partial negative charge
In HCl, a polar covalent bond, the pairs of electrons are pulled closer to Cl than H, giving H a…
Partial positive charge
Polar covalent bonds are due to…
Differences in electronegativity between the elements
In the PTE, across a period, electronegativity
Increases
In the PTE down a group, the electronegativity
Decreases
Why does the electronegativity increase across the PTE
Increasing nuclear charge makes it easier to attract a pair
Decreasing atomic radius, electrons nearer the nucleus
Why does electronegativity decrease down the PTE
Extra levels of electrons=more screening
Increase in atomic radius
(Increasing nuclear charge cancelled out by^^)
EN difference greater than 1.7
Ionic
EN difference between 1.7 and 0.4
Polar covalent
EN difference less than 0.r
Non-polar covalent
Dipole moment
The ability of a molecule to turn in an electric field
When can molecules with polar covalent bonds make up a non polar covalent molecule
When the centres of positive and negative charge coincide
Why can molecules with polar covalent bonds make up a non polar covalent molecule when their centres of + and - charge coincide?
Charges cancel each other out No overall charge No dipole movement Will not turn in an electric field Not a polar covalent molecule NON-POLAR COVALENT
Simple test to determine if a liquid is polar or non-polar
Note the effect of a charged plastic rod on a stream of liquid falling from a burette, if the liquid deflects, then it consists of polar molecules
Why would falling water be deflected towards a positively charged plastic rod
Water molecules are polar covalent and their partial negative charges cause them to be deflected towards positively charged rod
What happens with the water if a positive plastic rod is switched with a negatively charged plastic rod
Water molecules rearrange themselves so that the partial positive charges are deflected towards the negative rod.
Water is still deflected towards it,
At the level of atomic orbitals, what does a covalent bond consists of?
2 half full atomic orbitals
2 half full atomic orbitals overlapping into each other forms a
Molecular orbitals containing 2 shared electrons
Sigma bond
Head on overlapping
Double covalent bonds consist of
1 sigma bond and 1 pi bond
Pi bonds
Overlap sideways
Which bond is stronger sigma or pi and why
Sigma because there is more overlapping
Why in a double bond isn’t there 2 sigma bonds
Orbitals at 90°, a second sigma bond would break the first one
A triple covalent bonds is made up of
1 sigma bond
2 pi bonds
Why is a O=O bone not twice as strong as a O-O bond?
Because sigma bonds are stronger than pi bonds and the second bond in that molecule is a pi bond
Recrystallization
Technique used to purify certain crystalline substances
Melting point determination
Used to find out how pure a substance is
Certain substances are more soluble …
In hot solvent than cold solvent
Benzodiazepines acid crystals are very soluble in
Hoy water but only slightly soluble in cold water
Why is it important to use minimal water in purifying benzoic acid experiment
Saturated solution needed
Too much water, crystallization won’t occur
Why do u have to do the benxoic acid crystal experiment quickly
To prevent recrystallization occurring on the filter paper when filtering charcoal
Impurity’s consist of both..
Soluble and insoluble ones
What speeds up recrystalisation
Scraping the inside of the flask with a glass rod
Air suction device in second filtration
Büchner funnel
Why is a Büchner funnel used
To speed up process
Why would you wash last bit of benzoic acid crystals before taking them out of filter paper?
Still damp, that water could be contaminated with NaCl
Covalent bonds soluble
Only in hot water
Ionic bonds soluble in
Any water
Benzoic acid type of bond
Covalent
What do benzoic acid crystals look like?
White crystals
What is benzoic acid used for
Food preservative
Melting point of benzoic acid
122°C
Melting point determination
Determines how pure a substance is
How would you gather a sample of benzoic acid crystals in a capillary tube?
Gently push and rotate the open end of the tube into the crystals and take out, tap with fingernail to get the to the bottom
Repeat until 1cm of crystals in tube
Impure if 2
Melt over large range eg 6°
Melt at a lower melting point to the crystals average
2 uses for melting point determination
Check purity
Identify a substance
