7 - bonding and structure Flashcards
Define ionic bond
The electrostatic attraction between 2 oppositely charged ions
What do ionic bonds form between?
A metal and a non-metal
How do you draw dot and cross diagrams for atoms?
Draw a circle for the nucleus and put the symbol of the element inside it
Don’t draw any rings
Draw the outer shell without a ring around the nucleus
Draw the electrons as crosses in one atom and as dots around the other element
Put electrons in spin pairs whenever possible
How do you draw ions as dot and cross diagrams and how do you draw their reaction?
Draw the metal nucleus with no electrons on the outside (all been taken)
Put square brackets around it and put the charge as a superscript to the brackets
Draw the non-metal nucleus with the outer electrons of the atom as crosses and the added electrons (from the metal) as dots
Put electrons in spin pairs
Put brackets around the non-metal ion etc. as well
If you need more than one of an ion, but a big number before the brackets
What is the charge of the ions in each of the groups?
Group 1 have +
Group 2 have 2+
Group 13 have 3+ except for boron (a non-metal)
Group 14 doesn’t form ions
Only the non-metals in group 15 form 3-
Only the non-metals in group 16 form 2-
Group 17 have -
Group 18 don’t form ions because they are noble gases
Define giant ionic lattice
A 3D structure of oppositely charged ions, held together by strong ionic bonds
What are the 2 main properties of ionic compounds?
Conduct electricity when molten or liquid
High melting and boiling points
Exist as giant ionic lattices
Why do ionic compounds conduct electricity as liquid or molten?
When you melt or boil, you pull apart ionic bonds
Allowing the ions (charged particles) to move and so they can carry a current
As a solid, the ions aren’t free to move so current isn’t carried
Why do ionic compounds have high melting and boiling points?
Very strong electrostatic attractions between oppositely charged ions so a large energy is needed to pull them apart
What effect does the charges of ions have on the ionic bonds?
A bond made by higher charges is harder to overcome
Requires more energy to overcome
What is a polar solvent?
A solvent which has slight negative and positive charges
What is the most common polar solvent?
water
How are ionic compounds and polar solvents linked?
Ionic compounds dissolve in polar solvents
Why do ionic compounds dissolve in polar solvents?
The ions of the compound are attracted to the small positive and negative charges of the polar solvent
What are metallic bonds?
Strong electrostatic attractions between positive metal ions and delocalised electrons
What is a giant metallic lattice?
The 3D structure of positive metal ions and delocalised electrons which are bonded by strong metallic bonds
Where do the delocalised electrons in metals come from?
Electrons leave the outer shells of metal atoms so that they are free to move about the structure
The loss of electrons from the metal atoms makes them positive metal ions
What are the 8 properties of metals?
Conduct electricity and heat
Malleable
Ductile
Strong
High melting and boiling points
Sonorous
Shiny
Not soluble in water
Why do metals conduct electricity and heat?
The free, delocalised electrons can carry the current through the structure
What do malleable and ductile mean?
Malleable means that it can be compressed without breaking
Ductile means it can be stretched / pulled out without breaking
Why do metals have high melting and boiling points?
The strong metallic bonds require a large amount of energy (and high temperature) to overcome
What does sonorous mean?
It makes a ringing noise when hit
define covalent bond
What is a covalent bond?
When does covalent bonding occur and why?
Reactions between 2 non-metals
So that all atoms have a full outer shell
What are the 2 ways of drawing covalent bonds?
The symbols for the atoms with lines between them to represent a shared pair
Dot and cross diagrams showing shared pairs between bonded atoms
What is a bonded pair?
A pair of electrons which is shared between 2 atoms
What is a lone pair?
An outer shell pair of electrons which is not involved in covalent bonding
What is a multiple covalent bond?
When there are more than 1 shared pairs of electrons between 2 atoms
How are multiple covalent bonds shown in each type of covalent diagram?
Multiple lines between atoms for simple diagram
Multiple shared pairs between atoms on dot and cross diagram
What are dative covalent bonds?
A shared pair of electrons which has been provided by one of bonding atoms only
What do dative covalent bonds usually form between
A lone pair is shared between its originator and a H+ ion (proton) usually
What is the other word for dative covalent bond?
Coordinate bonds
How do you draw dative covalent bonds in dot and cross diagrams?
Draw the pair as 2 dots or 2 crosses
If there are 2 bonded pairs around a central atom, what shape is formed and what is it called?
A linear shape
The electron pairs go on to the opposite sides of the central atom forming a straight line
The bond angle is 180 degrees
What is the shape called when there are 3 bonded pairs and what does it look like?
The shape is trigonal planar
The 3 bonds are on a flat plane
120 degrees bond angle
What is the shape for 4 pairs?
3D Tetrahedral shape
Bond angle is 109.5 degrees
What is the shape for 6 pairs?
Octahedral shape
Bond angle is 90 degrees
What determines the shape of a molecule?
The electron pairs surrounding a central atom
Why do the electron pairs surrounding a central atom form the shapes they do?
Each electron pair repels other electron pairs
The pairs push apart as much as possible
What is strange about beryllium?
Beryllium forms covalent bonds even though it’s a metal
E.g BeCl~2
What is the octet rule?
Covalent bonds form so that there are 8 electrons around each atom (in the outer shell)
In which cases is there not quite 8 electrons and how many outer electrons are there in these cases?
BF~3
The boron has only 6 electrons
BeCl~2
The beryllium only has 4 outer electrons
In which cases is there more than 8 electrons in the outer shell? (Expansion of octet rule)
SF~6
12 outer electrons to the fluorine
What is the average bond enthalpy?
A measure of the energy needed to break a given bond
unit: kJ mol^-1
When there are 3 bonded pairs and 1 lone pair, what is the 3D shape called?
Pyramidal
How do you draw pyramidal molecules?
The same as tetrahedral but you don’t draw the bond at the top at all
The same as tetrahedral but you don’t draw the bond at the top at all
For each lone pair in a molecule, the bond angle is reduced by 2.5 degrees from the bond angle if all pairs were bonded
What is the bond angle of pyramidal molecules and why?
107 degrees
Because tetrahedral molecules have a bond angle of 109.5 degrees
But there is 1 lone pair in a pyramidal molecule
109.5 - 2.5 = 107
So the bond of angle of pyramidal molecules is 107 degrees
What is the shape called when there are 2 bonded pairs and 2 lone pairs?
Non-linear
What is the bond angle of non-linear molecules?
104.5 degrees
The same as tetrahedral but there are 2 lone pairs
109.5 - 2.5 - 2.5 = 104.5 degrees
How do you draw a non-linear molecule?
Draw it like a trigonal planar molecule without one of the bonds
(Even though non-linear is completely unrelated to trigonal planar)
What is the shape called if there are 4 bonded pairs and 2 lone pairs?
Square planar
What is the bond angle of square planar molecules and why?
If all pairs were bonded, it would be octahedral with bond angle 90 degrees
In square planar molecules, the lone pairs go to opposite sides of the central atom so the bond angle isn’t affected by them
The bond angle is still 90 degrees for square planar molecules
How do you draw a square planar molecule?
4 bonds around the central atom each with 90 degrees bond angle
How does a multiple bond affect the bond angles?
Bond angles aren’t affected
A double bond is treated as a single bonded pair
How do dative covalent bonds affect shape?
A dative bond is treated as a normal covalent bond
It makes no changes to the bond angle and it is drawn as a normal covalent bond
Define electronegativity
A measure of the attraction of a given bonded atom to a shared pair of electrons in a covalent bond
When is electronegativity at its maximum and why?
When the nucleus is as large as possible (more protons means more positive attraction)
When the number of shells is as small as possible (shared pair is closer to nucleus)
Where is the electronegativity highest on the periodic table?
When the group is highest and when the period is smallest
What is the most electronegative element?
Flourine
What is the significance of 2 bonded atoms having the same electronegativity?
The molecule of the 2 atoms is non-polar
There are no dipoles
When do 2 atoms have the same electronegativity?
only when they are the same element
Or by chance
What is the Pauling scale?
The scale which measures electronegativity
When is a covalent molecule polar and why?
When the difference in electronegativity between the bonded atoms is 0.4 or above, the shared pair of electrons will move closer to the more electronegative atom
This gives the more electronegative atom a delta negative charge and the less electronegative atom a delta positive charge
This is what makes the molecule polar
What are the 3 bond types on the Pauling scale and what are their electronegativity difference values?
Non-polar covalent (less than 0.4 difference between the atoms)
Polar covalent (0.4 to 1.8 difference)
Ionic bonds (1.8 or above)
What is the polarity of a C-H bond? You need to remember this
Non-polar
Unusual as it seems
What happens when a molecule is symmetrical?
The small delta charges are in opposite directions so they all cancel out meaning the molecule is non-polar despite the electronegativity differences
What is a dipole?
A dipole is simply 2 opposite, corresponding delta charges
What happens when the molecule is not symmetrical?
The dipoles aren’t symmetrical so the molecule is polar (if the electronegativity difference is greater than 0.4)
How do lone pairs affect the symmetry and polarity?
A lone pair means that the molecule isn’t symmetrical so the molecule will be polar
What do we know about polarity if there are lone pairs?
If there are polar bonds, the molecule will be asymmetrical and therefore polar
What are the 3 types of intermolecular force?
Permanent dipole-dipole forces
London forces
Hydrogen bonding
When can permanent dipole-dipole forces occur?
When the molecules that are going to be held together are all polar
Describe how permanent dipole-dipole forces work
The polar molecules have opposite delta charges on each side
The positive deltas of each molecule are attracted to the negative deltas of neighbouring molecules and vice versa
When do London forces occur?
They occur in every substance (between any combination of polar and non-polar molecules)
Only talk about London forces if no other IMFs are present
What will London forces sometimes be called?
INDUCED dipole-dipole forces
Describe how London forces work
Because of the fast movement of electrons around atoms
At an instant, the electrons will be on one side of a molecule more than the other
This gives the “more electrons” area a delta - charge and the opposite side a delta + charge
These instantaneous dipoles induce a matching instantaneous dipole in neighbouring molecules
These matching dipoles cause there to be electrostatic attractions between molecules for an instant
These London forces are constantly breaking and forming between different molecules
When are London forces stronger and why?
When the atoms have more electrons in total because the overall negative charge of the atoms will be greater
The London forces cause LARGER INDUCED DIPOLES
What is a hydrogen bond?
An attraction between a lone pair of electrons and a delta + hydrogen atom
What has to be true for hydrogen bonding to occur?
Hydrogen is bonded to something more electronegative (usually N, O or F)
The other element has to have lone pairs
What is the order of strength of the different IMFs and why?
Hydrogen bonding is strongest due to the greater attraction of lone pairs
Then permanent dipole-dipole forces because the delta attractions are constant
Then London forces are the weakest because the attractions only remain for instants
What are the 4 main properties decided by IMFs between covalent molecules?
Solubility
Electrical conductivity
Melting point
Boiling point
Why is ice less dense than water?
The hydrogen bonding in ice holds the water molecules apart in a fixed open lattice
They are held apart so ice is less dense
What is the other unusual property of water and why does this happen?
It has higher melting and boiling points because hydrogen bonds are stronger than the other IMFs
So more energy is needed to overcome them
Why do ionic compounds dissolve in water?
Water is polar
So the dipoles of water are attracted to the positive and negative ions in the ionic compound
The water molecules surround the ions so the compound is dissolved
What is the structure of simple covalent substances?
Covalently bonded molecules held together with IMFs
What are the 2 properties of all covalent substances and why?
Low melting and boiling points due to weak IMFs
Don’t conduct electricity because there are no charges particles
When do covalent substances dissolve?
Non-polar covalent substances dissolve in non-polar solvents
Polar covalent substances dissolve in polar solvents
What are the 5 giant covalent substances?
Diamond
Graphite
Graphene
Silicon
Silicon dioxide
What are the melting and boiling points for giant covalent substances?
High because strong covalent bonds need to be broken
What is the strength for giant covalents?
Strong due to strong covalent bonds
What are the 2 properties of only graphite and why?
It is soft because the separate layers of carbon atoms have weak IMFs between them so they can slide over each other
It can conduct electricity because of the delocalised electrons between its layers
What is the structure of graphite?
Each carbon covalently bonded to 3 others
Delocalised electrons between layers
What is the structure of graphene?
A single layer of graphite
Hexagonally arranged atoms
Delocalised electrons above the layer
What is the structure of diamond?
Each carbon bonded to 4 others
Pyramid sort of shape
Which other giant covalent has a similar structure to diamond?
Both silicon and silicon dioxide
What is the conductivity of giant covalents and why?
No charged particles so don’t conduct
Except for graphite
What is the solubility like for giant covalents and why?
All insoluble
Because strong covalent bonds need to be broken
What do you have to remember about the polarity of H~2 S?
It doesn’t have dipoles
What is the IMF for HCl?
Permanent dipole-dipole forces
You have to remember this
What is interesting about the lone pairs on carbon?
Always no lone pairs
