Bonding And Shapes Of Molecules UNIT 1 Flashcards
What is ionic bonding
The electrostatic force of attraction between ions of opposite charge formed by the transfer of electrons
What is metallic bonding
The electrostatic force of attraction between 2 positive metal atoms and the surrounding sea of delocalised electrons
What is covalent bonding
The electrostatic force of attraction between 2 negative non metal atoms, consists of sharing a pair of electrons
Examples of giant covalent compounds
Graphite
Diamond
What are lone pairs
Pairs of electrons in the outer shell which aren’t involved in the covalent bond
What is a similarity between coordinate/ dative covalent bonding and normal covalent bonding
They have the same length and strength bonds between the same atoms
Describe coordinate/ dative covalent bonding
Normally electrons shared between pair of atoms. In this case only 1 atom provides the pair.
The atom that accepts the pair is an atom that doesn’t have a full outer shell (electron deficient)
The atom that’s donating the pair has a pair of electrons which are not used in bond (lone pair)
What is electron deficiency
An atom that doesn’t have a full outer shell
Example of coordinate/dative covalent bonding
Ammonia into ammonium
The nitrogen atom in an ammonia molecule donates a pair of electrons to a proton from the hydrogen atom to form ammonium
What is bond polarity
Electron pair not always shared equally. If joined atoms are different the nucleus from one atom may attract the electron stronger than other
What is electronegativity
The ability to attract the bonding electrons in an atom in a covalent bond
Which is the most electronegative element
Flourine
Example of a double covalent bond
Oxygen
What is a diatomic molecule
They are non polar as atoms have same electronegativities attracting electrons to nucleus
What affects electronegativity
Nuclear charge
Distance between nucleus and outer electrons
Shielding of nuclear charge by electrons in inner shells
What results in greater electronegativity
The smaller the atom the closer the nucleus is to the shared outer main level electrons
What are dipoles
The difference in charge between two atoms which is caused by a difference in electron density
Do all molecules with polar atoms form polar molecules
NO depends on shape of molecules and polarity of bonds
What are the three intermolecular forces
Van der waal (tempory dipole)
Permanent Dipole-dipole
Hydrogen
Order the intermolecular forces, From strongest to weakest
Hydrogen
Permanent dipole dipole
Vdw
Permanent dipole dipole bonding happens between……..
Polar molecules
What happens in permanent dipole dipole forces
Happens between polar molecules
2 molecules with dipoles will orientate themselves so that atoms with opposite charges are next to each other, so the 2 molecules attract
What type of molecules are needed for permanent dipole dipole forces
Polar molecules
Which of the intermolecular forces is present in everything
Vdw
Vdw forces are very….
Weak
How strong are vdw forces
Very weak
The size of the vdw increases with…
Number of electrons
What increases the vdw
Number of electrons
How do vdw forces work
In an atom the electrons are not fixed in a particular place. They occupy orbitals. Electrons can be found anywhere in orbital. Means in any instant the electron density may be more in one direction= temporary dipole. This dipole can cause another temporary dipole in the opposite direction on a neighbouring atom. The 2 dipoles are then attracted to each other. The second dipole can cause another dipole in a third atom. Because the electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
Why does H2S have a much lower boiling point compared with H2O
It only contains vdw forces, compared with H2O which contains hydrogen bonds
What compound is the exception to the rule of hydrogen bonding
Trichloromethane
Why can trichloromethane form hydrogen bonds
The combined effect of the 3 chlorines ( which are quite electronegative) is enough to leave the hydrogen virtually electron less
Which element can form hydrogen bonds
Nitrogen
Oxygen
Fluorine
Why can only nitrogen , oxygen and fluorine form hydrogen bonds
The are the only atoms that are electronegative enough to leave hydrogen virtually electron less
When are hydrogen bonds formed
When hydrogen is bonded to a very electronegative element
What happens during hydrogen bonding
The very electronegative element pulls the bonding electrons away from the hydrogen atom, leaving it with virtually no electron density. This leaves an exposed proton. The bond is so polarised, and the hydrogen
has such a high charge density because it’s so small, that the hydrogen atoms form weak bonds with lone pairs of electrons. Then begins to form a dative covalent bond with atom that has a lone pair
Example of hydrogen bonded compound
Water
How does oxygen form hydrogen bonds
Oxygen is very electronegative so pulls shared electrons strongly leaving the hydrogen virtually electron less/no electron density and an exposed proton. It can start to from a dative covalent bond with hydrogen
Which is the only atom that can take place in hydrogen bonding
Hydrogen, due to its small size
Why can only hydrogen take place in hydrogen bonding
Due to its small size
What are compounds
Atoms of different elements bonded together
What are the 2 main types of bonding in compounds
Ionic and covalent
What happens when atoms of different elements bond together
A compound is formed
In ionic bonding, what are the simplest ions
The single atoms, which have either lost or gained 1,2 or 3 electrons so they have a full outer shell
An easy way to work out how many electrons are lost from elements
Look at periodic table. Elements in same group all have same number of outer electrons. means they lose or gain same number of electrons to achieve full outer shell. Means they all form atoms with same charges
What does electrostatic attraction hold together
Positive and negative ions
What force holds together positive and negative ions
Electrostatic force of attraction
Sodium chloride and magnesium oxide are…
Ionic compounds
Examples of ionic compounds
Sodium chloride
Magnesium oxide
Example of giant ionic lattice structure
Sodium chloride
What is sodium chloride an example of
Giant ionic lattice structure
Ionic crystals are what shape
Giant lattices of ions
What type of crystal is a giant ionic lattice
Ionic crystal
How is the giant lattice structure ‘giant’
As it is made up of the same basic unit repeated over and over
Different ionic compounds have ………. …….. Structures
Differently shaped
What does the structure of the ionic compound affect
Their physical properties
What affects physical properties in ionic compounds
The structure of the ionic compound
3 physical properties of ionic compounds
Conduct electricity
High melting points
Tend to dissolve in water
Info on ionic compounds conducting electricity
They conduct when molten or dissolved- not when solid
The ions in a liquid are free to move (and they carry a charge). In a solid they are fixed in position by strong ionic bonds
Why can’t solid ionic compounds conduct electricity
The ions are fixed in a position by strong ionic bonds
What allows molten or dissolved ionic compounds to conduct electricity
The ions are free to move and also carry a charge
Info on ionic compounds and high melting points
The giant ionic lattices are held together by strong electrostatic forces. It looks a large amount of energy to overcome these forces, meaning a high melting point (801c for sodium chloride)
What causes high melting points in ionic compounds
Strong electrostatic forces within the giant lattice
Info on ionic compounds and dissolving in water
Water molecules are polar, part of the molecule has a small negative charge, and the other part has small positive charge. The water pulls the ions away from the lattice and causes it to dissolve
Why do ionic compounds tend to dissolve
The water molecules pulls the ions away from the lattice
What is a molecule
A Group of atoms bonded together
What happens when groups of atoms are bonded together
A molecule is formed
Molecules are the ……….. Parts of a compound that can take part in chemical reactions
Smallest
What is the smallest part of a compound that can take part in a chemical reaction
A molecule
Does it matter if the atoms are the same or different in a molecule
No, they will still bond together to form a molecule
What type of bond holds together molecules
Strong covalent bonds
Strong covalent bonds hold together…
Molecules
What are giant covalent structures
They are structures which have a huge network of covalently bonded atoms.
What is the name of the structure with a huge network of covalently bonded atoms
Giant covalent structure
Another name for giant covalent structure
Macromolecular structures
Macromolecular structures are the same as….
They are made up of what….
Giant covalent structures
A huge network of covalently bonded atoms
Why can carbon atoms form giant covalent structures
They can each form 4 strong covalent bonds
Because carbon atoms can each form 4 strong covalent bonds, this means they can form….
Giant covalent structures/ Macromolecular
Structure of graphite
The carbon atoms are arranged in sheets of flat hexagons covalently bonded with 3 bonds each. The fourth outer electron of each carbon atom is delocalised.
The sheets of hexagons are bonded together by weak vdw forces
Properties of graphite
The weak bonds between the layers are easily broken, so layers can slide over each other. Graphite feels slippery, used as dry lubricant and pencils
The delocalised electrons aren’t attached to any particular carbon atoms and are free to move along the sheets so can carry an electric current
The layers are quite far apart compared to the length of the covalent bonds, so graphite has a low density. Used to make strong, lightweight sports equipment
Because of the strong covalent bonds in the hexagon sheets, graphite has a very high melting point ( it sublimes at over 3900K)
Graphite is insoluble in any solvent. The covalent bonds in the sheets are too difficult to break
What does sublimes mean
The molecule changes straight from a solid to a gas, skipping the liquid stage
Why is graphite insoluble in any solvent
The covalent bonds within the sheets are too difficult to break
Why can graphite conduct electricity
The delocalised are not attached to a particular carbon atom so are free to move along the sheets, so can carry an electric current
Why does graphite have such a high melting point
Due to the strong covalent bonds within the hexagon sheets
Why does graphite have a low density
The layers are quite far apart compared with the length of the covalent bonds
Why does graphite feel slippery
There are weak bonds between the layers which are easily broken, so the layers can slide over each other
Structure of diamond
Each carbon atom is covalently bonded to 4 other carbon atoms. The atoms arrange themselves in a tetrahedral shape (crystal lattice structure)
Why do diamond gemstones sparkle
It’s structure makes it reflect light
Properties of diamond
Very high melting point (sublimes at over 3800K)
Extremely hard (used in diamond-tipped drills and saws)
Vibrations travel easily through the stiff lattice = good thermal conductor
It can’t conduct electricity- all the outer electrons are held in localised bonds
Insoluble
What does molecular shape depend on
The number of pairs of electrons in the outer shell of the central atom
Electron pairs exist as
Charge clouds
What is a charge cloud
An area where you have a really large chance of finding an electron pair
What do the electrons do inside the charge cloud
They don’t stay still, they are constantly moving around
Bonding pairs and lone pairs of electrons exist as….
Charge cloud
Do Electron charge clouds repel or attract each other? Why?
Repel, as they are all negatively charged
What affects how much a charge cloud repels other charge clouds
The shape of the charge cloud
Which repels the most? Lone pair charge clouds or bonding pair charge clouds?
Lone pair charge clouds. So the greatest angles are between lone pairs of electrons, and bond angles between bonding pairs are often reduced because they are pushed together by lone pair repulsion
What does the Valence-Shell Electron-Pair Repulsion Theory mean
That the shapes and bond angles of many molecules can be predicted
What is the name of the rule that means that shapes and bond angles of molecules can be predicted
The Valence Shell Electron Pair Repulsion Theory
Write down the order of the strength of repulsion between different kinds of electron pair
Lone pair/ lone pair bond angles are the biggest
Lone pair/ bonding pair bond angles are the second biggest
Bonding pair/ bonding pair bond angles are the smallest
Molecules containing hydrogen bonding are usually…..
organic, containing OH or NH groups
Properties of molecules containing hydrogen bonding
High melting and boiling point because of the extra energy needed to break the hydrogen bonds
Why is ice less dense then liquid water
Ice has more hydrogen bonds, hydrogen bonds are quite long. So the H2O molecules in ice are further apart, making it less dense
The greater the difference in electronegativity the ……….. …….. The bond
More polar
When the elements have pretty similar electronegativities, will the bonds be polar or non polar
Non polar, as they are attracted to the nuclei by almost the same amount
What happens if you put an electrostatically charged rod next to a jet of a polar liquid
The liquid will move towards the rod, as it is polar and the molecule contains permanent dipoles. It doesn’t matter if the rod is positively or negatively charged, the polar molecules in the liquid can turn around so the oppositely charged end is attracted towards the rod
Van der Waals forces causes ……. atoms and molecules to be ……… to each other
All
Attracted
Van der Waals forces can hold molecules in a…..
Lattice
Example of van der waal lattice
Iodine….iodine atoms are held together in pairs by strong covalent bonds to form molecules
But the molecules are then held together in a molecular lattice arrangement by weak van der waals attractions
Stronger van der waals forces mean …….. boiling points
Higher
Not all van der waals forces are the same strength-larger molecules have larger electron clouds meaning stronger vdw forces. Molecules with greater surface areas also have stronger vdw forces because they have a more exposed electron cloud. Stronger vdw= higher boiling point
Metal atoms exist as…..
Giant metallic lattice structures
Structure of metals
Giant metallic lattice structure
The outermost shell of electrons of a metal atom is delocalised-the electrons are free to move about the metal. This leaves a positive metal ion. The positive metal ions are attracted to the delocalised negative electrons. They form a lattice of closely packed positive ions in a sea of delocalised electrons
Properties of metals
The number of delocalised electrons per atom affect the melting point. The more there are the stronger the bonding will be and the higher the melting point. Mg2+ has two delocalised electrons per atom, so it’s got a higher melting point that Na+, which only has one. The size of the metal ion and the lattice structure also affect the melting point.
As there are no bonds holding specific ions together, the metal ions can slide over each other when the structure is pulled, so metals are malleable and ductile (can be drawn into a wire).
The delocalised electrons can pass kinetic energy to each other= good thermal conductor
Metals also good electrical conductors because the delocalised electrons can carry a current.
Metals are insoluble, except in liquid metals, because of strength of metallic bonds.
Physical properties of solids
Particles very close together= high density, incompressible
The particles vibrate about a fixed point and can’t move about freely
Physical properties of liquids
Similar density to solids, virtually incompressible
The particles move about freely and randomly within the liquid, allowing it to flow
Physical properties of gases
The particles have loads of energy, much further apart compared sign solids and liquids. The density low, very compressible. The particles move about freely, with not a lot of attraction between them, so they’ll quickly diffuse to fill a container
Melting and boiling points depend on ………… Between ………
Attraction
Particles
The closer the particles are the……
Greater the density
What do delocalised electrons enable structures to do
Conduct electricity
As water is a polar substance, it tends to only dissolve….
Other polar substances
Polar substances will be dissolved by water because….
Water is also a polar molecule
If a solid has a regular structure it’s called a…..
Crystal. The structure is a giant lattice
What is a crystal
A solid with a regular pattern. It has a crystal lattice
What is the exception to the rule of covalent bonds not breaking during boiling and melting
Giant molecular structures, like diamond
Why don’t covalent bonds break during melting and boiling
To melt or boil a simple covalent compound you only have to overcome the vdw forces or hydrogen bonds that hold the molecules together
You don’t need to break the much stronger covalent bonds that hold the atoms together in the molecules
That’s why simple covalent compounds have low melting and boiling points
Example of a simple covalent molecule in terms of melting and boiling point
Chlorine has stronger covalent bonds than bromine. But under normal conditions, chlorine is a gas and bromine is a liquid. Bromine has the higher boiling point because it’s molecules have more electrons, giving stronger vdw forces
Bonding models explain…
How substances behave
Example of a bonding model
‘The physical properties of ionic compounds provide evidence that supports the theory of ionic bonding’
They have high melting points-tells you that the atoms are held together by a strong attraction. Positive and negative ions are strongly attracted, so the model fits the evidence
They are often soluble in water but not in non-polar solvents-tells you that the particles are charged. The ions are pulled apart by polar molecules like water, but not by non-polar molecules. Again, the model of ionic structures fits the evidence
Limitations of bonding models
Dot and cross models of ionic and covalent bonding are great for explaining what’s happening. But it can be oversimplified
Most bonds aren’t purely ionic or purely covalent but somewhere in between. This is due to bond polarisation. Most compounds end up with a mixture of ionic and covalent properties
Why is the electron distribution in a covalent bond not symmetrical
It can be symmetrical if two identical nonmetal atoms are attracted to the same pair of electrons but if two different elements are bonded, one is always higher in electronegativity and pulls the electron pair closer to itself causing a slight asymmetry
