Topic 3: Bonding and Structure

Question 1

physical evidence for exisiting ions:

Answer 1

-ionic substances are brittle and they are unable to conduct electricity in solid form
-however, when ionic substances are molten or aqueous the ions are free to move and so can conduct electricity in these states
-these properties, in comparison to metals which are malleable and conduct electricity when solid, can be explained by the model of oppositely charged ions in a giant ionic lattice

Question 2

electron density maps: ionic bonding

Answer 2

-electron density maps show the region around the nucleus in which electrons are distributed
-a high density corresponds to a high probability of an electron being there
-different types of chemical bonds have different electron density maps
-in covalent bonds, for instance, there is a high electron density between the bonding nuclei, whereas in ionic bonds there is a low electron density between ions
-this is explained by the fact that ionic bonds are formed by the physical transfer of electrions whereas covalent bonds are formed by the sharing of electrons
-electron density maps therefore allow chemists to work out the type of bondingpresent, and also the distance between ions in an ionic lattice

Question 3

ion migration:

Answer 3

-in electrolysis, ions are free to move and ions move to the oppositely charged electrode- where they gain or lose electrons to form atoms
-this ion migration can be explained by the electrostatic attraction that results between oppositely charged species

Question 4

ionic bonding:

Answer 4

-ionic bonding occurs between a metal and a non-metal
-electrons are transferred from the metal to the non-metal to achieve full outer shells, according to the octet rule
-the octet rule states that atoms will often react to form outer shell containing 8 electrons
-when the electrons are transferred, it creates charged particles called ions
-oppositely charged ions attract through strong electrostatic forces to form a giant ionic lattice
-positive ions are formed when an atom loses at least one electron
-negative ions are formed when an atom gains at least one electron

Question 5

ionic bond strength:

Answer 5

-the charge of an ion is related to the strength of the ionic bond that forms
-ions with a greater charge will have a greater attraction to the other ions, resulting in stronger forces of attraction and therefore stronger ionic bonding
.larger ions that have a greater ionic radius will have a weaker attraction to the oppositely charged ion because the ttractive forces have to act over a greater distance

Question 6

dot and cross diagrams:

Answer 6

-cations (+ve) and anions (-ve) can be represented using dot and cross diagrams-and so can ionic bonding
-the electrons being transferred from the cation can be displayed on the outer shell of the anion

Question 7

ionic radii: down a group

Answer 7

-as you move down the group, the number of electron shells increases, and therefore the ionic radius increases

Question 8

ionic radii: across a period

Answer 8

-groups 1 and 2: the atoms in these groups lose electrons to form positive ions
-as you go across the period, the magnitude of the positive charge increases for the same amount of electron shielding
-this means there is a greater electrostatic attraction which pulls the outer electrons to form this configuration
-the ions formed are said to be isoelectronic as they have the same electronic configuration (but lose different numbers of electrons to form this configuration)
-groups 6, 7 and 8: the atoms in these groups gain electrons to form negative ions
-as you go across the period the additional proton makes less of a difference, so the only change is a slight decrease in ionic radius

Question 9

polarisation:

Answer 9

-in ionic bonds, polarisation refers to the distortion of the electron cloud
-in the perfect ionic model, ions are spherical with no distortion, in reality, the positive ion will attract some of the negative ion’s electron cloud towards itself

Question 10

polarisation power:

Answer 10

-the polarising power of a cation and the polarisability of an anions depends on the radius and charge of ions
-a high charged cation will have a strong electrostatic attraction with an anion which will distort the anion’s electron cloud, if a cation had a smaller radius, there will generally be a greater concentration of charge in a small area, causing a distortion of the electron could therefore, the cation’s polarising power increases with an increase in charge and decrease in radius
-the more negatively charged an anion is, the greater the electron cloud, the larger the electron cloud, the more easily it is distorted. Also a larger radius means the outer electrons are held more loosely, allowing them to be more easily distorted however, the anion’s polarisability increases as the radius increases and charge becomes more negative

Question 11

covalent bonding: physical evidence

Answer 11

-giant covalent structures have high melting and boiling points due to strong covalent bonds
-they have no free ions or electrons (except graphite and graphene) so generally cannot conduct electricity
-simple covalent molecules have low melting and boiling points because, although the covalent bonds are strong, the intermolecular forces between molecules are weak
-the properties of these covalent structures can all be explained by the model of covalent bonding

Question 12

electron density maps: covalent bonding

Answer 12

-as described before, electron density maps show the regions around nuclei in which electrons are likely to be found
-they can be used to identify the type of bonding present
-covalent bonds involve the sharing of electrons between nuclei, so the electron density map shows a high density of electrons between bonding atoms

Question 13

covalent bonding:

Answer 13

-covalent bonds form between two non-metals
-there is a strong electrostatic attraction between the two nuclei and the shared electrons between them
-electrons are shared between the 2 outer shells in order to form a full outer shell, multiple electron pairs can be shared to produce multiple covalent bonds

Question 14

Covalent bonding: dot and cross diagrams

Answer 14

-the shared paired of electrons can be represented using dot and cross diagrams
-the overlap includes a covalent bond
-the number of electrons within the overlap tells you the nature of the covalent bond:
-2 electrons (1 from each atom): single (sigma) bond, displaced formula represented as -
-4 electrons (2 from each atom): double (sigma and pi) bond, displayed formula represented as =
-6 electrons (3 from each atom): triple bond (sigma, pi, pi) bond, displayed formula represented as ≡
-double and triple bonds can also be shown on dot and cross diagrams with the multiple electron pairs being displayed in the shared segment between two atoms
-the length of a covalent bond is strongly linked to its strength
-short bonds tend to be strong as the atoms are held close together so the forces of attraction are greater, requiring more energy to be overcome
-double and triple bonds are shorter than single covalent bonds, explaining why they are so much stronger

Question 15

dative bonding:

Answer 15

-dative or coordinate bonds form when both of the electrons in the shared pair are supplied from a single atom, it is indicated using an arrow from the lone electron pair
-once a dative bond has formed, it is treated as a standard covalent bond because it reacts in exactly the same way and has the same properties regarding length and strength
-since both electrons come from the same atom in a dative covalent bond, in dot and cross diagrams both electrons in that bond will have the same shape
-in order words, they will both be dots or both be crosses

Question 16

Giant covalent structures:

Answer 16

-macromolecular covalent substances are covalently bonded into a giant lattice structure
-each atom has multiple covalent bonds which are very strong, giving the substance a very high melting point

Question 17

Giant covalent structures: diamond

Answer 17

-is a macromolecular structure made up of carbon atoms each bonded to 4 other carbon atoms
-this forms a rigid tetrahedral structure, making diamond one of the hardest, strongest materials known-which is often why it is used on the tips of drills

Question 18

Giant covalent structures: graphite

Answer 18

-graphite is another macromolecular structure made up of carbon atoms
-however, in graphite, each carbon atom is bonded to 3 others in flat hexagonal sheets
-this means there is 1 delocalised electron per carbon atom
-these electrons can move freely, allowing graphite to conduct electricity
-graphite can therefore be used in an electrode
-the intermolecular forces between layers are weak and can easily slide over each other, meaning graphite can be used as a lubricant

Question 19

Giant covalent structures: graphene

Answer 19

-graphene consists of single, 2D sheets of graphite that are just one atom thick
-these sheets formed of hexagonal carbon rings that create a very strong, rigid material that is extremely lightweight
-delocalised electrons move through each layer allowing it to conduct electricity, making graphene a useful material in electronics

Question 20

Bond polarity in covalent bonds:

Answer 20

-the negative charge around a covalent bond is not spread evenly around the orbitals of the bonded atoms

Question 21

Electronegativity: definition

Answer 21

The power of an atom to attract the electron pair in a covalent bond towards itself

Question 22

Electronegativity: trend

Answer 22

-this ‘power’ is different for every atom depending on its size and nuclear charge
-Electronegativity increases along a period, as you move across a period, atoms have a greater nuclear charge and a smaller covalent radius
-this allows the nucleus to attract the bonding electrons more strongly
-electronegativity decreases down a group, going down a group, atoms increase in size due to the extra electron shells, increasing shielding towards the bonding electrons
-ionic and covalent bonding are the extremes in a continuos scale of bonding
-if the electronegativity between two atoms is great enough, an ionic bond will form between them
-the bond polarity can be permanent or induces depending on the molecule and how it interacts with things around it

Question 23

Permanent dipole:

Answer 23

-if the two atoms that are bonded have sufficiently different electronegatives, a polar bond forms
-the more electronegative atom draws more of the negative charge towards itself and way from the other atom, producing a a- region and a+ region which produces a permanent dipole
-hydrogen fluoride is a polar molecule as fluorine is a lot more electronegative than hydrogen, this causes electrons to be drawn towards the fluorine atom
-polar molecules with a permanent dipole can align to form a lattice of molecules similar to an ionic lattice

Question 24

Polarisability of ions:

Answer 24

-where electronegativity difference is large enough the electrons will not be shared between atoms but will instead spend most of their time localised around one of the atoms only - forming ions
-a cation is able is able to distort the shape of the electron could on a nearby anion
-the extent by which the electron cloud is distorted is known as polarisability
-smaller cations with a greater charge are the most polarising
-anions with a larger ionic radius can have their electron clouds distorted more easily

Question 25

Polar bonds and polar molecules:

Answer 25

-a polar bond results from a large difference in electronegativity between two atoms forming a covalent bond
-tables of electronegativity can be used to workout if a bond will be polar or not
-if the difference in electronegativity between bonding atoms is between 0.4 and 1.7, the bond will be a polar covalent bond
-if the difference in electronegativity is greater than that, the bond will be ionic
-since electronegativity is a periodic trend, elements that are close together on the periodic table will not form polar bonds

Question 26

Polar molecules:

Answer 26

-polar molecules arise when there is an overall difference in polarity across the molecule, due to the arrangement of polar bonds and the geometry of the molecule
-polar molecules must have polar bonds, however a molecule with polar bonds may not necessarily be a polar molecule
-CO2 - the C=O bonds in CO2 are polar, however the molecule is linear so the dipoles created by each polar bond cancel out
-H2O - the O-H bonds in water are polar and the geometry of the molecule is bent, so overall there is a polarity and the molecule is polar

Question 27

Metallic bonding:

Answer 27

-metallic bonding consists of a giant lattice of positively charged ions surround by a ‘sea’ of delocalised electrons
-there are very strong electrostatic forces of attraction between the oppositely charged particles
-the greater the charge on the positive ion, the stronger the attractive force since more electrons are released into the ‘sea’
-ions that are larger in size, such as barium, produce a weaker attraction due to their greater atomic radius

Question 28

Metallic structure (e.g. aluminium):

Answer 28

-substances with metallic structure are often good conductors
-the ‘sea’ of delocalised electrons is able to move and carry a flow of charge
-metals are also malleable because the uniform layers of positive ions are able to slide over one another
-the delocalised electrons prevent fragmentation as they can move around the lattice
-the electrostatic forces of attraction between the positive ions and delocalised electrons are very strong and therefore require a lot of energy to overcome
-this means metallic substances have high melting points and are nearly always solid a room temperature
-Mercury is the only metal which is liquid at room temperature