chemistry - chemical bonding

Question 1

what is the definition of the electronegativity of an atom?

Answer 1

the electronegativity of an atom is a measure of its ability to attract the electron in a covalent bond to itself.

Question 2

what are the factors that affect electronegativity? how do they affect electronegativity?

Answer 2

1. effective nuclear charge, across the period the nuclear charge increases while shielding effect remains relatively constant. effective nuclear charge increases, and electronegativity increases across the period
2. atomic radius, elements in the same group experience roughly the same effective nuclear charge as both nuclear charge and shielding effect increase down the group. however, as the number of quantum shells increase, atomic radius increases and electronegativity decreases down the group

Question 3

what bond is formed between atoms of similar electronegativity?

Answer 3

atoms with similar electronegativity experience electron sharing and thus form covalent bonds

Question 4

what bond is formed between atoms with large difference in electronegativity?

Answer 4

atoms with large difference in electronegativity experience electron transfer and thus form ionic bonds

Question 5

what are the factors that affect the strength of metallic bonding? how do they affect metallic bonding?

Answer 5

1. number of valence electrons contributed per metal atom, the larger the number of valence electrons contributed per atom, the greater the number of delocalised electrons and the stronger the metallic bonding
2. charge and radius of the metal atom, the higher the charge and the smaller the radius of the metal cation, the higher its charge density and the stronger the metallic bonding

Question 6

what is the definition of charge density?

Answer 6

charge density is the amount of charge per unit surface area of that ion. we can compare the charge densities of two ions by comparing their ionic charge/ionic radius ratio

Question 7

what are the directional differences between ionic bonding and covalent bonding?

Answer 7

ionic bonding is non-directional and extends indefinitely in all directions while a covalent bond is directional as one covalent bond can only be formed between two specific atoms

Question 8

what is the definition of the coordination number of an ion?

Answer 8

the coordination number of an ion is the number of ions that surround another ion of the opposite charge in an ionic lattice

Question 9

what factors do the coordination number depend on?

Answer 9

1. relative sizes of the ion, a small ion typically has a lower coordination number as there will be less space for ions of the opposite charge to surround it
2. relative charges, a cation a higher charge will have a higher coordination number as compared to a cation with a lower charge to gain charge neutrally

Question 10

what is the definition of lattice energy?

Answer 10

lattice energy is defined as the heat evolved when 1 mol of a pure ionic solid is formed from its constituents gaseous ions.

Question 11

what are the factors which determine the magnitude of lattice energy?

Answer 11

1. charge of the ion, the higher the charge of the ion the larger the magnitude of lattice energy and the stronger the ionic bonding
2. radius of the ion, the smaller the radius of the ion the larger the magnitude of lattice energy and the stronger the ionic bonding

Question 12

why are ionic compounds hard and rigid?

Answer 12

in an ionic lattice, oppositely charged ions are held in fixed positions throughout the crystal lattice by strong ionic bonding. moving the ions out of position requires large amounts of energy to overcome these bonds

Question 13

why are ionic compounds brittle?

Answer 13

if a strong enough force is applied, it will force ions of like charges to move next to each other, and repulsion between ions of like charges will cause the lattice to shatter.

Question 14

what is the condition required for covalent bonding?

Answer 14

in order for a covalent bond to be formed, the orbitals of the two atoms must overlap. the shared electrons of bond pairs occupy the region between the two nuclei formed by the overlap

additionally, there can only be one sigma bond between two atoms as it is not possible for a second pair of orbitals to overlap head-on. for a pi bond to form, a sigma bond must be formed first

Question 15

what is a sigma bond formed by?

Answer 15

a sigma bond is formed by ‘head-on’ overlap of two orbitals

Question 16

what is a pi bond formed by?

Answer 16

a pi bond is formed by ‘side-on’ overlap of two orbitals

Question 17

why is a pi bond weaker than a sigma bond?

Answer 17

a side-on overlap for a pi bond is poorer than head-on overlap for a sigma bond, as the degree of overlap is smaller for the former.

Question 18

what are the requirements for the formation of a pi bond?

Answer 18

1. between two atoms, there can only be one sigma bond as it is not possible for a second pair of orbitals to develop head-on as well
2. for a pi bond to form, a sigma bond must first be formed

Question 19

what is defined as the bond energy?

Answer 19

bond energy is the average amount of energy required to break 1 mole of a covalent bond in the gaseous state to form gaseous atoms

Question 20

what does bond strength depend on?

Answer 20

1. degree or orbital overlap between the two bonded atoms. smaller atoms form shorter and stronger bonds, as the overlap between orbitals is more effective
2. how strongly the electrons in the overlap region are attached to the nuclei of the atoms. multiple bonds are stronger and shorter than single bonds. when more electrons are being shared, the attraction between the two positively charged nuclei and the shared electrons is stronger.

Question 21

what are the exceptions to the octet rule?

Answer 21

1. in beryllium chloride and boron fluoride, the central atoms have fewer than 8 electrons in their valence shell. as such, they are electron deficient species
2. in phosphorus pentachloride, the central phosphorus atom has 10 electrons in its valence shell. phosphorus can form more than 4 bonds by using its d orbitals to accommodate additional electrons. phosphorus and other elements from period three can expand its octet configuration by using energetically accessible d orbitals
3. in some molecules with odd total number of electrons, the central atom has unpaired electrons in the valence shell. species with unpaired electrons are called radicals which are very reactive

Question 22

why can’t period 2 elements accommodate more than four pairs of valence electrons in their valence electrons?

Answer 22

period 2 elements have only four orbitals in their valence shells, and their 3d orbitals are not energetically accessible because they are in an outer quantum shell which is at a much higher energy level

Question 23

what is defined as a dative bond?

Answer 23

a dative bond is a covalent bond where both bonding electrons come from the same atom

Question 24

how is a dative bond formed?

Answer 24

a dative bond is formed when the donor atom has a lone pair of electrons while the acceptor atom has en empty orbital in its valence shell. a dative bond is represented by an arrow from the donor atom to the acceptor atom, or by two dots/crosses

Question 25

what are the two main principles of the valence shell electron pair repulsion theory?

Answer 25

1. the electron pairs around the central atom of a molecule arrange themselves as far as possible so as to minmise their mutual repulsion 2. the repulsion between lone pair and lone pair > lone pair and bond pair > bond pair and bond pair

Question 26

why does a lone pair repel other electron pairs around it more than a bond pair would?

Answer 26

a lone pair is attraction by only one nucleus while a bond pair is attracted by two nuclei. the electron cloud of a lone pair is less ‘elongated’ than a bond pair, and a lone pair of a central atom takes up more space around the atom so it ends up closer to the central atom

Question 27

how many bond pairs and lone pairs does a linear shaped molecule have?

Answer 27

2 bond pairs and 0 lone pairs OR 2 bond pairs and 3 lone pairs

Question 28

how many bond pairs and lone pairs does a trigonal planar molecule have？

Answer 28

3 bond pairs and 0 lone pairs

Question 29

how many bond pairs and lone pairs does a bent molecule have？

Answer 29

2 bond pairs and 1 lone pair OR 2 bond pairs and 2 lone pairs

Question 30

how many bond pairs and lone pairs does a tetrahedral molecule have？

Answer 30

4 bond pairs and 0 lone pairs

Question 31

how many bond pairs and lone pairs does a trigonal pyramidal molecule have？

Answer 31

3 bond pairs and 1 lone pair

Question 32

how many bond pairs and lone pairs does a trigonal bipyramidal molecule have？

Answer 32

5 bond pairs and 0 lone pairs

Question 33

how many bond pairs and lone pairs does a see saw molecule have？

Answer 33

4 bond pairs and 1 lone pair

Question 34

how many bond pairs and lone pairs does a T-shape molecule have？

Answer 34

3 bond pairs and 2 lone pairs

Question 35

how many bond pairs and lone pairs does a octahedral molecule have？

Answer 35

6 bond pairs and 0 lone pairs

Question 36

how many bond pairs and lone pairs does a square pyramidal molecule have？

Answer 36

5 bond pairs and 1 lone pair

Question 37

how many bond pairs and lone pairs does a square planar molecule have？

Answer 37

4 bond pairs and 2 lone pairs

Question 38

describe the sp3 hybridisation of carbon

Answer 38

both carbon atoms in ethane are sp3 hybridised, with each carbon atom sharing four electrons forming 4 sigma bonds. mutual repulsion of the electrons in the four sigma bonds tends to place them as far as possible. the CC bond is an sp3-sp3 sigma bond. each CH bond is a s-sp3 bond.

Question 39

describe the sp2 hybridisation of carbon

Answer 39

each carbon atom shares three electrons, forming 3 sigma bonds. the fourth electron on each carbon atom occupies a 2p orbital which forms a pi bond. mutual repulsion of the electrons around each carbon atom tends to place them as far apart as possible. the CC double bond consists of one sp2-sp2 sigma bond and one p-p pi bond from the unhybridised p orbitals. there are four CH bonds, each of which is a s-sp2 sigma bond.

Question 40

describe the sp hybridisation of ethyne

Answer 40

each carbon atom shared two electrons, forming two sigma bonds. the remaining two electrons on each carbon atom occupies a 2p orbital each which overlaps side-on to form two pi bonds. mutual repulsion of the two groups of electrons around each carbon atom tends to place them as far apart as possible. the CC triple bond consists of one sp-sp sigma bond, and two p-p pi bonds formed by unhybridised p orbitals. each CH bond is an s-sp sigma bond formed

Question 41

why is the HH bond a non-polar covalent bond?

Answer 41

in the hydrogen molecule, the bonding electrons are equally shared between the two hydrogen atoms as they have the same electronegativity.

Question 42

why is the HCl bond a polar covalent bond?

Answer 42

the covalent bond is formed by two different atoms, and the bonding electrons are not shared equally as the atoms have different electronegativities. the Cl atom is more electronegative than the H atom, and bonding electrons will be found on average nearer to Cl than to H, resulting in a partial charge separation. Cl acquires a partial negative charge while H acquires a partial positive charge

Question 43

what does the degree of covalent character depend on?

Answer 43

1. polarising power of the cation. cations that are small and highly charged have higher charge densities and high polarising power, and have a high tendency to distort the anion’s electron cloud, resulting in greater covalent character 2. polarisability of the anion. anions that are relatively large have high polarisability. their valence electrons are further from and less strongly attracted by the nucleus so the electron cloud is easily distorted by a cation, resulting in greater covalent character in the ionic bonding.

Question 44

what are the three types of intermoleular forces between molecules?

Answer 44

1. instantaneous dipole-induced dipole attractions (dispersion forces） 2. permanent dipole-permanent dipole attractions 3. hydrogen bonding

Question 45

how are dispersion forces induced?

Answer 45

the electrons in a molecule are constantly moving.. fluctuations in electron distribution can cause an instantaneous dipole in one molecule, which induces a dipole in a nearby molecule. an attraction arises between the two molecules. this short lived and generally weak attraction is called dispersion forces.

Question 46

what does the strength of dispersion forces depend on?

Answer 46

1. the number of electrons in the molcule. the larger the number of electrons, the more polarisable the electron cloud, the stronger the dispersion forces. 2. the surface area of contact between adjacent molecules. the larger the surface area of contact, the more easily induced dipoles are formed, the stronger the dispersion forces

Question 47

how are permanent dipole-permanent dipole attractions induced?

Answer 47

polar molecules have a permanent dipole, with an uneven electron distribution. the attraction between the oppositely charged ends of two polar molecules is the called the permanent dipole-permanent dipole attractions.

Question 48

what are the two conditions for hydrogen bonding to occur?

Answer 48

1. the molecule must contain a hydrogen atom bonded to a highly electronegative atom 2. there must be an atom with a lone pair of electrons

Question 49

what are the factors that affect the strength and extent of hydrogen bonding?

Answer 49

1. the electronegativity of the atom that is bonded to the H atom. the more electronegative the atom, the stronger the hydrogen bond 2. the number of lone pairs available. the more lone pairs available, the stronger the hydrogen bond

Question 50

when does intramolecular hydrogen bonding occur?

Answer 50

intramolecular hydrogen bonding occurs when the electron deficient H atom in a molecule is in close proximity to the lone pairs on atoms in the same molecule.

Question 51

why is ice less dense than liquid water?

Answer 51

in ice, each water molecule is bonded to four other molecules in a roughly tetrahedral arrangement, producing an open lattice with empty spaces between the water molecules. the more random arrangement of hydrogen bonding in liquid water takes up less space as the water molecules are closer together.

Question 52

describe the structure of graphite using the concept of hybridisation

Answer 52

in graphite, each sp2 hybridised C atom is covalently bonded to three other C atoms, forming a 2D layer of hexagonal C rings. each C atom in graphite has a 2p orbital containing 1 electron. within each layer, the 2p orbitals side-on overlap with each other to form a delocalised pi bonding system. between the layers of C atoms, there is significant dispersion forces due to the large surface area.

Question 53

why does the melting point increase from Na to Al?

Answer 53

from Na to Al, melting point increases as more energy is needed to overcome the stronger metallic bonding due to the increase in number of valence electrons contributed per atom, and the increase in charge density of the cation

Question 54

how do you tell if a solute is soluble in terms of energy changes?

Answer 54

a solute is soluble in a solvent if the energy released from solute-solvent interactions is greater than or comparable to the energy needed to overcome solute-solute interactions and solvent-solvent interactions.