2.1 ionic, covalent & metallic bonding

Question 1

what do ionic bonds form between? what type of attraction is present in an ionic bond?

Answer 1

• ionic bonds form between metals and non-metals in order to achieve the nearest noble gas electron configuration.
• the electrostatic attraction between ions of opposite charges results in ionic bonding.

Question 2

which type of ions do metals form as opposed to non-metals?

Answer 2

metals form cations (positively charged ions), whereas non-metals from anions (negatively charged ions)

Question 3

describe and explain three properties of ionic compounds.

Answer 3

high melting and boiling temperatures - strong electrostatic attractions between ions must be overcome in order to separate the ions.

electrical conductivity when molten or in solution - ions become free to move, so carry an electrical charge.

strong, brittle crystalline substances - break with applied force as same ions repel and force apart the lattice, meaning that ionic compounds cannot be shaped.

Question 4

using your knowledge of the structure of sodium chloride, describe and explain the structure of sodium fluoride.

Answer 4

• when sodium, a metal, reacts with fluoride, a non metal, the giant ionic lattice sodium fluoride is formed.
• both anions and cations are found within giant ionic lattices, with the oppositely charged ions attracting each other.
• the strong electrostatic force between the oppositely charged ions means that sodium fluoride is difficult to break apart and has a range of properties, including high melting and boiling temperatures.

Question 5

what two factors does the strength of the electrostatic attraction between ions depend on?

Answer 5

the charges of the ions (oppositely charged ions attract) and the size of the radii of the ions.

Question 6

why do metal ions have smaller ionic radii than their corresponding atoms?

Answer 6

• more protons attract less electrons.
• therefore, metal ions are smaller than metal atoms.

Question 7

why do non-metal ions have larger ionic radii than their corresponding atoms?

Answer 7

• more electrons are attracted by the same number of protons.
• therefore, non-metal ions are larger than non-metal atoms.

Question 8

why does ionic radius decrease down a group?

Answer 8

ionic radius decreases down a group because extra electron shells are added.

Question 9

what are isoelectronic ions?

Answer 9

ions of different atoms with the same number of electrons and electronic configuration.

Question 10

why does the ionic radius of a set of isoelectronic ions decrease as atomic number increases?

Answer 10

• this trend occurs because of increasing nuclear attraction.
• the electrons are pulled closer to the nucleus which causes the ionic radius to decrease.

Question 11

why is the melting temperature of sodium fluoride (993°C) lower than that of magnesium oxide (2852°C)?

Answer 11

• the electron structures of Na⁺ and Mg²⁺ are the same and the two ions have approximately the same radii.
• the electron structures of F⁻ and O²⁻ are the same and these two ions also have approximately the same radii.
• charges on Mg²⁺ and O²⁻ are twice as large as those on Na⁺ and F⁻.
• ionic bonding in MgO is much stronger than in NaF so MgO has a higher melting temperature.

Question 12

what is a covalent bond?

Answer 12

a strong electrostatic attraction between two nuclei and the shared pair of electrons found between those nuclei.

Question 13

what is an octet of electrons?

Answer 13

a set of eight electrons in the outer energy level of an atom.

Question 14

what is the octet rule?

Answer 14

when forming compounds, atoms tend to gain, lose or share electrons in order to obtain the nearest noble gas configuration.

Question 15

what does the term ‘bond length’ refer to?

Answer 15

the distance between two nuclei in a covalent molecule, where the attractive and repulsive forces balance each other.

Question 16

why do smaller atoms form shorter bond lengths than larger atoms?

Answer 16

• higher electron density and stronger attraction between smaller atoms.
• this results in a higher bond enthalpy, and therefore a shorter bond length.
• larger atoms form longer bond lengths as there are more electrons to shield the nuclei, which reduces nuclear attraction.

Question 17

why are multiple bonds shorter and stronger than single bonds?

Answer 17

• multiple bonds are shorter than singular bonds as nuclei are closer together.
• multiple bonds are stronger than singular bonds because there is an increase in the strength of the electrostatic attraction between the positively charged nuclei and negatively charged electrons.

Question 18

what is a pure covalent bond?

Answer 18

the even sharing of electron pairs between two atoms of the same element.

Question 19

what is a dative coordinate bond?

Answer 19

a dative coordinate (or covalent) bond is where one atom donates both electrons to a bond.

Question 20

describe and explain three properties of giant covalent structures.

Answer 20

good thermal conductors - vibrations can travel easily through the stiff lattice arrangement.

insoluble - insoluble in all polar solvents, as covalent structures do not contain ions. due to the covalent bonds, atoms are more strongly attracted to other atoms in the molecule as opposed to solvent molecules.

poor electrical conductors - all bonding electrons in a covalent lattice are held in localised covalent bonds, so there are no charged ions or delocalised electrons present in the lattice.

Question 21

one feature of giant covalent structures is that they are poor electrical conductors. explain why graphite is an exception to this rule.

Answer 21

• graphite is an allotrope of carbon.
• carbon atoms form sheets, which are held together by relatively weak forces.
• each carbon atom shares three of its outer shell electrons with three other carbon atoms, leaving the fourth outer electron in each atom fairly free to move between the sheets. this makes graphite an electrical conductor.

Question 22

metal elements exist as giant metallic lattice structures. explain how giant metallic lattices are held together by metallic bonding.

Answer 22

in metallic lattices, the electrons in the outermost shell of the metal atoms form a ‘sea’ of delocalised electrons with a lattice of metal cations.

Question 23

with reference to metallic bonding, explain why the melting points of metals are generally high.

Answer 23

• the melting points of metals are generally high due to strong metallic bonding.
• the number of delocalised electrons present in a lattice affects the melting point.
• the more electrons present, the stronger the bonding will be and the higher the melting point.

Question 24

with reference to metallic bonding, explain why metals are malleable and ductile.

Answer 24

• there are no bonds holding specific ions together, and the layers of positive metal ions are separated by layers of electrons.
• the layers of metal ions can slide over each other without disrupting the attraction between the cations and electrons.
• this makes metals both malleable and ductile.

Question 25

with reference to metallic bonding, explain why metals are good electrical conductors, and why impurities can reduce electrical conductivity.

Answer 25

• the delocalised electrons can pass kinetic energy to each other, and carry a current.
• impurities can reduce electrical conductivity by reducing the number of electrons that are free to move and carry a charge. this causes the electrons to transfer to the impurities and form anions.