chapter 7: covalent and metallic bonding Flashcards
what is a covalent bond?
- is formed by the sharing of electrons between two atoms
what can covalent bonds be formed between? and what do they form?
can be formed between:
- atoms of the same element
- atoms of different elements
> when atoms combine by sharing electrons, molecules are formed
what are diatomic molecules and what are some examples of it?
- oxygen, chlorine, hydrogen
> each of these elements is made up of two atoms that are joined together by sharing electrons
what are molecules of compounds?
-when atoms of different elements are joined together by covalent bonding
draw the dot-and-cross diagram of methane and carbon dioxide.
answer is on page 111 :)
what are the physical properties of substances with simple molecular structures?
- have low melting and boiling points
- soluble in organic solvents and insoluble in water
- do not conduct electricity in solid, liquid or gaseous state
why do simple molecular substances have low melting and boiling points?
- within each molecule, atoms are held together by strong covalent bonds
- BUT between the molecules, are only weak intermolecular forces holding them together
> can be easily overcomed
> low bp and mp
what is an example simple molecular substances that are insoluble in water and soluble in organic solvents?
- oil dissolves in dichloromethane but not water
exceptions: alcohol and sugar are covalent compounds that can be dissolved in water
why do simple molecular substances not conduct electricity in any state?
- simple molecular substances do not have any free moving electrons or mobile ions to act as charge carriers
what is a giant molecular structure and what are some examples of substances with it?
- a giant network of atoms that are covalently bonded
- eg. diamond, graphite and silicone dioxide
- *diamond & graphite - allotropes of carbon
> different forms of carbon
what are the physical properties of substances with giant molecular structures?
- very high melting points
- do not conduct electricity (except for graphite)
- are insoluble in both water and organic solvents
why do giant molecular substances have high melting and boiling points?
- consists of a large number of atoms that are held together by strong covalent bonds
- to melt or boil, strong bonds must be broken
> requires a large amount of energy - solids at room temperature
why do giant molecular substances not conduct electricity?
- all the outer electrons of the atoms are used
> to form covalent bonds
> no free electrons that move through the structure to conduct electricity
why are giant molecular substances insoluble in both organic solvents and water?
- all atoms are held together by strong covalent bonds
- any forces of attraction between the giant molecule and solvent molecules
> not strong enough to break the strong covalent bonds
what is diamond and what is it used for?
- one of the allotropes of carbon
- when well cut and polished, > valuable gemstone
- tips of drills & the cutting edges of tools
> exceptional hardness and high bp and mp
what is graphite what is it used for?
- another allotrope of carbon
- soft, slippery, fairly unreactive & good conductor of electricity
> used to make dry lubricants, inert electrodes - can be mixed with clay to make pencil lead
what are the physical properties of graphite and why?
- high m.p and b.p
- each carbon atom covalently bonded to 3 other C atoms
> which is bonded to 3 more carbon atoms
> forms a continuous layer of hexagons
- difficult to break strong covalent bonds - soft and slippery
- layers of carbon atoms held loosely by weak intermolecular forces of attraction
> layers of carbon atoms can slide over each other when force is applied - conducts electricity
- C atom has 1 outer electron not used to form covalent bonds
- electrons can move freely along the layers from one C atom to another
> delocalised
-free moving electrons act as charge carriers and allows graohite to conduct electricity
what is a metallic bond?
- force of attraction between positive ions and the ‘ sea of mobile electrons’
what are the physical properties of metals?
- good conductors of electricity
- good conductors of heat
- high densities, melting points and boiling points
- malleable and ductile
why are metals good conductors of electricity?
- while atoms of metal - tightly packed
> outer electrons can break away easily from the atoms - outer electrons able to move freely within the metal lattice
> mobile electrons allow metals to conduct electricity
why are metals good conductors of heat?
- due to the movement of mobile electrons within the metal lattice
> heat energy transferred easily by mobile electrons in structure
why do metals have high densities, melting points and boiling points?
- atoms packed tightly in layers & held together by strong metallic bonds
> large amount of energy needed to break these strong metallic bonds - this accounts for high densities, melting pints and boiling points of metals
why are metals malleable and ductile?
malleable: can be hammered into different shapes
ductile: drawn into wires without breaking
- when force applied on metal, layers of metal atoms
> can slide over each other through ‘ sea of electrons’
a) before force applied,
> metal atoms form an orderly and rigid arrangement
b) when force applies, one layer of metal ions slides over the though the ‘ sea of electrons’
