Bonding Flashcards
What are the three types of strong chemical bonds?
Ionic, covalent and metallic
Definition of an ionic bond
An ionic bond is the electrostatic attraction between oppositely charged ions
Definition of a covalent bond
A covalent bond is the electrostatic force of attraction between the negatively charged pair of electrons and the positively charged nuclei of the adjacent atoms
Explain why nitrogen is a gas at room temperature
Answer in terms of nitrogen’s structure
- Nitrogen is bonded with covalent bonds- very strong
- However, nitrogen is a simple covalent substance
- So intermolecular forces between nitrogen are v. weak
- So v. little energy is required to break them
- Meaning the boiling/melting point of nitrogen is very low
- So at room temperature, the temperature is high enough for nitrogen to be a gas
Graphite and fullerenes are forms of carbon.
Graphite is soft and is a good conductor of electricity. Explain why graphite has these properties.
Answer in terms of structure and bonding.
- In graphite, each carbon atom has 3 electrons in its outer shell, used in covalent bonds with other carbon atoms
- Leaves 1 electron that is free
- Because it is a charged particle that can move
- Electricity could be conducted in this way, making graphite a good conductor
- Graphite is also arranged in layers, that can slide over each other
- Making graphite soft as bonding between these layers isn’t very strong
- This means graphite layers can move and slide, making it soft
A lubricant is a substance that allows materials to move over one another easily. Suggest why graphite is a good lubricant
- Graphite is soft
- because the layers in graphite aren’t bonded strongly together
- So they can slide
- makes graphite a good lubricant
- as it can ease movement between materials
- the layers aren’t bonded strongly because only 1 electron per carbon atom is used for this bonding
Silicon dioxide, SiO2 has a similar structure to diamond. Explain why its melting point in 1710°C
- Silicon dioxide is a giant covalent substance
- so throughout the substance, there are strong covalent bonds that require lots of energy to break, requiring a high melting point.
- all 4 of the electrons in the outer shell of silicon are bonded with strong covalent bonds, increasing energy needed to break the bonds rather than just having 1 strong covalent bond
- This means the melting points is very high, 1710°C
Magnesium reacts with fluorine to produce the ionic compound magnesium fluoride. Describe what happens in terms of electrons when magnesium reacts with fluorine.
- Magnesium is a metal and so has few electrons in its outer shell (for magnesium, 2)
- Means that it is easier for magnesium to lose electrons rather than gain them
- Fluorine is a non-metal so it is easier for it to gain 1 electron than lose all 7 of the electrons in its outer shell
- When they react, each magnesium reacts with 2 fluorine to lose both of its 2 electrons, giving one electron to each
- This means magnesium and fluorine are now stable with a full outer shells, so they form the ionic compound magnesium fluoride
Magnesium oxide is a compound formed from the metal magnesium and the non-metal oxygen. Describe what happens when a magnesium atom reacts with an oxygen atom.
You should refer to electrons in your answer
- Magnesium atom loses electrons
- oxygen atom gains electrons
- two electrons are transferred
- Magnesium ions and oxide ions are formed
Give one limitation of using a dot and cross diagram to represent an ammonia molecule
- Does not show the shape
Ionic formula of ammonium
NH4+ (+1)
Ionic formula of carbonate
CO₃²⁻
(2-)
Ionic formula hydroxide
OH− (1-)
Ionic formula of nitrate
NO3- (1-)
Ionic formula sulfate
SO42- (2-)
What is a compound?
A pure substance made from more than one type of element chemically bonded together
How do elements form compounds?
Through chemical reactions
How are covalent bonds formed?
A covalent bond is formed when a pair of electrons is shared between two atoms
What is ionic bonding?
- Bonding between particles that are oppositely charged ions. Ionic bonding occurs in compounds formed from metals combined with non-metals
What is covalent bonding?
- Bonding in which the particles are atoms which share pairs of electrons. Covalent bonding occurs in most non-metallic elements and in compounds of non-metals
What is metallic bonding?
- Bonding where the particles are atoms which share delocalised electrons. Metallic bonding occurs in metallic elements and alloys
What is an ionic structure?
Ionic compounds are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions in the lattice and this is called ionic bonding.
What is the empirical formula of an ionic compound?
The empirical formula of an ionic compound is the ratio of each type of ion present. In sodium chloride there is one sodium ion for every chloride ion, meaning the ration is 1:1. This gives sodium chloride the formula NaCl
Properties of Ionic Compounds
- Ionic compounds have regular structures (giant ionic lattices) in which there are strong electrostatic forces of attraction in all directions between oppositely charged ions
- These compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds
- When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and so charge can flow.
Structure and bonding: Covalent bonding
- When atoms share pairs of electrons, they form covalent bonds
- These bonds between atoms are strong
- Covalently bonded substances may consist of small molecules
- Some covalently bonded substances have very large molecules, such as polymers
- Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide.
How can the molecular of a covalent substance be worked out?
The molecular formula of a substance can be worked out from a given model or diagram by counting the number of atoms of each type. For example, ammonia is NH3
How can covalent bonds be represented?
Covalent bonds can be represented in a number of different ways:
- The simplest format is a straight line. One line (-) represents a single bond, two lines (=) a double bond, and so on. This model shows how many bonds are present very easily. These models do not show 3D shape. This model also ignores electrons that are not involved in bonding.
Properties of Simple Molecular Substances
- usually gases or liquids
- have relatively low melting points and boiling points
- have only weak forces between the molecules (intermolecular forces)
- It is these intermolecular forces that are overcomes, not the covalent bonds, when the substance melts or boils
- The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
- do not conduct electricity because the molecules do not have an overall electric charge
- Intermolecular forces are weak compared with covalent bonds. This explains the bulk properties of molecular substances
Giant covalent substances: Diamond
In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard, has a very high melting point and does not conduct electricity
Giant Covalent Substances: Graphite
- each carbon atom forms three covalent bonds with three other carbon atoms
- forming layers of hexagonal rings which have no covalent bonds between the layers
- one electron from each carbon atom is delocalised
- Graphite is similar to metals in that it has delocalised electrons
Giant Covalent Substances: Graphene
Graphene is a single layer of graphite and has properties that make it useful in electronics and composites
Giant Covalent Substances: Fullerenes
- molecules of carbon atoms with hollow shapes
- The structure of fullerenes is based on hexagonal rings of carbon atoms but they may also contain rings with five or seven carbon atoms
- first fullerene to be discovered was Buckministerfullerene (C60) which has a spherical shape
- Fullerenes are finding use as transport mechanism for drugs to specific sites in the body, as catalysts and as reinforcements for composite materials
Giant Covalent Substances: Nanotubes
- cylindrical fullerenes
- very high length to diameter ratios.
- Their properties make them useful for nanotechnology, electronics and materials
Structure and bonding: metallic bonding
Metals consist of giant structures of ions arranged in a regular pattern. The electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure. The sharing of delocalised electrons gives rise to strong metallic bonds.
Properties of Metallic Substances
- Metals have giant structures of ions with strong metallic bonding. This means that most metals have high melting and boiling points
- In pure metals, ions are arranged in layers, which allows metals to be bent and shaped.
- Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder. Alloys are harder than pure metals as the differing sized ions prevent the layers of ions from being able to slide over one another
- Metals are good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal. Metals are good conductors of thermal energy because energy is transferred by the delocalised electrons
Why do ionic compounds have high melting and boiling points?
Ionic compounds have giant structures in which the ions are held tightly together by the electrostatic attraction between oppositely charged ions. A lot of thermal energy is needed to overcome the strong ionic bonds. Therefore ionic compounds have high melting and boiling points
Why are ionic compounds brittle?
Ionic compounds are brittle as any small deformation of the lattice can result in like charges being arranged next to one another. This results in repulsion
Why don’t ionic compounds conduct electricity when solid?
In solid ionic compounds, the ions are held firmly in place by the strong electrostatic force, so are unable to move and carry charge
Why do ionic compounds conduct electricity when dissolved in liquid or molten?
When ionic compounds are dissolved in water the ions are separated from one another by the water molecules/ they are freer to move when molten. The ions are then free to move and able to carry charge.
