C2 - Bonding, structure, and the properties of matter Flashcards
What are the three states of matter?
The three states of matter are solid, liquid and gas. The particle model represents particles by small, solid spheres. It describes the arrangement, movement and energy of particles in a substance. The model can be used to explain the physical properties of solids, liquids and gases.
Why do Solids have the properties they do?
have a fixed shape and cannot flow, because their particles cannot move from place to place
cannot be compressed (squashed), because their particles are close together and have no space to move into
Why do Liquids have the properties they do?
Liquids:
- flow and take the shape of their container, because their particles can move around each other
- cannot be compressed, because their particles are close together and have no space to move into
Why do Gases have the properties they do?
- flow and completely fill their container, because their particles can move quickly in all directions
- can be compressed, because their particles are far apart and have space to move into
What does it mean if the forces of attraction between the bond are stronger?
It means that more energy is required to break the bonds so there is a higher melting point.
What are the limitations of the particle model?
The particle model assumes that particles are solid spheres with no forces between them. However:
- particles are not solid, since atoms are mostly empty space
- many particles are not spherical
What are Ionic, Covalent and Metallic bonds?
- They do this in order to achieve a greater level of stability, which is reached when the atom obtains a full outer shell of electrons
- Ionic bonds:
- Takes place when metals and non-metals react by transferring electrons
- The atoms involved are oppositely charged particles (known as ions) in which electron transfer occurs
- The opposite charges attract through electrostatic forces
- Covalent bonds:
- non-metal atoms share pairs of electrons between each other
- Metallic bonds:
- This type of bonding occurs in metals and metal alloys (mixtures of metals)
What is Ionic Bonding?
- All metals lose electrons to other atoms to become positively charged ions
- All non-metals gain electrons from other atoms to become negatively charged ions
The positive and negative charges are held together by the strong electrostatic forces of attraction between oppositely charged ions
* This is what holds ionic compounds together
How do you represent ionic compounds?
- Ionic bonds can be represented diagrammatically using dot and cross diagrams
- These are a simple and quick way to show the formation of an ionic compound
- The electrons from each atom should be represented by using solid dots and crosses
- If there are more than two atoms, then hollow circles or other symbols / colours may be used to make it clear
- The large square brackets should encompass each atom and the charge should be in superscript and on the right-hand side, outside the brackets
- For larger atoms with more electron shells, only the valence shell needs to be drawn
Draw the dot and Cross diagram for Magnesium oxide?
- Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons
- A positive ion with the charge +2 is formed
- Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons
- Two electrons will be transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom
- Oxygen atom will gain two electrons to form a negative ion with charge -2
- The ions are then attracted to one another and held together by electrostatic forces
- The formula of the ionic compound is thus MgO
What happens to Group 1 and 7 Electrons during Ionic bonding?
Group 1 metals lose 1 electron forming a 1+ ion
Group 7 non-metals gain 1 electron forming a 1- ion.
- One Electron
- Passes from the sodium atom
- To the Chlorine Atom
- Both atoms achieve a full outer shell.
What are ionic lattices?
- The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions in which the ions are tightly packed together
- Strong electrostatic forces of attraction are present between oppositely charged ions, holding the lattice together
- Electrostatic forces are strong, acting in all directions - they form the basis of ionic bonding
- As a result of so many electrostatic forces existing in this lattice structure, ionic compounds have high melting and boiling points
- The lattice arrangement exists in three dimensions which allows solid ionic compounds to form regular shapes
- Solid ionic crystals contain huge numbers of ions and so are referred to as giant ionic lattices
- Ions are incredibly small - a single grain of sodium chloride contains trillions of sodium and chloride ions - so models are used to represent the structure of the ionic compound
What are the limitations of the 3D ball and stick model and the 2d space-filling model?
Different types of model are used to represent giant ionic structures. Each has its advantages and limitations. For example:
- the two-dimensional space-filling model clearly shows the arrangement of ions in one layer, but it does not show how the next layer of ions is arranged
- the three-dimensional ball and stick model shows the arrangement of ions in a larger section of the crystal, but using sticks for bonds is misleading because the forces of attraction between ions actually act in all directions
- the three-dimensional model is also misleading because it shows lots of free space between the ions, which there isn’t
- 3D drawings and models depict the arrangement of ions in space, showing the repeating pattern of ions throughout lattice structures (whereas 2D models only show the arrangement of ions in one layer)
- The 3D ball and stick model shows the arrangement of oppositely charged ions but represents ionic bonds as sticks between ions; in reality an ionic bond is an electrostatic force of attraction that acts in all directions around an ion
- Another limitation of the 3D ball and stick model is that it incorrectly depicts space existing between individual ions whereas the 3D space-filling model is more accurate (there is not much space between separate ions)
- It is difficult to represent the relative sizes of the ions in relation to each other correctly in any model
Why do Ionic compound have very high melting and boiling points?
- Ionic compounds are made of charged particles called ions which form a giant lattice structure
- Ionic substances have high melting and boiling points due to the presence of strong electrostatic forces acting between the oppositely charged ions
- These forces act in all directions and a lot of energy is required to overcome them
The strength of the ionic bonds depends on the charge on the ions. Ions with higher charge will have stronger forces between them, so will need more energy in order to overcome these forces.
Why cannot ionic compound conduct electricity when they are solid?
- For electrical current to flow there must be present freely moving charged particles such as electrons or ions
- Ionic compounds can conduct electricity in the molten state or in solution as they have ions that can move and carry charge
- They cannot conduct electricity in the solid state as the ions are in fixed positions within the lattice and are unable to move
A substance can conduct electricity if:
- it contains charged particles, such as ions, and
- these particles are free to move from place to place
What is Covalent bonding?
- A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the outer shells of the atoms. An atom that shares one or more of its electrons will complete its outer shell. Covalent bonds between atoms are very strong
- Covalently bonded substances may consist of small molecules or giant molecules
- Weak intermolecular forces exist between individual molecules
- Electrons on the outer shell which are not involved in the covalent bond(s) are called non-bonding electrons
Simple covalent molecules do not conduct electricity as they do not contain free electrons
What are simple Molecules?
- Covalent substances tend to have small molecular structures, such as Cl2, H2O or CO2
- These small molecules are known as simple molecules
- You need to be able to describe and draw the structures of the following molecules using dot-and-cross diagrams: hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2), hydrogen chloride (HCl), water (H2O), ammonia (NH3) and methane (CH4)
- The correct dot and cross diagrams for these molecules are shown below:
What are Giant Covalent Structures?
- Not all covalent molecules are small; covalent molecules can also be very large
- For example, polymers and giant covalent structures
- Common polymers include polythene which is used extensively in plastic bags and polyvinyl chloride (PVC) which has many industrial applications, most notably in the production of water pipes.
Some covalently bonded substances have giant covalent structures, such as graphite, diamond, and silicon dioxide
* These substances form giant crystal structures made from many atoms held together by covalent bonds
Pros and Cons of the ball and stick model
Ball and Stick Model
- Advantages:
- Useful for illustrating the arrangement of atoms in 3D space
- Especially useful for visualizing the shape of a molecule
- Disadvantages:
- Fails at indicating the movement of electrons
- The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller
Pros and Cons of 2D models
- dvantages:
- Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model
- Adequately indicate what atoms are in a molecule and how they are connected
- Disadvantages:
- Fail to illustrate the relative sizes of the atoms and bonds
- Cannot give you an idea of the shape of a molecule and what it looks like in 3D space
Draw a dot and Cross diagram for Ammonia
To work out how many circles to draw for a simple molecular substance and how to label them, look at the formula. For example, the formula for ammonia is NH3. For this, draw four circles, one labelled N and three labelled H. Each of the three H circles overlaps the N circle.
Nitrogen is in group 5 so it forms three covalent bonds. There are three shared spaces between the circles, so add a dot and cross to each one.
Finally, add in the non-bonding outer electrons. Nitrogen atoms have five outer electrons. Three of these are shared, which leaves two electrons that do not take part in bonding.
Double and Triple Bonds
Some molecules contain a double bond, which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms between them.
A nitrogen molecule is made up of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons.
Why are small molecules poor conductors of electricity?
- They are poor conductors of electricity as there are no free ions or electrons to carry the charge. There is no overall electric charge.
- Most covalent compounds do not conduct at all in the solid state and are thus insulators
- Common insulators include the plastic coating around household electrical wiring, rubber and wood
What is the difference between Intermolecular forces and Covalent bonds?
- Small molecules have covalent bonds joining the atoms together, but intermolecular forces that act between neighbouring molecules
- They have low melting and boiling points as there are only weak intermolecular forces acting between the molecules
- These forces are very weak when compared to the covalent bonds and so most small molecules are either gases or liquids at room temperature
- Often the liquids are volatile
- As the molecules increase in size the intermolecular forces also increase as there are more electrons available
- This causes the melting and boiling points to increase
What is Metallic Bonding?
- Metals consist of giant structures of atoms arranged in a regular pattern. The electrons from the outer shells of the metal atoms are delocalised, and are free to move through the whole structure. This sharing of delocalised electrons results in strong metallic bonding. The valence electrons no longer belong to any specific metal atom and are said to be delocalised. This means there are strong electrostatic forces of attraction which are called metallic bond between the metal and non-metal ions.
- Lots of energy is required to break the bonds.
What are the properties of Metallic substances?
- Metallic bonds are very strong and are a result of the attraction between the positive metal ions and the negative delocalised electrons within the metal lattice structure
- Metals thus have very high melting and boiling points and are solids at room temperature, with the exception of mercury which is a liquid
- They are usually insoluble in water although some do react with it
- Metals are good conductors of heat and electricity due to the delocalised electrons
- The layers of atoms in metals can slide over each other meaning metals are malleable and can be hammered and bent into shapes or rolled into flat sheets
Why do Alloys more harder?
- Alloys are mixtures of metals, where the metals are mixed together physically
- They can also be made from metals mixed with nonmetals such as carbon
- Alloys often have properties that can be very different to the metals they contain, for example they can have greater strength, hardness or resistance to corrosion or extreme temperatures
- Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal
Brass is a common example of an alloy which contains 70% copper and 30% zinc
Why are metals good conductors of Electricity?
- Metals have free electrons available to move and carry charge throughout the metal lattice structure
- Electrons entering one end of the metal cause a delocalised electron to displace itself from the other end
- Hence electrons can flow so electricity is conducted
- Copper is used extensively in the production of electrical wiring due to its excellent malleability and electrical conductivity
Why are metals good conductors of Heat?
- Similarly, metals are also good conductors of heat
- The delocalised electrons are free to move and can also carry thermal energy throughout the metal lattice structure
- Some metals are better conductors of heat energy than others
What is Melting, Boiling, Freezing, Evaporation, Condensation
Melting
- Melting is when a solid changes into a liquid
- The process requires heat energy which transforms into kinetic energy, allowing the particles to move
- It occurs at a specific temperature known as the melting point which is unique to each pure solid
Boiling
- Boiling is when a liquid changes into a gas
- This requires heat which causes bubbles of gas to form below the surface of a liquid, allowing for liquid particles to escape from the surface and from within the liquid
- It occurs at a specific temperature known as the boiling point which is unique to each pure liquid
Freezing
- Freezing is when a liquid changes into a solid
- This is the reverse of melting and occurs at exactly the same temperature as melting, hence the melting point and freezing point of a pure substance are the same
- Water for example freezes and melts at 0 ºC
- It requires a significant decrease in temperature (or loss of thermal energy) and occurs at a specific temperature which is unique for each pure substance
Evaporation
- When a liquid changes into a gas
- Evaporation occurs only at the surface of liquids where high energy particles can escape from the liquids surface at low temperatures, below the boiling point of the liquid
- The larger the surface area and the warmer the liquid/surface, the more quickly a liquid can evaporate
- Evaporation occurs over a range of temperatures, but heating will speed up the process as particles need energy to escape from the surface
Condensation
- When a gas changes into a liquid, usually on cooling
- When a gas is cooled its particles lose energy and when they bump into each other, they lack energy to bounce away again, instead grouping together to form a liquid
Sublimation
- When a solid changes directly into a gas
- This happens to only a few solids, such as iodine or solid carbon dioxide
- The reverse reaction also happens and is called desublimation or deposition
What is Particle Theory?
- Particle theory explains how matter changes state depending on the energy and forces present between the particles in the substance
- The amount of energy needed to change from a solid to a liquid and from a liquid to a gas depends on the relative strength of the forces acting between the particles
- There are many different types of substances which contain different amounts of elements and compounds
- Since each substance contains different particles, then the amount of energy needed to induce a change of state is different for each individual substance
- The stronger the forces between the particles, the higher the energy needed for melting and boiling to occur
- When substances are heated, the particles absorb thermal energy which is converted into kinetic energy
- Heating a solid causes its particles to vibrate more and as the temperature increases, they vibrate so much that the solid expands until the bonds break and the solid melts
- On further heating, the now liquid substance expands more and some particles at the surface gain sufficient energy to overcome the intermolecular forces and evaporate
- When the b.p. temperature is reached, all the particles gain enough energy for the intermolecular force to break and the molecules to escape as the liquid boils
- While changing state, the temperature of the substance remains the same as the heat energy goes into breaking the bonds between the particles
- This is called latent heat
- The entire process can be summarized in a diagram called a heating and cooling curve
What are Polymers?
- Polymers are large molecules of high relative molecular mass and are made by linking together large numbers of smaller molecules called monomers
- Each monomer is a repeat unit and is connected to the adjacent units via strong covalent bonds
- The intermolecular forces acting in between polymer chains are larger than those in between simple molecules so polymers are usually solid at room temperature
- Examples of polymers include polythene and polychloroethene, commonly known as PVC
- Many everyday materials such as resins, plastics, polystyrene cups, nylon etc. are polymers
- These are manufactured and are called synthetic polymers
- Nature also produces polymers which are called natural or biological polymers
- Examples include DNA, proteins, silk and wool
How do you draw Polymers?
- Polymers are represented using a specific notation which is shown below using polythene as an example
- You can spell it polythene or polyethene - both are acceptable
- The bonds on either side of the polymer must extend outside the brackets (these are called extension or continuation bonds)
- A small subscript n is written on the bottom right hand side to indicate a large number of repeat units
The relationship between the monomer, repeating unit and polymer is illustrated in this second example using polychloroethene
* Notice that the chlorines do not necessarily have to be drawn ‘up’ or ‘down’ from the chain, as long as every carbon has the correct atoms attached
What are the Properties of Giant Covalent Structures?
- They have high melting and boiling points as they have many strong covalent bonds & Large amounts of heat energy are needed to overcome these forces
- Most cannot conduct electricity as they do not have free electrons nor charged particles but there are some exceptions such as graphite and graphene
- Diamond, graphite, buckminsterfullerene and graphene are all made from carbon
What is a diamond?
- Diamond and graphite are allotropes of carbon
- Both substances contain only carbon atoms but due to the differences in bonding arrangements they are physically completely different
- NO ELECTRONS free
- In diamond, each carbon atom bonds with four other carbons, forming a tetrahedron
- All the covalent bonds are identical, very strong and there are no intermolecular forces
What is Graphite?
- Each carbon atom in graphite is bonded to three others forming layers of hexagons, leaving one free electron per carbon atom
- These free electrons migrate along the layers and are free to move and carry charge, hence graphite can conduct electricity
- The covalent bonds within the layers are very strong, but the layers are attracted to each other by weak intermolecular forces, so the layers can slide over each other making graphite soft and slippery
- High Melting and boiling point due to the many covalent bonds
What is Graphene?
- Graphene consists of a single layer of graphite which is a sheet of carbon atoms covalently bonded forming a continuous hexagonal layer
- It is essentially a 2D molecule since it is only one atom thick
- It has very unusual properties make it useful in fabricating composite materials and in electronics
What are the properties of Fullerenes?
Fullerenes are molecules of carbon atoms with hollow shapes. Their structures are based on hexagonal rings of carbon atoms joined by covalent bonds. Some fullerenes include rings with five or seven carbon atoms. Two examples of fullerenes are buckminsterfullerene and nanotubes.
Buckminsterfullerene
Buckminsterfullerene was the first fullerene to be discovered. Its molecules are made up of 60 carbon atoms joined together by strong covalent bonds. Molecules of C60 are spherical.
There are weak intermolecular forces between molecules of buckminsterfullerene. These need little energy to overcome, so buckminsterfullerene is slippery and has a low melting point.
What are Nanotubes?
- Graphene can also be rolled into a cylinder to produce an interesting type of fullerene called a nanotube
- These have high tensile strength and are resistant to breaking or stretching
- As in graphene, nanotubes can also conduct electricity which makes them useful in composites and specialised materials, electronics and nanotechnology
