Topic 2 Bonding, Structure and the Properties of Matter Flashcards
C3
What is ionic bonding?
Ionic bonding is the electrostatic attraction between positive and negative ions.
It is a relatively strong attraction.
How are ionic compounds held together and what are their properties?
- 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 greater the charge on the ions, the stronger the electrostatic forces and the higher the melting point will be. For example, magnesium oxide consists of Mg2+ and O2- so will have a higher melting point than sodium chloride which contains the ions, Na+ and Cl-
- Strong electrostatic forces act in all directions in an ionic solid
- Ionic compounds are usually solid at room temperature and are non-volatile
- They are usually water soluble as both ionic compounds and water are polar substances
- 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
State properties of ionic substances
● High melting and boiling point (strong electrostatic forces between oppositely charged ions)
● Do not conduct electricity when solid (ions in fixed positions).
● Conduct when molten or dissolved in water - ions are free to move.
Give 5 examples of positive ions and 5 examples
of negative ions (give names of negative anions).
What is important when working out a formula of
an ionic compound?
E.g. Positive: Na+, Mg2+, Al3+, Ca2+, Rb+
E.g. Negative: Cl−, Br−, SO42−, NO3−,OH−
(chloride, bromide, sulfate, nitrate, hydroxide).
Ionic compounds are electrically neutral, i.e. positive and negative charges balance each other.
How are ionic compounds formed? Explain in terms of MgO case.
- Reaction of a metal with a non-metal.
- Electron transfer occurs - metal gives away its outer shell electrons to non-metal.
- Mg is in Group II, so has 2 available outer shell electrons.
- O is in Group VI, so can accept 2 electrons to get a full outer shell configuration.
- Mg becomes Mg2+ and O becomes O2− (oxide).
What is a covalent bond?
Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons.
When two atoms share pairs of electrons, they form covalent bonds
Describe the structure and properties of simple molecular covalent substances
- Do not conduct electricity (no ions)
- Small molecules
- Weak intermolecular forces (Weak intermolecular forces exist between individual molecules
For example, in methane, each molecule consists of four hydrogen atoms covalently bonded to a carbon atom, and in between individual methane molecules there are weak intermolecular forces), therefore: - Low melting and boiling points
How are ionic compounds represented?
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
Exam Tip
Remember that in ionic lattice structures, positively charged and negatively
charged ions are arranged in an alternating pattern.
When are molecules formed?
When two or more atoms are covalently bonded together, they form ʻmoleculesʼ
What are shared electrons called in covalent bonding?
Shared electrons are called bonding electrons and occur in pairs
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 is a key difference between covalent bonds and ionic bonds?
A key difference between covalent bonds and ionic bonds is that in covalent bonds the electrons are shared between the atoms, they are not transferred
(donated or gained) and no ions are formed.
What type of molecular structure do covalent substances have?
Covalent substances tend to have small molecular structures, such as Cl2 , H2 O or CO2
These small molecules are known as simple molecules
Small covalent molecules can be represented by dot and cross diagrams
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 (HCl2), water (H2 O), ammonia (NH 3) and methane (CH4 )
See dot and cross diagrams on savemy exams notes pg15
Exam Tip
Exam Tip
Simple covalent molecules are small and can be separated into individual molecular units without breaking any chemical bonds (although there will
still be strong covalent bonds holding the atoms in each individual molecule
together.)
Giant ionic and covalent structures form huge continuous networks of atoms that are bonded together and cannot be separated into individual units without breaking bonds.
How do intermolecular forces change as the mass/size of the molecule increases?
They increase. That causes melting/boiling points to increase as well (more energy needed to overcome these forces).
What are polymers? What are thermosoftening polymers?
Polymers are very large molecules (>100s, 1000s of atoms) with atoms linked by covalent bonds.
Thermosoftening polymers - special type of polymers; they melt/soften when heated. There are no bonds between polymer chains. Strong intermolecular forces
ensure that the structure is solid at room temperature. These forces are overcome with heating - polymer melts.
Common polymers are…
- Polymers are large molecules of high relative molecular mass and are made by linking together large numbers of smaller molecules called monomers.
- Polymers are made from very long carbon chains with a repeating unit
- 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
- 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.
- Synthetic polymers are manifactured (eg materials such as resins, plastics, polystyrene cups, nylon etc.)
- Nature also produces polymers which are called natural or biological polymers Examples include DNA, proteins, silk and wool
Properties: Giant Covalent Substances
- 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 and break down bonds
- 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, buckminsterfuller and graphene are all made from carbon
See also printed table
What are giant covalent substances? Give examples
-Giant covalent structures have a huge number of non-metal atoms bonded to other non-metal atoms via strong covalent bonds
- These structures can also be called giant lattices or macromolecules and have a fixed ratio of atoms
in the overall structure
- Three common macromolecules you should know about are diamond, graphite and silicon dioxide
- Solids
Describe and explain the properties of Diamond (allotropes of carbon).
Diamond
– four, strong covalent bonds for each carbon atom
– very hard (Strong bonds)
– very high melting point (strong bonds)
– does not conduct (no delocalised electrons)
Describe and explain the properties of Graphite (allotropes of carbon).
Graphite
– three covalent bonds for each carbon atom
– layers of hexagonal rings
– high melting point
– layers free to slide as weak intermolecular forces
between layers; soft, can be used as a lubricant
– conduct thermal and electricity due to one delocalised
electron per each carbon atom
Describe and explain the properties of Fullerenes (allotropes of carbon).
– hollow shaped molecules
– based on hexagonal rings but may have
5/7-carbon rings
– C60 has spherical shape, simple
molecular structure (Buckminsterfullerene)
Describe and explain the properties of Nanotubes (allotropes of carbon).
Nanotubes
– cylindrical fullerene with high length to
diameter ratio
- High tensile strength (strong bonds)
- Conductivity (deloc. electrons)
What is Graphene?
a single layer of graphite.
What is metallic bonding?
Forces of attraction between delocalised electrons and nuclei of metal ions.
Metals consist of giant structures of atoms arranged in a regular pattern
Electrons in the outer shell of metal atoms are lost
Individual metal atoms are held together by strong metallic bonds forming a lattice
structure
This type of bonding occurs in metals and metal alloys, which are mixtures of
metal
Within the metal lattice, the atoms lose their valence electrons and become
positively charged metal ions
The valence electrons no longer belong to any specific metal atom and are said to
be delocalised
This means they can move freely between the positive metal ions and act like a
“sea of electrons”
Are Delocalised electrons are transferred as in ionic bonding?
Exam Tip
Delocalised electrons are not transferred as in ionic bonding, they are merely moving freely in the spaces in-between the nuclei of metal atoms.
Thermal Conductivity of Metals
- 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
- Copper is an excellent thermal conductor. For this reason many older cooking utensils were made of copper before the advent of alloys and composites.
How does electrical conductivity work in metals?
- Metals have free electrons available to move and carry charge throughout the metal lattice structure
- Free electrons can also be called mobile or delocalised
- 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
Describe properties of metals
- High melting/boiling points (strong forces of attraction) -
- 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 - Good conductors of heat and electricity (delocalised electrons)
- Malleable, soft (layers of atoms can slide over each other whilst maintaining the attraction forces)
- 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
- 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
- The properties of metals can be modified, sometimes significantly, by mixing it with another metal or non-metal to create and alloy.
What are alloys? Why are they harder than pure metals?
Alloys:
- mixtures of metal with other elements, usually metals but they can also be made from metals mixed with non-metals such as carbon. The metals are mixed together physically but are not chemically combined
- 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
- Exam Tip
Questions on this topic often give you a selection of particle diagrams and ask you to choose the one which represents an alloy. It will be the diagram with uneven sized particles and distorted layers or rows of particles.
See printed picture
Complete the table:
See print out
