Chapter 5: Structure and Properties of Materials Flashcards
What is an element?
It is a pure substance that cannot be broken down into two or more simpler substances by chemical processes.
How are elements represented?
Chemical symbols are used to represent elements.
Each symbol is unique, consisting of one or two letters.
Rules for writing chemical symbols of elements:
- A chemical symbol is usually the first one of two letters
- The first letter of a symbol is always a capital letter.
- The second letter of a symbol is always a small letter.
***Metals are always placed first when writing chemical symbols of elements.
What do elements exist as?
Elements can exist as atoms or molecules.
A molecule is a group of two or more atoms that are chemically combined together.
There are monoatomic (elements formed by only one atom), diatomic (elements formed by two atoms), and polyatomic (elements formed by three or more atoms) elements.
What are the 8 diatomic elements?
- Hydrogen
- Nitrogen
- Oxygen
- Fluorine
- Chlorine
- Iodine
- Bromine
- Astatine
What is a compound and what are its characteristics?
A compound is a pure substance that contains two or more elements that are chemically combined in a fixed ratio.
Properties of compounds:
A compound has different properties from the elements that form it (constituent elements).
Chemical formula:
A compound can be represented by a chemical formula.
The chemical shows:
- the type of elements present;
- the ratio of the different atoms present
Composition of compounds:
- Every compound has fixed compositions of the different elements it contains.
- Chemical formula can be deduced by the ratio of atoms present in a compound.
Decomposition of compounds:
- A chemical reaction is necessary to separate the elements in a compound.
- In thermal decomposition, heat is used to break down compounds into elements or simpler compounds.
- Compounds can also be broken down by using electricity. This process is known as electrolysis.
What are mixtures and what are some of its types?
Mixtures are made up of two or more substances that are not chemically combined.
Types of mixtures:
A mixture can exist as a combination of
- two or more elements;
- two or more compounds;
- element(s) and compound(s).
What are some differences between a mixture and compound?
Separation
Mixture:
- The components of a mixture can be separated by physical processes such as magnetic separation, filtration or distillation.
Compound:
- A compound can only be broken down into its elements or into simpler compounds by chemical processes (thermal decomposition or electrolysis).
Properties
Mixture:
- The chemical properties of a mixture are the same as those of its components.
Compound:
- The physical and chemical properties of a compound are different from those of its constituent elements.
Energy changes
Mixture:
- No chemical reaction takes place when a mixture is formed. Usually there is little or no energy change.
Compound:
- A chemical reaction takes place when a compound is formed, usually there is an energy change, e.g the reactants get hot or cold.
Composition
Mixture:
- The components of a mixture can be mixed in any proportion.
Compound:
- The elements in a compound are always combined in a fixed proportion.
Melting and boiling point
- A mixture may melt or boil over a range of temperatures
- A compound melts and boils at a fixed temperature.
What are some characteristics of ionic substances?
- Ionic substances have giant ionic crystal lattice structures.
- The structure consists of a 3-dimensional lattice of large number of alternating cations and anions held together by strong electrostatic forces of attraction (ionic bonds).
Example: Sodium chloride
Structure of sodium chloride
- Each chloride ion is surrounded by six sodium ions.
- Each sodium ion is surrounded by six chloride ions.
- The ratio of sodium ions to chloride ions is 1:1. Hence, the formula unit of sodium chloride is NaCl.
Melting & boiling points of ionic substances
Using NaCl as an example:
- Strong electrostatic forces of attraction exists between ions in the crystal lattice.
- A large amount of heat is required to overcome the strong electrostatic forces of attraction.
- High melting and boiling points.
- Exist as solids at room temperature.
Electrical conductivity of ionic substances:
- Ionic compounds conduct electricity in the molten and aqueous states.
- They do not conduct electricity in the solid state.
- In the molten and aqueous states, free moving (mobile ions) are present.
- Mobile ions act as charge carriers to conduct electricity.
- In the solid state, ions are held in fixed positions. Thus, no mobile charge carriers are present.
Solubility of ionic substances:
- Usually soluble in water
- Usually insoluble in organic solvents
Hardness of ionic substances:
Ionic substances are hard and brittle
- Ionic substances are hard as the strong electrostatic forces of attraction cause the ions to resist motion.
- Ionic substances are brittle as the ionic lattice is deformed when a strong enough force is applied which results in ions of the same charge repelling each other and causing the lattice structure to shatter.
What are some characteristics of simple covalent substances?
Bonding and structure:
- Most covalent substances consist of simple covalent molecules.
- Atoms within the molecules are held by strong covalent bonds.
- Molecules are held together by weak intermolecular forces of attraction. (between molecules)
Melting and boiling points:
Simple covalent substances have low melting & boiling points.
- Only a small amount of heat is required to overcome the weak intermolecular forces holding the molecules together.
- Hence, they have low melting and boling points and usually exist as gases or volatile liquids at rtp. (room temp. = 25 degrees celcius)
- The intermolecular forces of attraction become stronger as the molecules get larger. As such, simple covalent substances with larger molecules have higher melting and boiling points.
Solubility:
- Simple covalent substances are usually insoluble in water but soluble in organic solvents.
- Some exceptions: ammonia, alcohol and sugar are simple covalent substances but are soluble in water.
Electrical conductivity:
Most simple covalent substances do not conduct electricity in any state.
- Simple covalent substances usually have no free moving ions/electrons in any state that can act as charge carriers to conduct electricity.
- However, hydrogen chloride, sulfur dioxide and ammonia form ions when dissolved in water. The ions formed would be free moving/mobile and thus act as charge carriers to conduct electricity.
What are some characteristics of giant covalent substances?
Bonding and structure:
- Some covalent substances consist of giant covalent molecules made up of a large network of atoms that are connected via strong covalent bonds.
- Such substances have giant covalent structures (also called giant molecular substances).
- Examples of substances with giant covalent structures: diamond, graphite and silicon dioxide.
Melting and boiling points:
- A large network of atoms are being held together by strong covalent bonds.
- A large amount of heat is required to overcome the strong covalent bonds for a change of state (such as melting or boiling) to occur, resulting in the high meling and boiling points of giant covalent substances.
Solubility:
Giant covalent substances (such as diamond, graphite and silicon dioxide) are insoluble in both water and organic solvents.
- Atoms are held together by strong covalent bonds that cannot be overcomed by solvent molecules.
*** Electrical conductivity:
Most giant covalent substances (graphite is an exception) do not conduct electricity.
- Valence electrons are either used up in covalent bonding or are localised to atoms present in the structure (except for graphite).
- As such, there are no free moving (mobile) electrons that can act as charge carriers to conduct electricity.
What are some examples of giant covalent substances?
Diamond and graphite:
- Diamond and graphite are allotropes of carbon.
- Allotropes are different forms of the same element with different structural arrangement of atoms.
Diamond:
- Each carbon atom is covalently bonded to four other carbon atoms. (Tetrahedral arrangement)
- One of the world’s hardest materials
- Large forces are required to overcome the extensive strong covalent bonds holding many carbon atoms together, making diamond very hard and rigid.
Graphite:
- Each carbon atom is covalently bonded to only three other carbon atoms.
- Graphite has a layered structure of hexagonal rings of carbon atoms.
- Between each layer of carbon atoms, weak intermolecular forces of attraction are present.
- Only a small force is required to overcome the weak intermolecular forces of attraction between the layered hexagonal rings of carbon atoms.
- These layers of carbon atoms can slide over each other easily when a force is applied.
- As such, graphite is soft and slippery.
- Each carbon atom is covalently bonded to only three other carbon atoms.
- Thus, each carbon atom has one valence electron not used in covalent bonding.
- The unutilised electrons are delocalised and move freeely between the layers of carbon atoms. These electrons act as charge carriers to conduct electricity.
Silicon dioxide:
- Found in sand
- Each carbon atom is covalently bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms.
- Has properties that are similar to that of diamond.
Graphene - A carbon allotrope:
- Obtained by extracting one layer of carbon atoms from graphite.
- Graphene is the thinnest material known to man at one atom think, and also incredibly strong.
- It is a good conductor of heat and electricity with high melting/boiling point.
Carbon nanotubes - A carbon allotrope:
- It is a good conductor of heat and electricity with high melting/boiling point.
- Strong and flexible
What are macromolecules and what are its characteristics?
A macromolecule is a very large molecule that is formed from many small molecules.
A polymer is a type of long chain macromolecule that is covalently linked together by many small repeat units called monomers.
- Most polymers are solids at room temperatures due to their large molecular size which makes the overall intermolecular forces of attraction relatively strong.
- Polymers do not have a fixed melting or boiling point.
- Polymers typically soften over a range of temperatures when the intermolecular forces of attraction are overcomed by the strong molecular vibrations due to heating.
- Polymers have varying hardness and flexibility that allows them to be made into different products.
- Most polymers are insoluble in water but soluble in some organic solvents.
- Most polymers cannot conduct electricity in any states due to the absence of mobile ions and electrons.
What are some physical properties of metals?
- High density
- Close packing of metal atoms in regular layers - High melting points
- A large amount of heat is needed to overcome strong metallic bonds - Good conductors of heat and electricity
- Consist of free moving electrons which allows for efficient transfer of thermal energy and act as charge carriers to conduct electricity - Malleable and ductile
- Layers of metal atoms of same size and arranged in neat regular layers that can slide over each other easily when a force is applied
How can the properties of metals be further improved?
A mixture containing a metal physically combined with one or more other elements is called an alloy.
When another metal/non-metal containing atoms of a different size is added:
- The regular arrangement of atoms is disrupted.
- The layers of atoms cannot slide over each other easily.
- This makes an alloy harder and stronger than the pure metals it is made from.
- It also makes an alloy more resistant to corrosion
