Science
Different substances and their properties
Ionic substances form giant ionic lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] containing oppositely charged ions. They have high melting and boiling points, and conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity when melted or dissolved in water.
Simple molecular substances consist of molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] in which the atoms are joined by strong covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ]. Their molecules are held together by weak forces, so these substances have low melting and boiling points. They do not conduct electricity.
Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.
Nanoparticles are 1-100 nm in size, typically the size of small molecules - far too small to see with a microscope. They have remarkable properties that are different to the same substance in bulk form.
Metals form giant structures containing free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], making them good conductors of heat and electricity.
Ionic compounds
Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] forces of attraction between oppositely charged ions.
Positively charged Na ions and negatively charged Cl ions
The oppositely charged ions are arranged in a regular way to form giant ionic lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ]. Ionic compounds [compounds: Substances formed by the chemical union (involving bond formation) of two or more elements. ] often form crystals as a result. The illustration shows part of a sodium chloride (NaCl) ionic lattice.
Properties of ionic compounds
- High melting and boiling points - Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
- Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.
Properties of ionic compounds
| Ionic compound | Properties |
|---|---|
| Sodium chloride, NaCl | High melting point: 800ºC Non-conductive in its solid state, but when dissolved in water or molten [molten: Molten means reduced to liquid form by heating. It is mainly used to describe rock, glass or metal. ] NaCl will conduct electricity. |
| Magnesium oxide, MgO | Higher melting point than sodium chloride: around 2,800ºC. This is because its Mg2+ and O2- ions have a greater number of charges, so they form stronger ionic bonds than the Na+ and Cl- ions in sodium chloride. Because magnesium oxide stays solid at such high temperatures, it remains non-conductive. It is used for high-temperature electrical insulation. |
Covalent compounds - simple molecules
Covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ] form between non-metal atoms. Each bond consists of a shared pair of electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], and is very strong. Covalently bonded substances fall into two main types:
- simple molecules [molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ] and
- giant covalent structures.
Simple molecules
A molecule of carbon dioxide
These contain only a few atoms held together by strong covalent bonds. An example is carbon dioxide (CO2), the molecules of which contain one atom [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. ] of carbon bonded with two atoms of oxygen.
Properties of simple molecular substances
- Low melting and boiling points - This is because the weak intermolecular forces break down easily.
- Non-conductive - Substances with a simple molecular structure do not conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity. This is because they do not have any free electrons or an overall electric charge.
Higher Tier only
Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds. All have very strong bonds between the atoms, but much weaker forces holding the molecules together. When one of these substances melts or boils, it is these weak 'intermolecular forces' that break, not the strong covalent bonds. Simple molecular substances are gases, liquids or solids with low melting and boiling points.
The animation shows how the weak intermolecular forces between water molecules break down during boiling or melting:
Covalent bonding - giant covalent structures
Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds [covalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons. ]. The atoms are usually arranged into giant regular lattices [lattice: A lattice is a regular grid-like arrangement of atoms in a material. ] - extremely strong structures because of the many bonds involved. The graphic shows the molecular structure of diamond and graphite: two allotropes [allotropes: Allotropes are structurally different forms of an element. They differ in the way the atoms bond with each other and arrange themselves into a structure. Because of their different structures, allotropes have different physical and chemical properties. ] of carbon, and of silica (silicon dioxide).
From left to right - graphite, diamond, silica
Properties of giant covalent structures
- Very high melting points - Substances with giant covalent structures have very high melting points, because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600ºC.
- Variable conductivity - Diamond does not conduct [conduct: To allow electricity, heat or other energy forms to pass through. ] electricity. Graphite contains free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], so it does conduct electricity. Silicon is semi-conductive - that is, midway between non-conductive and conductive.
Graphite
Graphite is a form of carbon in which the carbon atoms form layers. These layers can slide over each other, so graphite is much softer than diamond. It is used in pencils, and as a lubricant [lubricant: A substance used to reduce the friction between two solid surfaces. ]. Each carbon atom in a layer is joined to only three other carbon atoms. Graphite conducts electricity.
Diamond
Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure. As a result, diamond is very hard and has a high melting point. It does not conduct electricity.
Silica
Silica, which is found in sand, has a similar structure to diamond. It is also hard and has a high melting point, but contains silicon and oxygen atoms, instead of carbon atoms.
The fact that it is a semi-conductor makes it immensely useful in the electronics industry: most transistors are made of silica.
Buckminsterfullerene
Structure of a buckminsterfullerene molecule - a large ball of 60 atoms
Buckminsterfullerene is yet another allotrope of carbon. It is actually not a giant covalent structure, but a giant molecule in which the carbon atoms form pentagons and hexagons - in a similar way to a leather football. It is used in lubricants.
Nanoparticles
Measurements
The table shows some of the units used to measure length. As you go down the table, each unit is 1,000 times smaller than the one above it.
Units used to measure length
| Unit name | Unit symbol | Meaning |
|---|---|---|
| gigametre | Gm | one billion metres |
| megametre | Mm | one million metres |
| kilometre | km | one thousand metres |
| metre | m | one metre |
| millimetre | mm | one thousandth of a metre |
| micrometre | µm | one millionth of a metre |
| nanometre | nm | one billionth of a metre |
Nanotubes like this could be used to make tiny mechanical devices, molecular computers or extremely strong materials
Nanoparticles range in size from about 100 nm down to about 1 nm. They are typically the size of small molecules, and far too small to see with a microscope.
Working with nanoparticles is called nanotechnology.
Uses of nanoparticles
Nanoparticles have a very large surface area compared with their volume. So they are often able to react very quickly. This makes them useful as catalysts to speed up reactions. For example, they can be used in self-cleaning ovens and windows.
Nanoparticles also have different properties to the same substance in normal-sized pieces. For example, titanium dioxide is a white solid used in house paint and certain sweet-coated chocolates. Titanium dioxide nanoparticles are so small they do not reflect visible light, so cannot be seen. They are used in sunblock creams to block harmful ultraviolet light without appearing white on the skin.
Nanoscience may lead to the development of:
- new catalysts
- new coatings
- new computers
- stronger and lighter building materials
- sensors that detect individual substances in tiny amounts.
Metallic bonding - higher
Giant structures with free electrons
Metals form giant structures in which electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ] in the outer shells of the metal atoms [atoms: An atom is the smallest part of an element that still has the properties of that element, comprising electrons surrounding a nucleus of protons and neutrons. ] are free to move. The metallic bond is the force of attraction between these free electrons and metal ions [ions: Electrically charged particles, formed when an atom or molecule gains or loses electrons. ]. Metallic bonds are strong, so metals can maintain a regular structure and usually have high melting and boiling points.
Atomic structure of a metal
Metals are good conductors [conductor: An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance. A thermal conductor allows thermal energy to be transferred through it easily. ] of electricity and heat, because the free electrons carry a charge or heat energy through the metal. The free electrons allow metal atoms to slide over each other, so metals are malleable [malleable: the ability of a material to permanently deform, or lose its shape, in all directions without cracking. ] and ductile [ductile: able to deform, usually by stretching along its length. ].
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