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GIANT COVALENT COMPOUND PROPERTIES Noadswood Science, 2012 Tuesday, April 5, 2016 To understand the properties of giant covalent compounds Covalent Compounds Covalent bonds form between non-metal atoms Each bond consists of a shared pair of electrons and is very strong – covalently bonded substances fall into two main types: Simple molecules Giant covalent structures Simple Molecules – Recap 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 of carbon bonded with two atoms of oxygen O O O H Oxygen O C H Water Carbon dioxide O Simple Properties – Recap Simple molecular substances have the following properties: 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 electricity because they do not have any free electrons or an overall electric charge Solids are usually soft and brittle, shattering when hit, and insoluble in water (but soluble in other solvents, e.g. petrol) Simple molecular substances are gases, liquids or solids with low melting and boiling points Giant Covalent In some substances, millions of atoms join together by covalent bonding – this produces giant covalent structures, not molecules All the bonds are covalent, which means that giant covalent structures have a very high melting and boiling point This also means that almost all giant covalent structure are hard but brittle Allotropes Allotropes have the same chemical properties because they have the same number of electrons However, they have different physical properties because the electrons are shared in different ways with other atoms E.g. in the element carbon, atoms bond in different ways, creating different kinds of giant structures: diamond and graphite… Giant Covalent Giant covalent structures contain a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds The atoms are usually arranged into giant regular lattices – extremely strong structures because of the many bonds involved Giant covalent structures can vary, for example the molecular structure of carbon can have two allotropes – diamond and graphite (allotropes are structurally different forms of an element)… Graphite (left) and diamond (right) – allotropes of carbon Diamond Diamond is a form of carbon in which each carbon atom is joined to four other carbon atoms, forming a giant covalent structure All the electrons in the outer shell of the carbon atom (2.4) are used in covalent bonds – this affects diamond’s properties As a result, diamond is very hard and has a high melting point and it does not conduct electricity (the hardest naturally occurring substance on Earth) 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 This forms rings of six atoms, creating a giant structure containing many layers – these layers are held together by weak forces of attraction Only three of the four electrons in the outer shell of the carbon atom (2.4) are used in covalent bonds – graphite conducts electricity and it is also used in pencils, and as a lubricant Other Carbon Allotropes Buckminsterfullerene is yet another allotrope of carbon – it is actually not a giant covalent structure, but a giant molecule Buckminsterfullerene contains 60 carbon atoms, each of which bonds with three others by forming two single bonds and one double bond – these atoms are arranged in 12 pentagons and 20 hexagons to form spheres, which are sometimes called ‘bucky balls’ Silica Sand is an impure form of silicon dioxide (quartz) 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 Each silicon atom (2.8.4) is bonded to four oxygen atoms, and each oxygen atom (2.6) is bonded to two silicon atoms – it is a semi-conductor which makes it immensely useful in the electronics industry: most transistors are made of silica Giant Covalent Properties Giant covalent structures have the following properties: 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 no conduct electricity whilst graphite contains free electrons so it does conduct electricity and silicon is semi-conductive (midway between non-conductive and conductive Many are very hard They are chemically un-reactive Fullerenes Fullerenes are molecules of carbon shaped like hollow balls or closed tubes – the carbon atoms are arranged in hexagonal rings and they have different properties than carbon Fullerenes can also be joined together to form nanotubes (very strong but light structures) Summary