SIMPLE COVALENT COMPOUND PROPERTIES Noadswood Science, 2012

Friday, August 07, 2015  To understand the properties of simple covalent compounds

Covalent Bonding – Recap  A covalent bond forms when two non-metal atoms share a pair of electrons: the electrons involved are in the highest occupied energy levels (outer shells) of the atoms  An atom that shares one or more of its electrons will complete its highest occupied energy level  Covalent bonds are strong – a lot of energy is needed to break them  Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water

Covalent Bonding – Recap H H H H Incomplete outer shells Both atoms have a full outer shell  When non-metal atoms react together, they need to gain electrons to fill their outer shell and become stable  They can only do this if they share electrons with each other – they are covalently bonded

Example Compounds Hydrogen atoms can each form one covalent bond, while oxygen atoms can each form two covalent bonds Two pairs of electrons are shared in a water molecule (H 2 O) Hydrogen atoms and chlorine atoms can each form one covalent bond One pair of electrons is shared in a hydrogen chloride molecule (HCl) Hydrogen atoms can each form one covalent bond, while and nitrogen atoms can each form three covalent bonds Three pairs of electrons are shared in an ammonia molecule (NH 3 )

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  These contain only a few atoms held together by strong covalent bonds  An example is carbon dioxide (CO 2 ), the molecules of which contain one atom of carbon bonded with two atoms of oxygen  How would you draw out carbon dioxide?

Simple Molecules  Atoms that join together by covalent bonding can form different types of covalent structure  Oxygen, water and carbon dioxide are molecules – they have a simple structure because they only contain a few atoms  Draw out the above simple molecules… OOCOO H O H OxygenWaterCarbon dioxide

Simple Molecules – Iodine  Most molecular substances are gas or liquid at room temperature  A few are solid and these are called molecular solids, e.g. iodine…  Iodine is a molecular solid at room temperature – two iodine atoms form a single covalent bond to become an iodine molecule  Millions of iodine molecules are held together by weak forces of attraction to create a 3D molecular lattice Weak forces of attraction

Simple Properties  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

Intermolecular Forces  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