* GCSE Chemistry (& basic advanced) Revision Notes Part 3 COVALENT BONDING - small covalent molecules *
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Doc Brown's Chemistry - Chemical Bonding - Revision Notes Part 3 Covalent Bonding - small simple molecules Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10) Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA OCR Edexcel Salters CIE revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) Part 1 Introduction - why do atoms bond together? & sub-index for Parts 2-5 (read 1st) Part 2 Ionic Bonding - compounds and properties Part 3 Covalent Bonding -small simple molecules and properties (this page) Part 4 Covalent Bonding - macromolecules and giant covalent structures Part 5 Metallic Bonding - structure and properties of metals Part 6 More advanced concepts for advanced level chemistry (in preparation, BUT a lot on intermolecular forces in Equilibria Part 8)
Part 3. COVALENT BONDING small simple molecules and properties simple small molecule bonding e.g. water * physical properties of small molecules inter/intra (internal)-molecular forces Examples of covalent elements/compound examples described: hydrogen H2, chlorine Cl2, hydrogen chloride HCl, water H2O, ammonia NH3, methane CH4, oxygen O2, carbon dioxide CO2, ethene C2H4, nitrogen N2, ethane C2H6, chloromethane CH3Cl, methanol CH3OH More advanced level chemistry notes on the shapes and bond angles of molecules and ions
Covalent bonds are formed by atoms sharing electrons to form molecules. This type of bond usually formed between two non-metallic elements. The molecules might be that of an element i.e. one type of atom only OR from different elements chemically combined to form a compound. The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms of the bond, and the negative electrons between them. One single covalent bond is a sharing of 1 pair of electrons, two pairs of shared electrons between the same two atoms gives a double bond and it is possible for two atoms to share 3 pairs of electrons and give a triple bond. Note: In the examples it is assumed you can work out the electron configuration (arrangement in shells or energy levels) given the atomic number from the Periodic Table. This kind of bond or electronic linkage does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why molecules have a particular shape. In the case of ionic or metallic bonding, the electrical attractive forces act in all directions around the particles involved. The simplest molecules are formed from two atoms and examples of their formation are shown below. The electrons are shown as dots and crosses to indicate which atom the electrons come from, though all electrons are the same. The diagrams may only show the outer electron arrangements for atoms that use two or more electron shells. The electron structures are given in (). Examples of simple covalent molecules are … Example 1: two hydrogen atoms (1) form the molecule of the element hydrogen H2
Example 2: two chlorine atoms (2.8.7) form the molecule of the element chlorine Cl2
Example 3: one atom of hydrogen (1) combines with one atom of chlorine (2.8.7) to form the molecule of the compound hydrogen chloride HCl
Note: Hydrogen chloride gas is a true covalent substance consisting of small HCl molecules. If the gas is dissolved in a hydrocarbon solvent like hexane or methylbenzene it remains as HCl molecules and because there are no ions present, the solution does not conduct electricity. However, if hydrogen chloride gas is dissolved in water, things are very different and the HCl molecules split into ions. Hydrochloric acid is formed which consists of a solution of hydrogen ions (H+) and chloride ions (Cl-). The solution then conducts electricity and passage of a d.c. current causes electrolysis to take place forming hydrogen and chlorine. Reminder: How to work out formula of covalent compounds without going through some demanding electronic thinking is described on the "Elements, Compounds and Mixtures" page and it is followed by a section on naming compounds. Example 4: two atoms of hydrogen (1) combine with one atom of oxygen (2.6) to form the molecule of the compound water H2O
Example 5: three atoms of hydrogen (1) combine with one atom of nitrogen (2.5) to form the molecule of the compound ammonia NH3 three of Example 6: four atoms of hydrogen (1) combine with one atom of carbon (2.4) to form the molecule of the compound methane CH4 four of All the bonds in the above examples are single covalent bonds. Below are three examples 7-9, where there is a double bond in the molecule, in order that the atoms have stable Noble Gas outer electron arrangements around each atom. Carbon and silicon have a valency of 4. More complex examples can be worked out e.g. involving C, H and O. In each case link in the atoms so that there are 2 around a H (electronically like He), or 8 around the C or O (electronically like Ne). Example 7: Example 8:
Example 9: Examples 10-13: The scribbles below illustrate some more complex examples. Can you deduce them for yourself? Ex. 10 nitrogen N2; Ex. 11 ethane C2H6; Ex. 12 chloromethane CH3Cl and Ex. 13 methanol CH3OH. Electronic origin of the diagrams showing the outer electrons of N, C, Cl and O: N at. no. 7 (2.5), H at. no. (1), C at. no. 6 (2.4), Cl at. no. 17 (2.8.7) and O at. no. 8 (2.6) plus a variety of crosses and blobs! The valencies or combining power in theses examples are N 3, H 1, C 4, Cl 1 and O 2. From these you can work out others e.g. Ex. 12 can be used to derive the ox diagram for tetrachloromethane CCl4.
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