Chemical Formulae and Equation dddpp.pptx

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1. ● State the formulae of the elements and compounds named in the subject content. ● Define the molecular formula of a compound as the number and type of different atoms in one molecule. ● Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation. ● Define the empirical formula of a compound as the simplest whole number ratio of the different atoms or ions in a compound. ● Determine the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions. ● Construct symbol equations with state symbols, including ionic equations. ● Deduce the symbol equation with state symbols for a chemical reaction, given relevant information. ● Construct word equations and symbol equations to show how reactants form products, including state symbols. ● Describe relative atomic mass, Ar , as the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of 12C. ● Define relative molecular mass, Mr , as the sum of the relative atomic masses. Relative formula mass, Mr , will be used for ionic compounds. Learning objectives Chemical formulae and equations

2. Each element has its own chemical symbol . The symbol for an element is one of the most important pieces of information presented in the Periodic Table. All chemical symbols are either a single capital letter, such as hydrogen (H) , oxygen (O) and nitrogen (N) , or made up of two letters, such as aluminium (Al), argon ( Ar ) or magnesium (Mg). It is important to remember that the second letter of any symbol is always a small (lower case) letter. For some elements the symbol is taken from the Latin name of the element The use of symbols means that we can very quickly represent an element and its structure. For the elements whose structures are made up of individual atoms (the noble gases) , the formula of the element is simply its symbol

3. Where elements exist as giant structures, whether held together by metallic or covalent bonding , the formula is simply the symbol of the element (e.g. Cu, Mg, Fe, Na, K, C, Si, Ge). For convenience, the same applies to elements such as phosphorus (P) or sulfur (S) where the molecules contain more than three atoms (strictly speaking they should be P 4 and S 8 ). Iron (Fe) Diamond (C)

4. One important aspect of this is that there are some ions that are made up of more than two elements. These compound ions , or groups, are made up of atoms covalently bonded together . These groups have a negative charge because they have gained electrons to make a stable structure. In addition to these negative compound ions, there is one important compound ion that is positively charged, the ammonium ion.

5. In ionic compound : Write the name of the cation first, followed by the name of the anion. Do not indicate the quantity of ions in naming; the charges implicitly dictate the ratio. If the metal can form more than one ion, then the name indicates which ion is present. For example, iron(II) chloride contains the Fe 2+ ion, while iron(III) chloride contains the Fe 3+ ion. In covalent compound : Naming compounds use prefixes to tell you the number of that particular atom in the molecule. This is useful if two elements form more than one compound. Mono = 1, di = 2, tri = 3, tetra = 4 For example: carbon monoxide (CO) and carbon dioxide (CO 2 ), nitrogen dioxide (NO 2 ) and dinitrogen tetroxide (N 2 O 4 ), sulfur dioxide (SO 2 ) and sulfur trioxide (SO 3 ). The names for the important mineral acids follow a logical system but are best simply learnt at this stage, e.g. sulfuric acid (H2SO4). Some common and important compounds have historical names that do not fit with the above rules. Examples of these include water (H2O), ammonia (NH3) and methane (CH4)

6. Atoms of each element have a combining power , sometimes referred to as their valency. The combining power of an atom in a covalent molecule is the number of bonds that atom makes . This idea of an atom having a valency can be used to work out the formulae of covalent compounds. Note that the term valency is not required knowledge. The valency of an element in the main groups of the Periodic Table can be deduced from the group number of the element. The relationship between valency and group number is: For elements in Groups I-IV valency = group number For elements in Groups V-VII valency = 8 - the group number For elements in Group VIII valency = 0 Combining power

7. The 'cross-over' method for working out chemical formulae can be applied to simple covalent compounds where the molecules have a central atom, e.g. water, methane, carbon dioxide and ammonia

8. If you look closely at the third structure in Figure it has the molecular formula C 3 H 6 . We do not simplify the formula of this compound down to CH 2 as that would not represent the actual molecule. The method for working out the formulae of simple covalent molecules by crossing over the valencies does not work for large numbers of covalent molecules, particularly those that do not have a single central atom, e.g. H 2 O 2 , C 2 H 6 , C 3 H 6 . Molecular formula : A formula that shows the actual number of atoms of each element present in a molecule of the compound Empirical formula : A formula for a compound which shows the simplest ratio of atoms present Some non-metallic compounds form giant covalent structures. The most important example of this is silicon(IV) oxide. The formula for any giant covalent compound is always the simplest whole number ratio of the atoms present in the structure. Such compounds are always represented by their empirical formula. The formula of silicon(IV) oxide is therefore SiO2 as the atoms are present in a 1 : 2 ratio in the crystal structure.

9. Ionic compounds are solids at room temperature. Such compounds have giant ionic lattice structures and their formulae are simply the whole number ratio of the positive to negative ions in the structure. Thus, in magnesium chloride, there are two chloride ions (Cr) for each magnesium ion (Mg2+). The chemical formula is MgCl 2 . The overall structure must be neutral .

10. Ion Model (Whiteboard) - JavaLab