Symbols Symbols are a kind of shorthand system for writing down elements and compounds. Each element has a particular one or two letter symbol. The first letter of a symbol is always capital, and if there is a second letter, e.g. Mg (magnesium), this is written in lower case. A complete set of symbols is found in a periodic table. The following table lists some common elements, their symbols, and their physical state at room temperature and pressure. Table 1 A set of common elements, their symbols and physical state Element Symbol Physical State Hydrogen H gas Oxygen O gas Chlorine Cl gas Mercury Hg liquid Gold Au solid Silver Ag solid Molecules Atoms of elements usually join up either with atoms of the same element or with atoms of different elements to form molecules. In both cases the atoms are chemically bound together. On the other hand there are elements whose atoms are not found as molecules but as single atoms. These elements are said to be monatomic, e.g. He, helium; Ne, neon; Kr, krypton; Xe, xenon. Elements whose atoms are found joined up in twos as molecules are called diatomic, e.g. Cl 2, chlorine; O 2, oxygen; N 2, nitrogen. Atoms of one element may join up with atoms of another element to form a new substance. These are called compounds. For example, one atom of oxygen binds chemically with two atoms of hydrogen to form a molecule of water. Water is a compound made up of oxygen and hydrogen. Chemical formulae When we are talking about molecules of compounds we do not write symbols for them, but we write chemical formulae. Taking water as an example, since Symbols 1
one molecule of water is composed of two atoms of hydrogen and one of oxygen, the chemical formula can be written as H 2 O. When writing a chemical formula, metals and hydrogen are always written before non-metals, e.g. HCl, NaCl, MgO, etc. The chemical formula of a compound gives the ratio of the atoms of the different elements, e.g. NaCl: one atom of sodium : one atom of chlorine MgCl 2 : one atom of magnesium : two atoms of chlorine Not all chemical formulae are so easy. Look at the chemical formulae in the following tables: Note that for aluminium hydroxide the formula is Al(OH) 3 and not AlO 3 H 3. Why is this so question? The answer to this question comes out when we discuss radicals. A radical is a group of atoms that exists in several compounds but does not exist on its own. The next table contains some common radicals. Table 2: Some radicals Groups of atoms present Name of radical Examples of compounds NO 3 Nitrate NaNO 3, Cu(NO 3 ) 2 NO 2 Nitrite NaNO 2, Cu(NO 2 ) 2 CO 3 Carbonate Na 2 CO 3, K 2 CO 3 HCO 3 Hydrogencarbonate NaHCO 3, Mg(HCO 3 ) 2 SO 4 Sulfate CaSO 4, (NH 4 ) 2 SO 4 SO 3 Sulfite MgSO 3, K 2 SO 3 HSO 4 Hydrogensulfate Mg(HSO 4 ) 2, Ca(HSO 4 ) 2 NH 4 Ammonium NH 4 Cl, (NH 4 ) 2 CO 3 OH Hydroxide NaOH, KOH PO 4 Phosphate AlPO 4, Mg 3 (PO 4 ) 2 Cl Chloride NaCl, CuCl 2 S Sulfide ZnS, FeS O Oxide MgO, CO 2 So when we write the chemical formula of a compound that contains radicals we must consider the radical as one entity and not as separate atoms. Symbols 2
Note that when looking at the names of compounds there are certain trends in the endings. For example, the ending -ate indicates a radical containing oxygen, e.g. sodium hydrogencarbonate (NaHCO 3 ), magnesium sulfate (MgSO 4 ). The ending -ide indicates the presence of two elements only, e.g. sodium chloride (NaCl), potassium iodide (KI). Valency But how are we going to decide what is the ratio of atoms combined in a compound. This is decided by the valency of an element. For example, hydrogen has a valency of 1; chlorine has a valency of 1 as well. So hydrogen (H) and chlorine (Cl) will combine to form a molecule of hydrogen chloride in a ratio of 1 hydrogen atom : 1 chlorine atom. The chemical formula will therefore be HCl. One has to remember the valencies of the most common elements. A help to remember this is to know which Group in the Periodic table the element is found in. For example, sodium is in Group 1 and has a valency of 1. Magnesium is in Group 2 and has a valency of 2. Aluminium is in Group 3 and has a valency of 3. Carbon is in Group 4 and has a valency of 4. This rule has to be changed a bit when it comes to Groups 5, 6 and 7. For these groups, the valency is equal to the number of electrons required to achieve an outer shell of 8 electrons which is stable. For Group 5 elements, they have 5 outer electrons and hence need 3 to achieve a noble gas configuration, and so their valency is 3. The same is done for Groups 6 and 7, whose elements have valencies of 2 and 1 respectively. For more notes about valency refer to Topic 6. Also, i. there are some elements that have variable valencies, i.e. they may have more than one valency. This is a property of the transition metals, e.g. Copper has can have a valency of either 1 or 2; iron may have a valency of either 2 or 3; ii. even radicals have valencies (see Table 3 below); Symbols 3
iii. noble gases, i.e. the elements found in Group 8 or Group O have a valency of 0. Table 3: Valencies of radicals Radical Symbol Valency Hydroxide OH 1 Chloride Cl 1 Nitrate NO 3 1 Hydrogencarbonate HCO 3 1 Hydrogensulfate HSO 4 1 Ammonium NH 4 1 Oxide O 2 Carbonate CO 3 2 Sulfate SO 4 2 The following table summarises the symbols and valencies of common elements. Table 4 Common elements and their valency Elements Valency Hydrogen 1 Sodium 1 Magnesium 2 Calcium 2 Carbon 4 Oxygen 2 Iron 2 or 3 Steps for writing chemical formulae 1. Look at the name of the chemical compound and write the symbols of the elements and/or radicals that make it up. aluminium chloride: aluminium ion, Al 3+ chloride ion, Cl - Symbols 4
2. Note the valencies of the atoms and/or radicals making up the compound. Al - valency 3 Cl - valency 1 3. The valencies need to be balanced if they are not balanced already. Al - valency 3: one ion is needed (resulting valency = 3) Cl - valency 1: three ions are needed (resulting valency 1 x 3 = 3) 4. Therefore for every aluminium ion there is, there have to be three chlorine ions. 5. Resulting chemical formula: AlCl 3 Symbols for states of substances A substance can be either a solid, a liquid or a gas. A substance may also be in solution, usually in water. There is a symbol for all these four states. These symbols are usually written in brackets as subscripts after an element or compound. The symbol for solid is (s). The symbol for liquid is (l). The symbol for gas is (g). The symbol for in aqueous solution is (aq). This means that the substance is in solution in water. These symbols are usually used when we are writing chemical equations. Chemical equations Chemical equations represent what happens in chemical reactions. One can also represent what is happening in a chemical reaction by use of word equations, e.g. iron + sulphur iron sulfide On the left hand side of the equation, one writes the reactant/s, i.e. the substances that actively react in the reaction. Symbols 5
On the right hand side of the equation, one writes the product/s, i.e. the substances produced as a result of the reaction. The above is called a WORD EQUATION because words are used to show what is happening. A more accurate and scientific way of showing what happens in a chemical reaction, is to use symbols for elements and chemical formulae instead of words. Hence for the reaction above, we can write, Fe + S FeS We can also add the symbols that show the physical state for each substance, Fe(s) + S(s) FeS(s) Now, one must see that the number of atoms of an element (or radical) on the left hand side must equal those on the right hand side. For the above equation, this is so, and hence the chemical equation is all right. But what about the next one? Zn(s) + HCl(aq) ZnCl 2 (aq) + H 2 (g) Note that for Zn, the number of atoms on the left hand side balance those on the right hand side, but not so for Cl and H. For the equation to be balanced, there must be two HCl molecules on the left hand side, and the equation becomes: Zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g) Note that the 2" is written IN FRONT of the molecule and not anywhere else. REMEMBER THIS! Numbers in chemical formulae and equations: A,B are the elements or radicals and X is a number. Symbols 6
A X, B X : means that there are x atoms of A and X of B in a molecule; e.g. Cl 2, O 2, Na 2 O, AlCl 3. (AB) X : means that there x radicals of AB in a molecule, e.g. (NH 4 ) 2 SO 4, Al(OH) 3, Al 2 (CO 3 ) 3. X AB: means that there are X molecules of AB, e.g. 2 NaCl, 3 MgCO 3. NOTE: When balancing chemical equations, sometimes it is easier to multiply the whole equation, rather than have a fraction, e.g. KClO 3 KClO 3 KCl + O 2 is not balanced; KCl + 1 1 / 2 O 2 is balanced but not correct; 2 KClO 3 2 KCl + 3 O 2 is balanced and correct. Calculations from formulae and equations A chemical formula of a compound does not tell you only what types of atoms make up that compound but also the number of atoms present in that compound. For example, a molecule of magnesium chloride, MgCl 2, has one atom of magnesium and two atoms of chlorine chemically combined together. In moles, one mole of magnesium chloride molecules, consists of one mole of magnesium ions and two moles of chloride ions. In grams, the relative molecular mass (RMM) of MgCl 2 is (24+(35.5 x 2) = 95g. Therefore 95g of MgCl 2 contains 24g of Mg combined with 71g of Cl. These ideas can be used to calculate the empirical formula of a compound, i.e. the simplest ratio of atoms present in a molecule. Example: Find the empirical formula of hydrogen peroxide if 0.04g of hydrogen react with 0.64g of oxygen to form hydrogen peroxide. Atoms present: H O Masses given: H - 0.04g Symbols 7
O - 0.64g To find the empirical formula we have to obtain a ratio of moles. Number of moles of hydrogen in 0.04g:- 1g of hydrogen molecules contains 1 mole of hydrogen atoms 0.04g of hydrogen molecules contains? moles of hydrogen atoms 0.04 x 1 / 1 = 0.04 moles Number of moles of oxygen in 0.64g:- 16g of oxygen molecules contain 1 mole of oxygen molecules 0.64g of oxygen molecules contain? moles of oxygen molecules 0.64 x 1 / 16 = 0.04 moles Therefore the ratio of moles of hydrogen reacting with moles of oxygen to given hydrogen peroxide is 0.04:0.04 or 1:1 Therefore the empirical formula is HO Another way to calculate the empirical formula is shown in the following example. Example: Sodium sulfate has the following composition by mass: Na 32.4%, S 22.5%, and O 45.1%. What is the empirical formula question Na S O % composition 32.4 22.5 45.1 Divide by RAM 32.4/23 22.5/32 45.1/16 Relative number of atoms =1.41 =0.70 =2.82 Divide by smallest number 1.41/0.70 0.70/0.70 2.82/0.70 =2 =1 =4 Therefore empirical formula is Na 2 SO 4 Symbols 8
Note that for the final division answers may not be whole numbers. In this case round them to the nearest whole or half. Calculating the molecular formula from the empirical formula The molecular formula is different from the empirical formula. The latter gives the simplest ratio of atoms present in a molecule whereas the former gives the actual number of atoms present in a molecule. You can obtain the molecular formula if you know both the empirical formula and the RMM. For example, for hydrogen peroxide, the empirical formula is HO. The mass of this empirical formula is (1 + 16) 17. The RMM is 34. Therefore one immediately notes that the RMM is twice the mass of the empirical formula, and so the molecular formula is (OH) x 2, or H 2 O 2. Percentage composition from the chemical formula If the RMM of a compound is taken as 100%, then the relative atomic masses (RAM) of each element can be taken as a percentage of the total RMM. Example: Find the percentage composition by mass of ammonium nitrate. 1. Write down the formula of the compound: NH 4 NO 3 2. Calculate the RMM: 14 + (1 x 4) + 14 + (16 x 3) = 80 3. Take the TOTAL RAM of each element: N = 14 (x 2) = 28 O = 16 (x 3) = 48 H = 1 (x 4) = 4 4. Find what percentage of the RMM, each TOTAL RAM is. N = 28/80 x 100 = 35% Symbols 9
O = 48/80 x 100 = 60% H = 4/80 x 100 = 5% 5. Check that the percentages total to 100%. NOTE For hydrated compounds, i.e. compounds that have water of crystallisation, e.g. CuSO 4.5H 2 O, the H 2 O is taken as an entity on its own, and not as H and O on their own. E.g. Percentage composition of MgCl 2.6H 2 O RMM = 24 + (35.5 x 2) + (18 x 6) = 203 Mg = 24/203 x 100 = 11.82% Cl = 71/203 x 100 = 34.98% H 2 O = 108/203 x 100 = 53.2% Mass calculations and equations The masses of reactants or products can be calculated using chemical equations. For example, S(s) + O 2 (g) SO 2 (g) The equation indicates that 1 mole of S reacts with 1 mole of O 2 to produce 1 mole of SO 2. We can express this statement in masses. 32g of S (RAM of S) react with 32g (RAM of O x 2 = 16 x 2) of O 2 to produce 64g of SO 2. This idea can be used to work out masses of products or reactants. Example: Symbols 10
How much magnesium oxide will be produced when 48g of magnesium are burned in excess of oxygen. (RAM: Mg = 24; O = 16) 2 Mg(s) + O 2 (g) 2 MgO(s) i.e. 2 moles of Mg + 1 mole of O 2 2 moles of MgO 2 x 24 + excess O 2 2 x (24 + 16) 48g + excess O 2 80g Therefore when 48g of Mg react with excess O 2, 80g of MgO are produced. Example: Calculate the mass of zinc required to produce 81g of zinc oxide when heated in excess of oxygen. (RAM: Zn = 65; O = 16) 2 Zn(s) + O 2 (g) 2 ZnO(s) 2 moles of Zn + excess O 2 2 moles ZnO 2 x 65 + excess O 2 2 x (65+16) 130g + excess O 2 162g But we want to find out the mass of Zn required to produce 81g of ZnO and not 162g. Therefore, 162g of ZnO are produced by 130g of Zn 81g of ZnO are produced by? of Zn = 130 x 81 162 = 65g of Zn Symbols 11