We use formula mass to describe their relative masses

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We use formula mass to describe their relative masses. Formula mass carries no unit . The formula mass of a substance (or species) is the mass of one formula unit of it on the 12 C = 12.00 scale. For a pure substance with a formula, the simplest unit is its formula unit. In ionic compounds, like magnesium chloride, a formula unit of magnesium chloride is MgCl 2 , which consists of one Mg 2+ ion and two Cl ions. Take hydroxide ion (which is a species, not a substance) as an example. It has the formula OH . A formula unit of it corresponds to one OH ion. Take magnesium chloride (MgCl 2 ) as an example. Formula mass of MgCl 2 = 24.3 + 35.5 × 2 = 95.3 relative atomic mass of magnesium relative atomic mass of chlorine 8 Chemical bonding: covalent bonding 17 8 Never speak of the relative molecular mass of magnesium chloride, as there are no molecules in this compound. Learning tip Formula mass of a substance (or species) Key point Sum of relative atomic masses of all atoms present in a formula unit of the substance (or species). = Example 8.1 Determining the formula masses of some substances or species Calculate the formula mass of (a) C 6 H 12 O 6 (b) SO 4 2– (c) Al 2 (SO 4 ) 3 Solution C 6 H 12 O 6 (a) Formula mass of C 6 H 12 O 6 = 12.0 × 6 + 1.0 × 12 + 16.0 × 6 = 180.0 S O 4 (b) Formula mass of SO 4 2– = 32.1 + 16.0 × 4 = 96.1 Al 2 (SO 4 ) 3 (c) Formula mass of Al 2 (SO 4 ) 3 = 27.0 × 2 + (32.1 + 16.0 × 4) × 3 = 342.3 H 2 0 The formula mass of C 6 H 12 O 6 is equal to its relative molecular mass. This is because this compound consists of molecules. It is wrong to speak of the relative molecular masses of SO 4 2– or Al 2 (SO 4 ) 3 because they consist of ions only, not molecules. Learning tip formula mass 式量 formula unit 式單位
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Metallic bond, ionic bond and covalent bond — a summary In the previous sections, we have discussed metallic bond, ionic bond and covalent bond in different substances. Some of their information is summarized in Table 8.6 below. Formula mass is a general term applicable to all substances (or species) with a formula. In comparison, relative molecular mass only applies to molecular substances . 18 8 II Microscopic world I Ammonium nitrate (NH 4 NO 3 ) is made up of only non-metals, so it is considered as a covalent compound. Although ammonium nitrate is made up of only non-metals, it is an ionic compound. It consists of ammonium ion (NH 4 + ) and nitrate ion (NO 3 ). Concept check Chemical bonding and constituent particles in different substances — a summary 8.6 Metallic bond Ionic bond Covalent bond Nature electrostatic attraction between a ‘sea’ of negatively charged electrons and positively charged metal ions electrostatic attraction holding the oppositely charged ions together electrostatic attraction between the shared electrons and the two nuclei of the bonded atoms Formation escape of outermost shell electrons from metal atoms transfer of one or more electrons from one atom (or group of atoms) to another sharing of outermost shell electrons between two atoms Directional/ non-directional non-directional non-directional directional Table 8.6 A summary of information about metallic bond, ionic bond and covalent bond.
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