Chemistry_Grade_10-12 (1).pdf

221 representing molecules the structure of a

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2.2.1 Representing molecules The structure of a molecule can be shown in many different ways. Sometimes it is easiest to show what a molecule looks like by using different types of diagrams , but at other times, we may decide to simply represent a molecule using its chemical formula or its written name. 21
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2.2 CHAPTER 2. WHAT ARE THE OBJECTS AROUND US MADE OF? - GRADE 10 1. Using formulae to show the structure of a molecule A chemical formula is an abbreviated (shortened) way of describing a molecule, or some other chemical substance. In chapter 1, we saw how chemical compounds can be repre- sented using element symbols from the Periodic Table. A chemical formula can also tell us the number of atoms of each element that are in a molecule, and their ratio in that molecule. For example, the chemical formula for a molecule of carbon dioxide is: CO 2 The formula above is called the molecular formula of that compound. The formula tells us that in one molecule of carbon dioxide, there is one atom of carbon and two atoms of oxygen. The ratio of carbon atoms to oxygen atoms is 1:2. Definition: Molecular formula A concise way of expressing information about the atoms that make up a particular chemical compound. The molecular formula gives the exact number of each type of atom in the molecule. A molecule of glucose has the molecular formula: C 6 H 12 O 6 In each glucose molecule, there are six carbon atoms, twelve hydrogen atoms and six oxy- gen atoms. The ratio of carbon:hydrogen:oxygen is 6:12:6. We can simplify this ratio to write 1:2:1, or if we were to use the element symbols, the formula would be written as CH 2 O . This is called the empirical formula of the molecule. Definition: Empirical formula This is a way of expressing the relative number of each type of atom in a chemical compound. In most cases, the empirical formula does not show the exact number of atoms, but rather the simplest ratio of the atoms in the compound. The empirical formula is useful when we want to write the formula for a giant molecule . Since giant molecules may consist of millions of atoms, it is impossible to say exactly how many atoms are in each molecule. It makes sense then to represent these molecules using their empirical formula. So, in the case of a metal such as copper, we would simply write Cu, or if we were to represent a molecule of sodium chloride, we would simply write NaCl. Chemical formulae therefore tell us something about the types of atoms that are in a molecule and the ratio in which these atoms occur in the molecule, but they don’t give us any idea of what the molecule actually looks like, in other words its shape . Another useful way of representing molecules is to use diagrams. Another type of formula that can be used to describe a molecule is its structural formula .
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