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CHEMISTRY FORM 2

1. STRUCTURE OF THE ATOM, AND THE PERIODIC TABLE 2. CHEMICAL FAMILIES AND PATTERNS IN PROPERTIES 3. CHEMICAL BONDING AND STRUCTURE 4. SALTS 5. EFFECT OF AN ELECTRIC CURRENT ON SUBSTANCES 6. CARBON AND SOME OF ITS COMPOUNDS Content developer # Structure of the Atom, and the Periodic Table: Relative Atomic Mass and Isotopes

### 1.6 Relative Atomic Mass and Isotopes

What is relative atomic mass?

In Section 1.3, we learnt that an element with isotopes (atoms with the same number of protons but different numbers of neutrons) has different mass numbers. How can we have a single mass value for each element?

First, we should have a mass scale on which we can compare masses, because we are not dealing with actual values, which are very small. Scientists have agreed on a scale on which carbon-12 isotope is assigned a mass value of exactly 12 units, because it has a mass number of 12 (6 protons + 6 neutrons). Figure 1.6: Carbon-12 relative mass scale

The mass of an atom on this scale is called Relative Atomic Mass (R.A.M). Relative Atomic Mass is the mass of an atom on a scale where carbon-12 isotope weighs exactly 12 units

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Whereas mass number (sum of protons and neutrons) is a whole number, R.A.M is an average of the masses of various isotopes of an element; so it can be in decimals. It is a single value that represents an element. Certainly, one unit on this scale, called atomic mass unit (a.m.u), equals 1/12 of mass of carbon-12 isotope.

To determine R.A.M for elements with isotopes

Suppose among 40 atoms of an element, Q, 15 weigh 60 units each (mass number of 60) and 25 weigh 50 units each (mass number of 50). What is the R.A.M of Q.

Solution: We are to work out the average mass, called weighted mean. NB: As a relative quantity, R.A.M has no units.

Calculating R.A.M, involves working out the weighted mean. That is, we consider the number (in this case 15 and 25) and isotopic masses (in this case 60 and 50). Sometimes, the numbers are given as percentages; but the calculations are the same.

Questions 1.6 1. A sample of chlorine consists of 25% chlorine-37 and 75% chlorine-35 isotopes. Determine the relative atomic mass (R.A.M) of chlorine.
2. An element consists of two isotopes of mass number 12 and 14. The percent abundance of the heavier isotope is 5%. Determine the relative atomic mass (R.A.M) of the element.

Answers to Questions 1.6 NB: The percentages are also called percent abundances.