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In this video, we are going to learn how to calculate the relative atomic mass of an element from its isotopic abundances. If we consider carbon, its relative atomic mass is very close to 12 atomic mass units. This is because in nature 99 atoms out of 100 atoms of a carbon sample are carbon 12 isotopes. Another way of saying this is that carbon-12 has a 99% isotopic abundance.

However, not all relative atomic masses of elements are weighted in this way. Chlorine, for instance, exists as two isotopes: chlorine-35 and chlorine-37. How many protons and neutrons do you think each chlorine isotope has? Remember, the mass number for an isotope is equal to the number protons it has plus the number of neutrons. Pause and select from one of the following options. Use the chemical symbols to help you work this out.

• A) 17 protons/18 neutrons (chlorine-35); 17 protons/19 neutrons (chlorine-37);

• B) 17 protons/17 neutrons (chlorine-35); 17 protons/20 neutrons (chlorine-37);

• C) 17 protons/18 neutrons (chlorine-35); 17 protons/20 neutrons (chlorine-37);

Both chlorine-35 and chlorine-37 isotopes have 17 protons, but the chlorine-35 isotope has 18 neutrons, whereas the chlorine-37 isotope has 20 neutrons. The answer was C. Did you get it right?

In nature, out of 100 atoms of a chlorine sample, 75 atoms are chlorine-35 isotopes and 25 atoms are chlorine-37 isotopes. For you to be able to calculate the relative atomic mass for chlorine, you have to multiply the mass number of the isotope with its abundance (percentage), and add these products together. This gives the total mass number of the isotopes. Using this data, can you calculate the total mass of chlorine isotopes using their abundance and mass numbers? Pause, think, choose and continue when ready.

• A. (75x37)+ (25x35) = 2775 + 875 = 3650.

• B. (75x35)+ (25x37) = 2625 + 925 = 3550.

• C. (75x25)+(35x37) = 1875+ 1295 = 3170

The answer is B. You have to multiply a 75% abundance of the chlorine-35 isotope by its mass number, 35. You then have to multiply a 25% abundance of the chlorine isotope by its mass number, 37. When added together, this gives a total of 3,550 when the two products are added together. The final step in calculating the relative atomic mass is to divide your answer by 100. This is because the abundances are given as percentages. This means chlorine has a relative atomic mass of 35.50.

Here's a challenge. From the following data, calculate the relative atomic mass of uranium. Pause and continue when ready. Make sure you write your working out down as you do the calculation.

• Isotope : 238U and Relative abundance/%  : 99.27

• Isotope : 235U and Relative abundance/%  : 0.72

• Isotope : 234U and Relative abundance/%  : 0.01

Hint: you add the three products this time of (isotope mass number x relative abundance).

The correct answer and working out is displayed on the screen.

(99.27 x 238) + (0.72 x 235) + (0.01 x 234) = 23,626.26 + 169.2 + 2.34 = 23,797.8

Remember, you then have to divide this value by 100. This gives 237.99. Did you get it right?

In summary, the relative atomic mass is the average weighted mass for the isotopes of a particular element. To calculate this, you need to multiply the relative abundances by the mass number of the isotope, add these products together for each isotope, and then divide the total by 100 – and that's how you calculate the relative atomic mass of any element from its isotopes in the periodic table.

In this video, we are going to learn how to calculate the relative atomic mass of an element from its isotopic abundances. If we consider carbon, its relative atomic mass is very close to 12 atomic mass units. This is because in nature 99 atoms out of 100 atoms of a carbon sample are carbon 12 isotopes. Another way of saying this is that carbon-12 has a 99% isotopic abundance.

However, not all relative atomic masses of elements are weighted in this way. Chlorine, for instance, exists as two isotopes: chlorine-35 and chlorine-37. How many protons and neutrons do you think each chlorine isotope has? Remember, the mass number for an isotope is equal to the number protons it has plus the number of neutrons. Pause and select from one of the following options. Use the chemical symbols to help you work this out.

• A) 17 protons/18 neutrons (chlorine-35); 17 protons/19 neutrons (chlorine-37);

• B) 17 protons/17 neutrons (chlorine-35); 17 protons/20 neutrons (chlorine-37);

• C) 17 protons/18 neutrons (chlorine-35); 17 protons/20 neutrons (chlorine-37);

Both chlorine-35 and chlorine-37 isotopes have 17 protons, but the chlorine-35 isotope has 18 neutrons, whereas the chlorine-37 isotope has 20 neutrons. The answer was C. Did you get it right?

In nature, out of 100 atoms of a chlorine sample, 75 atoms are chlorine-35 isotopes and 25 atoms are chlorine-37 isotopes. For you to be able to calculate the relative atomic mass for chlorine, you have to multiply the mass number of the isotope with its abundance (percentage), and add these products together. This gives the total mass number of the isotopes. Using this data, can you calculate the total mass of chlorine isotopes using their abundance and mass numbers? Pause, think, choose and continue when ready.

• A. (75x37)+ (25x35) = 2775 + 875 = 3650.

• B. (75x35)+ (25x37) = 2625 + 925 = 3550.

• C. (75x25)+(35x37) = 1875+ 1295 = 3170

The answer is B. You have to multiply a 75% abundance of the chlorine-35 isotope by its mass number, 35. You then have to multiply a 25% abundance of the chlorine isotope by its mass number, 37. When added together, this gives a total of 3,550 when the two products are added together. The final step in calculating the relative atomic mass is to divide your answer by 100. This is because the abundances are given as percentages. This means chlorine has a relative atomic mass of 35.50.

Here's a challenge. From the following data, calculate the relative atomic mass of uranium. Pause and continue when ready. Make sure you write your working out down as you do the calculation.

• Isotope : 238U and Relative abundance/%  : 99.27

• Isotope : 235U and Relative abundance/%  : 0.72

• Isotope : 234U and Relative abundance/%  : 0.01

Hint: you add the three products this time of (isotope mass number x relative abundance).

The correct answer and working out is displayed on the screen.

(99.27 x 238) + (0.72 x 235) + (0.01 x 234) = 23,626.26 + 169.2 + 2.34 = 23,797.8

Remember, you then have to divide this value by 100. This gives 237.99. Did you get it right?

In summary, the relative atomic mass is the average weighted mass for the isotopes of a particular element. To calculate this, you need to multiply the relative abundances by the mass number of the isotope, add these products together for each isotope, and then divide the total by 100 – and that's how you calculate the relative atomic mass of any element from its isotopes in the periodic table.