Isotopes
1.6
All atoms of carbon have 6 protons, so its atomic number is 6. Most carbon atoms have 6 neutrons, so the mass number is 6 + 6 = 12. This form of the carbon atom is written as 126 C. Another form of carbon, 146 C, has an atomic number of 6 (6 protons) and a mass number of 14. It must therefore have 8 neutrons (14 – 6). 146 C is sometimes written as carbon-14. 126C and 146 C are isotopes of carbon. 3
Write the isotope symbol for an atom that has 17 protons and 18 neutrons.
4
Identify all the subatomic particles in an atom of carbon-13.
KEY INFORMATION In the symbol 126C, the smaller number (6) is the atomic number and the larger number (12) is the mass number.
Relative abundance of isotopes Most elements have two or more isotopes. For example, hydrogen has three common isotopes. Isotope
Electrons
Protons
Neutrons
Mass number
H
1
1
0
1
H
1
1
1
2
H
1
1
2
3
1 1 2 1 3 1
DID YOU KNOW?
The relative atomic mass of an element is the average mass of the different isotopes of an element. Chlorine’s A r of 35.5 is an average of the masses of the different isotopes of chlorine. Cl and 37 Cl. If there were 50% There are two main isotopes 35 17 17 of each of the isotopes what would be the average mass? The Cl isotopes. So we need answer is 36. But there are less of the 37 17 a relative abundance calculation: mass of first isotope mass of second isotope × e % of second isotope × % of first isotope Ar = 100
The mass numbers of the isotopes of hydrogen are 1, 2 and 3. However, there are unequal proportions of each type of isotope in a sample of hydrogen gas, so the average atomic mass of hydrogen is 1.008.
For example, for chlorine the abundance values are: 75% 35 Cl and 25% 37 Cl 17 17 Therefore: Ar = (75 × 35) + (25 × 37) 100 2625 + 925 = 100 = 35.5 5
Explain the similarities and differences between the three isotopes of hydrogen.
6
Element X has two isotopes, mass 27 and 29. Calculate the relative atomic mass of X if the first isotope has abundance of 65% and the second isotope has 35% abundance.
Google search: ‘webelements’
50952_P012_031.indd 25
25
8/10/16 1:50 PM