Module 2:Unit 5. Isotopes and Atomic Weights

Isotopes and Atomic Weights

Although John Dalton stated in his atomic theory of 1804 that all atoms of an element are identical, the discovery of the neutron began to show that this assumption was not correct. not all atoms of a given element are identical. Specifically, the number of neutrons can be variable for many elements.

The element carbon (C) has an atomic number of 6, which means that all neutral carbon atoms contain 6 protons and 6 electrons. In a typical sample of carbon-containing material, 98.89% of the carbon atoms also contain 6 neutrons, so each has a mass number of 12. An isotope of any element can be uniquely represented as ^A_ZX, where X is the atomic symbol of the element. The isotope of carbon that has 6 neutrons is therefore ₆¹²C. The subscript indicating the atomic number is actually redundant because the atomic symbol already uniquely specifies Z. Consequently, C612 is more often written as ¹²C, which is read as “carbon-12.” Nevertheless, the value of Z is commonly included in the notation for nuclear reactions because these reactions involve changes in Z, as described in Chapter 3 “Nuclear Chemistry”.

Isotopes are atoms that have the same number of protons but a different number of neutrons. Isotopes are chemical similar in nature since they have same number of electrons. Isotopes have identical atomic number but different mass number.

Addition to ¹²C, a typical sample of carbon contains 1.11% C613 (¹³C), with 7 neutrons and 6 protons, and a trace of C614 (¹⁴C), with 8 neutrons and 6 protons. The nucleus of ¹⁴C is not stable, however, but undergoes a slow radioactive decay that is the basis of the carbon-14 dating technique used in archaeology. Many elements other than carbon have more than one stable isotope; tin, for example, has 10 isotopes. The properties of some common isotopes are in Table 2.1 “Properties of Selected Isotopes”.

Another example:

Atomic Mass:

Most elements occur naturally as a mixture of two or more isotopes. Table below shows the natural isotopes of several elements, along with the percent natural abundance of each.

The atomic weight is the weighted average of the mass of the naturally occurring isotopes of a particular element. To determine the atomic weight of an element, two quantities are required, the mass of each isotope in atomic mass unit (1.6 *10^-24 g) and the abundance with which each isotope occurs.

Calculation of Atomic Weight:

To calculate the atomic weight or atomic mass of an atom, each exact isotopic mass is multiplied by its percent abundance (expressed as a decimal). Then, results are added together and rounded off to an appropriate number of significant figures. Values of each isotopic abundance is usually expressed in percentages and converted to decimal during calculations.

Example:

Carbon has two isotopes C-12 and C-13. The mass values of C-12 isotope C-13 isotopes are 12.00 and 13.003355 amu with abundance values 98.90% and 1.10%  respectively. Determine the average atomic mass of Carbon.

(12.000000) (0.9890) + (13.003355) (0.0110) = 12.011 amu

TRY THIS OUT!

https://phet.colorado.edu/en/simulation/isotopes-and-atomic-masshttps://phet.colorado.edu/sims/html/isotopes-and-atomic-mass/latest/isotopes-and-atomic-mass_en.html

• Go to the above activity and click on the mixture.
• Pick the element Nitrogen.
• Drag and drop one atom of each isotope of Nitrogen and then click on Nature’s mix.
• Hide the percent composition and average atomic mass boxes.
• Now estimate percent abundance from the visual represented and calculate average atomic mass.
• Open the Percent composition and average atomic mass boxes.
• Compare your value with the values provided in the boxes.

Table 2.1 Properties of Selected Isotopes

Sources of isotope data: G. Audi et al., Nuclear Physics A 729 (2003): 337–676; J. C. Kotz and K. F. Purcell, Chemistry and Chemical Reactivity, 2nd ed., 1991.

Questions:

1. Hydrogen has three isotopes that contain 0, 1, 2 neutrons. Write the isotope symbol of each isotopes of Hydrogen.

2. Calculate the atomic weight of Chlorine atom with the following information:

Ans: 1. ¹₁H, ²₁H, ³₁H

2.35.45 amu