Isotopes and Atomic Mass

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Presentation transcript:

Isotopes and Atomic Mass L.O. To understand isotopes and atomic mass

What do you notice about these elements?

Isotopes of carbon Most naturally-occurring carbon exists as carbon-12, about 1% is carbon-13 and a much smaller amount is carbon-14. 6 protons 6 neutrons 6 electrons 6 protons 7 neutrons 6 electrons 6 protons 8 neutrons 6 electrons

Why does the mass number change in an isotope? Particle Mass Charge proton 1 +1 neutron electron almost 0 -1 The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

Properties of isotopes The isotopes of an element are virtually identical in their chemical reactions. This is because they have the same number of protons and the same number of electrons. The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density. Natural samples of elements are often a mixture of isotopes.

Isotopes of hydrogen Hydrogen-1 makes up the vast majority of the naturally-occurring element but two other isotopes exist. hydrogen deuterium tritium 1 proton 0 neutrons 1 electron 1 proton 1 neutron 1 electron 1 proton 2 neutrons 1 electron

Isotopes of chlorine About 75% of naturally-occurring chlorine is chlorine-35 and 25% is chlorine-37. 17 protons 18 neutrons 17 electrons 17 protons 20 neutrons 17 electrons

What are the particle numbers in each isotope? Isotopes of oxygen Almost all of naturally-occurring oxygen is oxygen-16 but about 0.2% is oxygen-18. What are the particle numbers in each isotope? oxygen-16 oxygen-18 protons neutrons electrons 8 8 8 10 8 8

Atomic Mass of elements The atoms of each element have a different mass. The relative atomic mass is a way of saying how heavy atoms are in comparison to others. Carbon is given a relative atomic mass (RAM) of 12. The RAM of other atoms compares them with carbon. Eg. Hydrogen has a mass of only one twelfth that of carbon and so has a RAM of 1. Below are the RAMs of some other elements. Element Symbol Times as heavy as carbon R.A.M Helium He one third Beryllium Be three quarters Molybdenum Mo Eight Krypton Kr Seven Oxygen O One and one third Silver Ag Nine Calcium Ca Three and one third 4 9 96 84 16 108 40

Complete the Isotopes and atoms sheet

Isotopes and RAM Many elements are a mixture of isotopes. The RAM given in the periodic table takes account of this. To calculate the RAM of a mixture of isotopes, multiply the percentage of each isotope by its atomic mass and add them together. For example, chlorine exists as two isotopes: chlorine-35 (75%) and chlorine-37 (25%). = (0.75 x 35) + (0.25 x 37) = 26.25 + 9.25 = 35.5 RAM of chlorine = (75% x 35) + (25% x 37)

Calculating RAM Bromine contains 50.5% bromine-79 and 49.5% bromine-81. What is the RAM of naturally-occurring bromine? = (0.505 x 79) + (0.495 x 81) = 39.895 + 40.095 = 79.99 = 80 (the RAM is usually rounded to the nearest whole number) RAM of bromine = (50.5% x 79) + (49.5% x 81)

RAM Questions Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56. What is the average atomic mass of silicon if 92.21 % of its atoms have a mass of 27.977 amu, 4.07 % have a mass of 28.976 amu, and 3.09 % have a mass of 29.974 amu? Calculate the average atomic mass for neon if its abundance in nature is 90.5% neon-20 (19.922 amu), 0.3% neon-21 (20.994 amu), and 9.2% neon-22 (21.991 amu). Calculate the average atomic mass of silver if 13 out of 25 atoms are silver-107 and 12 out of 25 atoms are silver-109. Calculate the average atomic mass of chromium.