Chemistry 1.2 - Atomic Structure - Atomic Weights & Isotopes

Chapter 1 - Atomic Structure

Section 2 - Atomic Weights and Isotopes

Atomic Weights

• 1 Amu = 1/12 the mass of Carbon 12 atom which is approximately 1.66 x 10^-24 grams. Remember, amu = atomic mass units. 
• Carbon 12 has 6 protons and 6 neutrons. Mass of an atom is based upon protons and neutrons.  If you take the average of Carbon 12's protons and neutrons - 6+6/12 = 1 Amu. So, 1 Amu is just an average of the mass of a proton or a neutron. 
• Because remember from previous section, Neutron is only slightly larger than a proton, we can approximate the mass of a Neutron to be mass of a proton --> 1 Amu. 
• The mass number  is the same as the atomic mass (sum of protons and neutrons)
• On the other hand, atomic weight is the mass of grams of one mole of atoms of a given element
• Units = grams/mol
• Mole
• Everyone gets confused about mols. Mole is analogous to a Dozen. If I ask, how many pens are in a dozen? the answer is 12. If I ask, how many cars are in a dozen? the answer is 12. Dozen=12. Similarly, mole = 6.022 x 10^23. How many pens are in a mol? 6.022 x 10^23 pens. How many cars are in a mol? 6.022 x 10^23 cars. 
• So, 12 grams/mol means that, 6.022 x 10^23 atoms of Carbon combined have a mass of 12 grams. 

Isotopes

• An isotope of an element is when the atom has same number of protons but different number of neutrons
• Hydrogen's isotopes are:
• Protium --> 1 amu (1 proton, 0 neutrons) 
• Deuterium --> 2 amu (1 proton, 1 neutron)
• Tritium --> 3 amu (1 proton, 2 neutrons)
• Isotopes in general have same chemical properties because they have the same number of protons and electrons. Since neutrons have no charge on them, they don't play much role in chemical properties
• The masses on the periodic table are the weighted averages that take in to account of not just various isotopes of the respective elements, but also it's abundance. 
• If abundance of Protium is 20% and has a mass of 1 amu. Abundance of Deuterium is 70% and has a mass of 2 amu. Abundance of Tritium is 10% and has a mass of 3 amu. The atomic weight of Hydrogen would be: 0.2(1) + 0.7(2) + 0.1(3) = 0.2 + 1.4 + 0.3 = 1.9 g/mol.