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Measuring the Atom The Atom All atoms are made up of subatomic particles There are many subatomic particles, but we will limit our discussion to protons, neutrons, and electrons Protons and neutrons are found in the nucleus and are therefore called nucleons. The electrons are found outside of the nucleus (more on that in a month or so) Isotopes Not all the atoms of an element are equal They may differ in the number of neutrons in the nucleus. Example – – Carbon-12 – 6p+, 6n0, 6e- (6+6 = 12) Carbon-14 – 6p+, 8n0, 6e- (6+8 = 14) Weighing an Atom A proton weighs 1.673 x 10-24 g A neutron weighs 1.675 x 10–24 g An electron weighs 1.1 x 10-29 g All of these masses are very small so chemists needed a standard. They chose Carbon-12. Why? Atomic Mass Unit Chemists decided that Carbon-12 would weigh 12 atomic mass units (u) Making each neutron and proton about 1u Average Atomic Mass If we look on the periodic table we will see that atomic mass of Carbon is 12.01u But didn’t we just say that Carbon was the standard at 12u???? The standard was Carbon-12 not Carbon (which is made up of all the isotopes: 12, 13, and 14) Calculating Average Atomic Mass Carbon is made up of 3 isotopes. Carbon-12 is 98.9% Carbon-13 is 1.1% Carbon-14 is 0.0000000001% To find the atomic mass we must take a weighted average (12u x 98.9% + ~13u x 1.1% + ~14u x 1 x10-10) / 100% = 12.01u Homework Text pg 82 #19, 25