Chapter 17 Section 2

1. Chapter 17 Section 2 Masses of Atoms

2. Atomic Mass • The nucleus contains most of the mass of the atom • Protons and neutrons are much bigger than electrons • The unit of measurement for these particles is the atomic mass unit (amu) • About equal to the mass of 1 proton or 1 neutron

3. Protons Identify the Element • The number of protons in an atom determine which element it is • Every carbon atom has 6 protons • Every atom with 8 protons is oxygen • Atomic number = the number of protons in an atom

4. Mass Number • Mass number = the number of protons + the number of neutrons in the nucleus • # of neutrons = mass number – atomic number

5. Isotopes • Isotopes = atoms of the same element that have different numbers of neutrons • Atoms with different numbers of neutrons have different properties (radioactivity)

6. Identifying Isotopes • If the number of neutrons is different, that means the atomic mass is different also • To differentiate between isotopes, write the name of the element, followed by its atomic mass • Boron-10 or boron-11 • Carbon-12 or carbon-14

7. Identifying Isotopes • Average atomic mass = weighted-average mass of a mixture of isotopes • If 4 atoms are boron-11 and 1 atom is boron-10… • 4/5 (11 amu) + 1/5 (10 amu) = 10.8 amu • This is how they find the average atomic mass on the periodic table

8. Radioactive Isotopes and Half-Lives • Atoms can be used to measure geologic time • Half-life = the time it takes for half of the radioactive atoms in in a piece of rock or bone to change into another element • So, for example, it takes 5,715 years for half of the carbon-14 in a bone to decay into nitrogen-14