1. Chemistry:
The Nature of Matter

2. Elements
An element is a pure substance of one type of atom that cannot be broken down into other substances
Periodic Table of Elements
Over 100 elements known, but only about 2 dozen commonly found in living systems

3. Chemical Compounds
Substance formed by chemical combination of 2 or more elements in definite proportions
Chemical formulas used to write compounds
Example: H2O, NaCl
Compounds have different properties than the elements that form them
Example: H = gas, O = gas; H2O = liquid

5. Atoms Smallest unit of matter Composed of subatomic particles:
Protons – positively (+) charged
Neutrons – not charged
Electrons – negatively (-) charged

6. Neutrons and protons have about the same mass and pack together to form the atomic nucleus at the center of the atom (nucleus is thus + charged)
Electrons are in constant motion; are attracted to the + charged nucleus but are outside of the nucleus
Atoms have equal numbers of protons (+) and electrons (-) keeping the atom’s overall charge neutral

7. Atomic Number
Atomic number = # protons and is the top number on most Periodic Tables (also, since there are equal numbers of protons and electrons in an atom, the atomic number also tells the # electrons) 6
Atomic Number C
Carbon
12.011

8. Atomic Mass
Atomic mass = total mass of an atom (protons + neutrons) and is the bottom number on most Periodic Tables 6 6 C C
Carbon
Carbon
12.011
12.011
Atomic Mass

9. Number of Neutrons
From the Mass number, we can figure out the # neutrons
(atomic weight – atomic number = # neutrons) 6 C
Carbon
12.011

10. Atomic Number, Mass, and Mass Number
Atomic number = 6 (6 protons, 6 electrons)
Atomic mass =
6 neutrons (atomic mass – atomic number) 6 C
Carbon
12.011
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11. Energy Levels of Electrons
Only electrons are involved in chemical reactions
More distant an electron is from the nucleus, the greater the energy possible in the atom

12. Energy Levels of Electrons (Continued)
Different states of energy are called energy levels or electron shells
1st shell is closest to the nucleus, has the lowest energy, and holds only 2 electrons
2nd shell has a little more energy and holds 8 electrons
3rd shell has even more energy, holding 8 electrons, etc.

13. Valence Electrons
Chemical behavior of an atom depends on the number of electrons in outermost shell = valence electrons
Electron configuration – where electrons are within the atom
Example: Oxygen – atomic number of 8, with 6 valence electrons

14. Isotopes Elements with different numbers of neutrons
Example: isotopes of carbon can have 6, 7, or 8 neutrons
Nonradioactive carbon-12
Nonradioactive carbon-13
Radioactive carbon-14
6 electrons
6 protons
6 neutrons
6 electrons
6 protons
7 neutrons
6 electrons
6 protons
8 neutrons

15. Isotopes (continued)
Isotopes identified by mass numbers – weighted averages of the masses of an element’s isotope = atomic mass
“Weighted” means abundance of each isotope in nature is considered when average is calculated
Carbon-12 is most abundant, thus Carbon’s atomic mass is

16. Radioactive Isotopes
Some isotopes have unstable nuclei that break down at a constant rate over time – this “break down” can give off radiation
Use of radioactive isotopes:
Radioactive dating of rocks and fossils
Treat cancer
Kill bacteria
Medical tracers