Chemistry: The Nature of Matter

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

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

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

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

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

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

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

Atomic Number, Mass, and Mass Number Atomic number = 6 (6 protons, 6 electrons) Atomic mass = 12.011 6 neutrons (atomic mass – atomic number) 6 C Carbon 12.011 Go to Section:

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

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.

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

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

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 12.011

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