chemistry: the nature of matter. elements an element is a pure substance of one type of atom that...
TRANSCRIPT
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
CCarbon12.011
Atomic Number
Atomic Mass
• Atomic mass = total mass of an atom (protons + neutrons) and is the bottom number on most Periodic Tables
6
CCarbon12.011Atomic Mass
6
CCarbon12.011
Number of Neutrons
• From the Mass number, we can figure out the # neutrons
• (atomic weight – atomic number = # neutrons)
6
CCarbon12.011
6
CCarbon12.011
Go to Section:
Atomic Number, Mass, and Mass Number
Atomic number = 6 (6 protons, 6 electrons)
Atomic mass = 12.011
6 neutrons (atomic mass – atomic number)
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 electrons6 protons6 neutrons
6 electrons6 protons8 neutrons
6 electrons6 protons7 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