Chemistry:

The Nature of Matter

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 Weight

• Atomic weight = total mass of an atom and is the bottom number on most Periodic Tables

6

CCarbon12.011Atomic Weight

6

CCarbon12.011

Mass Number

• Mass number = # neutrons

• (atomic weight – atomic number = # neutrons)

6

CCarbon12.011

6

CCarbon12.011

Go to Section:

Atomic Number, Weight, and Mass Number

Atomic number = 6 (6 protons, 6 electrons)

Atomic weight = 12.011

Mass number = 6 neutrons (atomic weight – 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

Elements

• An element is a pure substance of one type of atom

• Periodic Table of Elements

• Over 100 elements known, but only about 2 dozen commonly found in living systems

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

• All isotopes of an element have the same chemical properties

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

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