Isotopes and Atomic Mass

The terms… The atom: smallest piece of matter with a distinct identity and is used to make up compounds and molecules. Sub-Atomic Particles: Atoms themselves are made of three types of matter. They are the: Proton….small, very heavy, postive charge Neutron…small, very heavy, no charge Electron…extremely small, very light, negative charge

Atomic Structure Nucleus Electrons All atoms are put together in a regular pattern. All have protons and electrons, and almost all have neutrons. Protons are in the middle, packed into a very small area called the nucleus Neutrons also fit into the nucleus The electrons fill up the rest of the volume of the atom in a large “cloud-like” volume. The nucleus has almost all the mass, but a very small volume The electrons have almost no mass, but make up almost all the volume n p p n n n p Nucleus Electrons

Rules for Atomic Structure For atoms, the number of protons determines the number of electrons. Almost all atoms have neutrons (except Hydrogen, the smallest). The number of neutrons is about the same or up to 50% more than the number of protons. The chemical properties of an atom depend on how the number of electrons and how they are arranged. The mass of the atom depends almost entirely on the number of protons and neutrons.

Properties of Sub-Atomic Particles Mass Relative Mass* Charge** Size Location Protons 1.67262 x 10-27 kg 1.00727 +1 Small (about 10-15 meters Nucleus Neutrons 1.67493 x 1.00866 Electrons 10-31 kg 0.00054858 -1 Very, very small (approaching zero volume) Entire atom * Realtive mass is based on 1 atomic mass unit (amu) = 1/12 the mass of carbon-12 ** Charge is relative to each other. Absolute charge is +/- 1.60218 Coulombs

Two new Terms Atomic Number (Z) = Number of protons Also equal to the number of electrons (for an atom) This determines the identity of the atom. ALL ATOMS WITH THE SAME ATOMIC NUMBER CHEMICALLY ACT THE SAME WAY Z = p = e(atom) Mass Number (A) = Number of protons plus the number of neutrons A = p + n

Element, defined Every element, (and there are 118 so far discovered), has a defined atomic number. All atoms with that atomic number (and so protons and electrons) is an example of that element. Because all atoms of that element have the same number of electrons, they chemically behave in exactly the same way. The number of neutrons may be different between different atoms, but that does NOT change their chemical identity.

Isotope, defined Atoms of the same element have the same number of protons may have different numbers of neutrons. Atoms of the same element, but different numbers of neutrons are called isotopes. Isotopes will chemically act the exact same way, but will have different masses. Some isotopes may be unstable, and after a period of time, fall apart. They are said to be radioactive. Many isotopes are stable and will never fall apart. Almost all elements are made of mixtures of isotopes.

Labeling Atoms and Isotopes Atoms are labeled to identify the element and isotope. Atomic number and mass number are used, in addition to the atomic symbol. “C” refers to the element carbon The atomic number is often given as a lower left subscript. The mass number is given in the upper left corner. This isotope can also be written as: Carbon-14 or C-14 C 14 - _________________ 8 neutrons 6 The number of neutrons can be calculated by subtracting the atomic number from the mass number