MCAT Study Guide/Atoms

John Dalton's Theories on the AtomEdit

a. An atom is the smallest representative unit of an element that retains all of the characteristics of that element. [Law of Conservation of Matter] b. Atoms of different elements have different properties. c. During a (non-nuclear) chemical reactions are neither created nor destroyed. d. Atoms of more than one element may react to form compounds; the number of atoms of each element is constant in a pure compound.

Components of the AtomEdit

  • Nucleus: center of the atom
    • Neutrons have no charge; have a mass of ~1 dalton; number of neutrons determines isotope
    • Protons have a relative charge of +1; have a mass of ~1 dalton; number of protons determines atomic number and element identity
  • Electron Orbitals: surround the nucleus
    • Electrons have a relative charge of -1; have a mass of ~0.0005 dalton; number of electrons is typically equal to the number of protons in the nucleus, except in ions

Energy Levels of Electron OrbitalsEdit

Quantum numbers determine placement of the electrons:

  • Principal quantum number (n) determines shell; start with 1, in increasing succession (1, 2, 3, 4, ...), in increasing distance from the nucleus and increasing energy. The K shell has a principal quantum number of 1. The total number of electrons in a shell is 2n2
  • Angular momentum quantum number (l) determines the subshell; starts with 0, in increasing succession (0, 1, 2, ..., n-1).
  • Magnetic quantum number (m1) determines the orbital; starts with l, in decreasing and increasing succession [-l, (1-l), ..., 0, ..., (l-1), l].
  • Spin quantum number (m2) determines the electron spin; value is always either -1/2 or +1/2.

Pauli's exclusion principle explains that no two electrons in an atom have exactly identical sets of quantum numbers.

Orbitals are filled from lower to higher energy levels.

Last modified on 13 November 2012, at 23:19