MCAT Study Guide/Atoms

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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.