Elements and the Periodic Table

Atomic properties

  • atomic mass: Count the number of neutrons + protons for the atomic mass. We write the atomic mass as a superscript in front of the atomic symbol. The most common isotope of carbon has a mass of 12, 12C.

  • atomic number: Count the number of protons in the nucleus for the atomic number. The number of electrons in a neutral atom is equal to the number of protons in the nucleus, or the atomic number. The atomic number of carbon is 6.

  • isotopes
    These are atoms of the same element that differ in the number of neutrons. Different isotopes have different natural abundance. For hydrogen, the isotope 1H has the highest natural abundance.
      1H, 2H, 3H
      12C, 13C, 14C


    Each electron in an atom is described by a unique quantum number.
    1. n, principle quantum number, value= 1, 2, 3...
    2. l, orbital quantum number, value= 0, 1, 2...(n-1)
    3. m, magnetic quantum number, integral values between -l and l
    4. s, spin quantum number, value= -1/2 or 1/2

    For l= 0, there is only one kind of orbital (m= 0), the s orbital. For l= 1, p, there are 3 kinds of orbitals (m= -1, 0, 1) that we know as the px, py, and pz orbitals. For l= 2, there are 5 kinds of d orbitals (m= -2, -2, 0, 1, 2). Two electrons at most can occupy each orbital with a spin quantum number of 1/2 or -1/2. The shapes of the simplest s, p, and d orbitals are shown below.

    The orbitals differ in the number of nodes. A nodal plane is a plane where electron density is exactly zero. Look at the sine wave at right. Think of this as a 2 dimensional representation of a p orbital. The part of the line above the base line encloses the mathematically positive part of the electron density function. The curve below the baseline encloses the mathematically negative part of the electron density function. This orbital has a nodal plane, a plane where the electron density is zero, at the nucleus.

    The 1s orbital has 0 radial nodes, 2p orbitals each have 1 node, 3d orbitals have 2 nodes, etc.

    When there are 2 electrons in an orbital, their spins (1/2, -1/2) cancel. Only when there are unpaired electrons in orbitals does the atom or molecule have a net electron spin.

    Oxidation = loss of one or more electrons

    Reduction = gain of one or more electrons

Orbital Energies
There is a different ordering of orbital energies for neutral atoms and for those same atoms in ions or molecules where there can be a partial charge on the atom.

  1. Neutral, isolated atoms
    1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 5d

  2. Ions and compounds
    1s < 2s < 2p < 3s < 3p < 3d < 4s < 4p < 4d < 5s < 5p < 5d

Periodic Table

The periodic table is a device that helps us classify elements according to their electronic configuration. Many of the properties of elements and their reactivity relate to their position in the periodic table.

  1. Each row in the periodic table corresponds to one principle quantum number, n. The row beginning with H has n=1; the row beginning with Li has n=2; etc.

  2. Counting along a row from the left gives the number of valence electrons. Titanium is 4 elements from the left and has 4 valence electrons. It is in a row corresponding to n=4 so its electronic configuration as a neutral atom is [Ar]4s24d2.

  3. If you ignore the noble gases (column including He, Ne, Ar...), electronegativity increases as you go from left to right across any row. It also increases as you go from bottom to top in any column. The opposite of electronegative is electropositive.

  4. Because they have the same valence shell electronic configuration, adjacent elements in any column make similar compounds. For example: H2O and H2S, NH3 and PH3.

  5. You should memorize the symbol and atomic number for the elements H through Ne.

Professor Patricia Shapley, University of Illinois, 2012