Humboldt State University ® Department of Chemistry

Richard A. Paselk

	Science 331
Fall 2004	Lecture/Activity	Office: SA560a
Notes: 27 October		Phone: x 5719 Home: 822-1116
		e-mail: rap1

Chemical Periodicity, cont.

Terms:

• Period: the rows of elements showing a repeating pattern of properties (e.g. Na - Ar).
• Group: a vertical column of elements on the table sharing a family resemblance of properties (e.g. Li - Fr).
• Representative elements: the elements of the s-block and p-block (blue and green on the table below).
• Transition metal elements: the elements of the d-block (yellow in the table below).
• Inner-transition metal elements: The Lanthanides and Actinides (not shown on the table below)
• Groups:
  • IA = alkali metals;
  • IIA = Alkaline earth metals;
  • VIIA = Halogens (note the generic symbol of X standing for any halogen);
  • VIIIA = Noble gases (older = inert gases).

You should know the terminology above and memorize the names and symbols for the elements shown in the table below.

Examples of Periodic Properties

Let's look at some of the elements and see what their properties are like:

• Group IA, on the left side of the chart, is known as the alkali metals because the react with water to produce strong bases (a base is alkaline). Note that all of them are soft (cut with a butter knife), low density (Li floats on oil, Na and K float on water), very reactive metals. All of them react with water with Li<Na<K<Rb<Cs. in each case the metal gives its electron to water leaving hydroxide ion (OH^- a base) and hydrogen gas. For example with sodium:

• Group VIIA, on the right side of the chart, is known as the halogens. The halogens form acids with water, are gases at the top of the Periodic Chart and high vapor pressure liquids, then solid going down; exist as diatomic molecules (X₂), and are very reactive towards metals. For example sodium reacts violently with chlorine gas to give table salt (NaCl):

• Look at the elements of Period 3 (samples of Na - Cl, alkali metals and halogens) Note how their properties change from metallic to non-metallic.

General Properties:

• Combining ratios with hydrogen. 1 - 4 - 1: LiH, BeH₂, B₃, C₄, H₃N, H₂O, HF.
• Ions:
  1. Group IA always +1
  2. Group IIA always +2
  3. Group IIIA commonly +3, Al always+3
  4. Group IV +4 or -4 (usually covalent)
  5. Group VA commonly -3
  6. Group VIA commonly -2
  7. Group VIIA commonly -1
• Metal oxides basic combined with water (e.g. NaOH, KOH), non-metal oxides acidic combined with water (e.g. SO₂ and water gives H₂SO₃, one of the important acids in acid rain).

Trends: Note the trends for

1. atomic size: decreases going from left -> right and from bottom -> top; overall have diagonal, Cs (largest) -> F (smallest).
   1. Size goes up with atomic number for any individual group.
   2. Size decreases irregularly as atomic number increases for any given period (more charge pulls electrons in to nucleus, but shielding reverses as subshells [s or p orbital sets] file.
2. ionization energy: increases from left -> right and from bottom -> top; overall have diagonal, Cs (lowest) -> F (highest).
   1. Ionization energy goes down with atomic number for any individual group.
   2. Ionization energy increases irregularly as atomic number increases for any given period (more charge pulls electrons in to nucleus, but shielding reverses as subshells [s or p orbital sets] fill.
3. Note metallic vs. non-metallic elements"
   1. Groups IA, IIA, all B groups, diagonal from Al - Po gives a divide between, with elements on line and next to line on right often semi-metallic.

Matter and Atoms

Why do we think atoms exist?

• Crystals.
• Gases.

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© R A Paselk

Last modified 27 October 2004