Isotopes

Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also different substantially in terms of their nuclear stability. Let's finish our table of examples of isotopes:

You should be able to fill in the blanks in a table like this with, the aid of a periodic table, on a quiz.

Determination of Atomic Mass:

We want to be able to figure out the atomic mass of a sample with a particular isotopic composition.

Example: Cu occurs as an isotopic mixture of 69.09% ⁶³Cu (mass = 62.93 amu) and 30.91% ⁶⁵Cu (64.93 amu). What is the atomic mass of copper in this sample.

Chemical Periodicity

Look at the Periodic Chart on the wall.

• The vertical columns of the charts are called groups. Group: a vertical column of elements on the table sharing a family resemblance of properties (e.g. Li - Fr).
• The rows are referred to as Periods. Period: a row of elements showing a repeating pattern of properties (e.g. Na - Ar & Li-Ne). The pattern arises due to a repetition or periodicity of chemical properties. For example:
  • Group VIII is known as the Noble Gases, or sometimes the Inert Gases because until the 1960's they had no known compounds. Very unreactive and only known compounds are with very reactive elements like F and O, and even they don't form compounds with smaller Noble gases such as He and Ne.
  • Combining ratios with Hydrogen: LiH, BeH₂, BH₃, CH₄, H₃N, H₂O, HF; NaH, ..., HCl; etc.
  • IA & IIA oxides are basic, combining with water to give compounds such as NaOH and Mg(OH)₂, while VIA & VIIA oxides form acids with water such as H₂SO₄ and HClO₄. Intermediate groups show transitional behavior, e.g. carbon froms a weak acid, nitrogen strong acid, aluminum a weak base, phosphorous a weak acid, sulfur a strong acid.
  • Properties go from metallic in Group IA to non-metallic in VIIIA.
• Note the numbering of the groups. The numbers from 1 - 18 are the internationally accepted numbers. We will also use the I - VIII "American" numbering system for the representative elements (the "A" groups on the chart).
  • Representative elements: the elements of the s-block and p-block ("tallest" columns, 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 (1) = Alkali metals;
    • IIA (2) = Alkaline earth metals;
    • VIIA (17) = Halogens (note the generic symbol of X standing for any halogen);
    • VIIIA (18) = 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.

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 they 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 hydrogen leaving hydroxide ion (OH^- a base) and hydrogen gas. For example with sodium:

2 Na + 2 H₂O 2 Na⁺ + 2 OH^- + H₂

• 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 solids 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):

2 Na + Cl₂ 2 NaCl

• Look at the elements of Period 3 (Na - Cl) Note how their properties change from metallic to non-metallic.

Predicting the charges of elemental ions.

• For the Main Group elements (Groups IA-VIIA) can easily guess the ionic charges:
  • for I, II, & III the Group number = the ion charge, e.g. N⁺, Ca²⁺, Al³⁺
  • for Group IV tend to get covalent (non-ionic) compounds need top of PT
  • for V - VII the Group number - 8 = the ion charge, e.g. P^3- (V -8 = 5 - 8 = -3), S^2-, Br^-.
• There are of course exceptions: B generally doesn't form ions, Groups in the middle tend to form multiple ion charges as move down chart.

Transition metal ions

• Tend to have multiple charges.
• Memorize charges for ions on Peridic Chart on p 101 of text:
  • +2 & +3: Cr, Mn, Fe, Co.
  • Cu⁺, Cu²⁺
  • Ag⁺
  • Zn²⁺, Cd²⁺, Hg₂²⁺, Hg²⁺
  • Sn²⁺, Sn⁴⁺, Pb²⁺, Pb⁴⁺

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© R A Paselk

Last modified 8 September 2009