Essential Concepts Chapter 3: The Periodic Properties of the Elements
Obj 3.1 Describe the historical development of the periodic table and explain the difference between Mendeleev's table and the modern periodic table.
§ In 1829 Dobereiner grouped some of the known elements in sets of three based on similar characteristics, which are referred to as Dobereiner's triads. For instance, Cl, Br, and I formed one triad, and Li, Na, and K another.
§ In 1864 Newland described what was termed the Law of Octaves, which stated that when elements were arranged by increasing atomic mass, the properties of the elements repeated with every eight elements, like the notes on a musical scale.
§ However, it is Mendeleev who is credited with developing the first useful periodic table, in 1869. Mendeleev, a Russian chemist, realized that the chemical and physical properties of the elements repeated in an orderly way when the elements were organized according to increasing atomic mass. He even correctly predicted the existence of unknown elements.
The modern periodic table looks a lot like Mendeleev's, except it is arranged by atomic number (the number of protons) instead of atomic mass (the average mass of atoms of an element).
Obj. 3.2 Predict similarities in properties of the elements by using the periodic table.
Elements within a column (called a group or family) have similar properties.
Obj 3.3 List the properties of groups of elements.
Group / Name / Valence electrons / Traits1 / Alkali metals / 1 / React vigorously with water. So reactive that they aren't found in nature in an uncombined state. Low melting point. Soft enough to be cut with a knife. Form strong bases with -OH groups.
2 / Alkaline earth metals / 2 / Release H2 gas when they react with acids. Harder and less reactive than alkali metals. Have a gray-white luster but tarnish quickly. Form weak bases with -OH groups..
3-12 / Transition metals / usually 1-2 / Hard and have high melting points. Useful as alloys. Most are colored and combine with other elements in multiple combinations.
17 / Halogens / 7 / React with metals to form salts. Found in nature in a combined state.
18 / Noble gases / 8 / Very unreactive and colorless gases. Are almost always in an uncombined state.
Within a group, the properties of the elements in the periodic table as you move down a group are:
§ atomic number increases
§ atomic mass increases
§ freezing point increases (thus elements at the bottom of the table are solid at room temperature)
§ boiling point increases (thus elements at the top of the table, where the boiling point is relatively low, are often gases at room temperature)
§ density increases
Obj. 3.4 Relate an element's valence electron structure to its position in the periodic table.
Elements within a group usually have the same number of valence electrons. Group 1 has 1 valence electron; group 2 has 2; group 13 has 3; 14 has 4; 15 has 5; 16 has 6; 17 has 7; and group 18, the noble gases, has none.
Obj. 3.5 Use the periodic table to classify an element as a metal, nonmetal, or metalloid.
Obj 3.6 Compare the properties of metals, nonmetals, and metalloids.
Where on periodic table / PropertiesMetals / left of "stair step" / Tend to form positive ions. Have high conductivity, high metallic luster, and are malleable and ductile. Except for mercury, metals are solid at room temperature.
Metalloids / along "stair step" / Some properties of metals and some of nonmetals
Non-metals / right of "stair step" / Form negative ions when reacting with metals, or share electrons when reacting with other elements. Tend to be brittle, have low conductivities (thus they make good insulators). They are gases, liquids, or brittle, dull solids.
Obj 3.7 Describe the trends in reactivity when observing the location of elements in the periodic table.
§ The reactivity of metals increases toward the lower left of the periodic table.
§ The reactivity of non-metals increases toward the upper right of the periodic table.