CHEMISTRY O LEVEL(FORM TWO) NOTES – PERIODIC CLASSIFICATION

PERIODIC CLASSIFICATION

PERIODIC TABLE

Definition: The periodic table is the chart or table that shows the arrangement of elements in order of increasing atomic numbers so that elements with similar properties fall in the same vertical column.

PERIODIC LAW

States that “The properties of elements are a periodic function of their atomic number in the periodic table.”

Periodicity

These are regular periodic changes of elements due to their atomic number.

In the periodic table, elements are arranged in groups and periods.

1. PERIODS – the horizontal rows of elements in the periodic table.
2. GROUPS – the vertical columns of elements in the periodic table.

GROUPS

There are eight groups in the periodic table. Groups are usually indicated by Roman numerals, e.g., I, II, III, IV, V, VI, VII, VIII. The group number signifies the number of electrons in the outermost shell. The groups are numbered from left to right.

NOTE: The lanthanide series are part of period 6, and the actinide series are part of period 7.

PERIODS

There are seven horizontal rows in the periodic table. Periods are usually indicated by normal numbers, e.g., 1, 2, 3, 4, 5, 6, 7. Elements with the same number of shells belong to the same period.

Properties of elements within a group

Group 1 elements (Alkali metals)

• Known as alkali metals; they include Li, Na, K, etc.
• Called alkali metals because they all react with water to form alkali.
• They have one electron in their outermost shell.

Group 2 elements (Alkaline earth metals)

• Called alkaline earth metals because their oxides are alkaline in nature and exist in the earth, e.g., Be, Mg, Ca.
• They have two electrons in their outermost shell.

Group 7 elements (Halogens)

• Called halogens because they react with metals to form salts.
• They have seven electrons in their outermost shells, e.g., chlorine, fluorine.

Group 8 elements (Noble gases)

• These elements are very stable. Their outermost shells are full of electrons.
• They have eight electrons in their outermost shell.

General periodic trends

The trends observed include variations in:

1. Melting point – The temperature at which a solid melts to form a liquid.
2. Boiling point – The temperature at which a liquid boils to form a gas.
3. Density – Mass per unit volume of a substance.
4. Electronegativity – Ability of an atom to attract an electron.
5. Ionization energy – The energy required to remove an electron from an atom or ion.
6. Atomic radius – The distance between the nucleus of an atom and the outermost stable energy level.

Trends across periods

1. The atomic radius of elements in a period decreases from left to right.
2. Elements on the left of the periodic table show metallic properties, while elements on the right show non-metallic properties.
3. Electronegativity increases from left to right.
4. The number of electrons and protons increases from left to right.
5. The physical states of elements at room temperature (20ºC) move from solid to gas.

General group trends

1. Atomic radius increases down the group as successive energy levels are filled.
2. Densities increase down the group.
3. Melting point decreases down the group as the element becomes less metallic in nature.
4. Electronegativity and ionization energy decrease down the group.

TRANSITION METALS

Properties:

1. They are denser metals.
2. They are strong with high melting points.
3. They form colored compounds.
4. They form insoluble oxides and hydroxides.
5. They can show variable valency (number of valence states or oxidation states).