10th Class Science Periodic Classification of Elements Periodic Classification of Elements

Periodic Classification of Elements

At present about 115 elements are known. All the elements are divided into groups such that the elements in the same group have similar properties. The periodic classification of elements helps in systematic study of elements and makes it easy to understand the properties of elements.

Dobereiners’s Triads

Johann Wolfgang Dobereiner, a German scientist, was the first to classify elements. He grouped the elements with similar chemical properties into groups of three called ‘Triads’. When elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was equal to the approximate arithmetic mean of the atomic masses of the other two elements of the triad. For example, Li, Na, K and Cl, Br, I.

Limitations of Triad Classification

• Large number of similar elements could not be grouped into triads. For example, iron, manganese, nickel, cobalt, zinc and copper have similar properties but cannot be placed in the triads.
• It was possible that quite dissimilar elements could be grouped into triads.
• Dobereiner failed to arrange all the elements known at that time in the form of triads.

Newlands’ Octaves

John Alexander Reina Newland arranged many of the known elements in the increasing order their atomic masses.

He noticed that every eighth element was similar in properties to the first element.

The eighth element after lithium is sodium. It is similar to lithium in many of its chemical properties. Similarly, the eighth element after sodium is potassium, whose properties are similar to sodium. The eighth element from fluorine is chlorine both of which are similar in their properties. The eighth element from nitrogen is phosphorus and both these elements are similar in properties.

Based on this observation, Newland stated his law of octaves. According to this law 'when elements are arranged in increasing order of their atomic mass, the eighth elements resembles the first in physical and chemical properties'. This repetition of properties of elements gave rise to a new term called periodicity. Periodicity is the recurrence of characteristic properties of elements arranged in a table, at regular intervals of a period

Advantages of the Law of Octaves

• The law of octaves was the first logical attempt to classify elements on the basis of atomic weights.
• Periodicity of elements was recognised for the first time.

Mendeleev's Periodic Law

Later, Mendeleev arranged the sixty-three elements known at that time in the increasing order of the atomic masses, in the form of a table called the Periodic Table. Mendeleev's periodic table further classified the elements by arranging the elements with similar properties together and separating the elements with dissimilar properties from one another.

Mendeleev stated the law of chemical periodicity as: "The physical and chemical properties of elements are periodic functions of their atomic masses."

Mendeleev's periodic table contains eight vertical columns of elements called 'groups' and seven horizontal rows called 'periods.

Achievements of Mendeleev's Periodic Table

• Mendeleev's Periodic table simplified the study of elements. The elements showing similar properties belonged to the same group, thus remembering properties of elements became simple.
• Mendeleev was able to predict and thus left three blanks for elements that were not discovered at that time. He was able to predict the properties of these unknown elements more or less accurately. He named them eka-boron, eka-aluminium and eka- silicon. He named them so, as they were just below boron, aluminium and silicon in respective sub-groups. Eka-boron was later named as scandium, eka-aluminium as gallium and eka - silicon as germanium.
• Mendeleev's periodic table could accommodate noble gases when they were discovered.

Limitation of Mendeleev's Periodic Table

• The position of hydrogen is not correctly defined. It is still not certain whether to place hydrogen in group I A or VII A.
• Isotopes are atoms of the same element having different atomic mass but same atomic number. For example., here are three isotopes of hydrogen with atomic mass 1, 2, and 3. According to Mendeleev's periodic table, these should be placed at three separate places. However isotopes have not been given separate places in the periodic table.
• Some similar elements are separated and dissimilar elements are grouped together e.g; copper and mercury have similar properties but are placed in different groups. On the other hand elements of group IA such as lithium, sodium and potassium were grouped with dissimilar- elements such as copper, silver and gold.
• Mendeleev's table was unable to explain the cause of periodicity among elements.
• Mendeleev's periodic table elements are not classified as normal elements, transition elements and noble gases.
• In 1913, Henry Gwyn Moseley showed that the atomic number of an element is a more fundamental property than atomic mass and hence atomic number is a better basis for the classification of elements.

Modern Periodic Law

The properties of elements are a periodic function of their atomic numbers. When the elements are arranged according to increasing atomic numbers, then the elements having same number of valence electrons occur at regular intervals (or periods).

Modern Periodic Table

The modern periodic table was prepared by Bohr.

The modern periodic table has 18 vertical columns known as groups and 7 horizontal known as periods.

Periods

• First period: The first period has two elements hydrogen and helium.
• Second and third period: The second and third periods are known as short periods. Each consists of eight elements. The second period starts with lithium (Z = 3) an ends with inert gas neon (Z = 10). The third period starts with sodium (Z = 11) and ends with argon (Z = 18).
• Fourth and fifth period: These two periods are long periods containing eighteen elements each. The fourth period starts with potassium (Z = 19) and ends with krypton (Z = 36). The fifth period starts with rubidium (Z = 37) and ends with xenon (Z = 54).
• Sixth period: The longest period is the sixth period that contains thirty-two elements starting with cesium (Z = 55) and ending with radon (Z = 86). This period includes 14 lanthanide elements (Z = 57 to 7l). The lanthanides are all placed in the same place due to their similar properties.
• Seventh period: The seventh period is incomplete and contains nineteen elements (Z = 87 to 103). The elements after actinium (Z = 89 to 91) are called actinides.

General Characteristics of Periods and Groups