Maharashtra Board Class 11 Chemistry Chapter 7 Modern Periodic Table PDF Download

Modern Periodic Table

7.1 Introduction

In the early nineteenth century about 30 elements were known and were classified into three types on the basis of their physical properties as: metals, nonmetals and metalloids. Subsequent noteworthy attempts for classification of the increasing number of elements based on atomic mass were Dobereiner's triads and Newlands' octaves.

Mendeleev arranged the elements known at that time in an increasing order of their atomic masses. The serial or ordinal number of an element in the increasing order of atomic mass was referred to as its atomic number. He folded this list in accordance with recurrence of properties of elements and formed his periodic table consisting of vertical groups and horizontal series (now called periods).

In Mendeleev's periodic table, elements belonging to the same group showed similar properties. Properties of elements in a series/period showed gradual variation from left to right. Mendeleev left some gaps corresponding to certain atomic numbers in the periodic table so as to maintain the periodicity of the properties. Mendeleev's periodic table was accepted by the scientific community since the newly discovered elements fitted well into the gaps with their properties as predicted by Mendeleev's periodic law. Inert gases, not predicted by Mendeleev and discovered in later years also could be accommodated in this periodic table by creating an additional group.

After the discovery of atomic structure, the atomic number, which was an ordinal number assigned to element in Mendeleev's periodic table, was recognized as the proton number, Z, of that element. This was the outcome of Henry Moseley's work (1913) on x-ray spectroscopic study of a large number of elements. Moseley showed that the frequency of x-ray emitted by an element is related to atomic number, Z, rather than the atomic mass. The atomic number, Z, was considered as more fundamental property of the atom than the atomic mass. As a result, Mendeleev's periodic law was modified. It is called the modern periodic law and is stated as: The physical and chemical properties of elements are a periodic function of their atomic numbers.

In 1869, Russian chemist Dmitri Mendeleev put forth periodic table of the 63 elements known at that time using the atomic mass and properties of elements. Mendeleev's periodic table was based on the Mendeleev's periodic law which is stated as The physical and chemical properties of elements are periodic function of their atomic masses.

Teacher's Note

Mendeleev created the first proper periodic table by arranging elements by their atomic mass. Just like we arrange students in a class from shortest to tallest, Mendeleev arranged elements by their mass and found a pattern in their properties.

Exam Trick

Remember: Mendeleev = atomic mass order. Modern periodic table = atomic number order. Atomic number is just the number of protons in the nucleus.

Points to Remember

Mendeleev arranged elements by atomic mass and found the periodic pattern.
Modern periodic law says properties repeat based on atomic number, not atomic mass.
The periodic table has 7 periods (rows) and 18 groups (columns).
All 118 elements now fit perfectly in the periodic table.
Elements in the same group have similar chemical properties.

7.2 Structure of the Modern Periodic Table

Over a long period of time many scientists have come up with different forms of periodic table. However, the so called long form of periodic table or the modern periodic table, which is a revised version of Mendeleev's periodic table, is the most convenient and widely used form of the periodic table of elements today.

The modern periodic table has horizontal rows intersecting the vertical columns giving rise to a number of boxes. The horizontal rows are called periods (which Mendeleev called series) and the vertical columns are called groups. There are seven periods (numbered 1 to 7) and eighteen groups (numbered 1 to 18) in the modern periodic table.

This numbering of the periods and groups is recommended by the International Union of Pure and Applied Chemistry, IUPAC. The boxes formed at the intersection of the periods and groups are the places for individual elements. Below the main table are placed two series containing fourteen elements each. There are in all 118 boxes to accommodate 118 elements in the modern periodic table. As on today all the 118 boxes are filled as a result of discovery of manmade elements. IUPAC has approved names and symbols of all the 118 elements.

The overall shape of the modern periodic table shows that it is divided into four blocks. Two groups on left form the s-block, six groups on the right constitute the p-block, ten groups in the center form the d-block and the two series at the bottom constitute the f-block.

Teacher's Note

The periodic table is like a telephone directory of all elements. It is organized so that similar elements are grouped together, making it easy to find information and predict how elements will behave.

Exam Trick

Remember: 7 periods = 7 shells or energy levels. 18 groups = 18 different types of elements. s-block is on the left, p-block is on the right, d-block is in the middle, and f-block is at the bottom.

Points to Remember

The periodic table has 7 horizontal rows called periods.
The periodic table has 18 vertical columns called groups.
Elements in the same group have similar properties.
The periodic table contains all 118 known elements.
The table is divided into four blocks: s, p, d, and f.

7.3 Periodic Table and Electronic Configuration

Along a period the atomic number increases by one and one electron is added to outermost shell which forms neutral atom of the next element. Every period ends with complete octet configuration (or duplet in the case of the first period) of the valence shell and the next period begins with addition of electron to the next shell of higher energy compared to the previous period. The first shell, thus, gets filled along the first period. As the first shell can accommodate only two electrons, there are two elements in the first period, namely, H (Z=1): \(1s^1\) and He (Z=2): \(1s^2\). The first period ends at He because He has complete duplet.

Electrons are filled in the second shell along the second period. The second period, thus, begins with Li (Z=3): \(1s^2 2s^1\) and ends up with Ne (Z=10): \(1s^2 2s^2 2p^6\). Ne with 8 electrons in its outermost second shell has complete octet. The second shell has electron capacity of 8. It gets filled along the second period, as the atomic number increases. Thus there are eight elements in the second period. Similarly there are eight elements (Na (Z=11) to Ar (Z=18)) having the condensed electronic configurations described together as [Ne]\(3s^{1-2}3p^{1-6}\) in the third period, as a result of completion of the third shell.

The fourth period begins with filling of 4s subshell. The first two elements of the fourth period are K (Z=19): [Ar] \(4s^1\) and Ca (Z=20): [Ar]\(4s^2\). According to the aufbau principle the next higher energy subshell is 3d, which can accommodate upto 10 electrons. Filling of the 3d-subshell results in the next 10 elements of the fourth period, from Sc: (Z=21) [Ar] \(4s^2 3d^1\) to Zn: (Z=30) [Ar] \(4s^2 3d^{10}\). After this the electrons enter the subshell 4p for the next six elements: Ga: (Z=31) [Ar] \(4s^2 3d^{10}4p^1\) to Kr: (Z=36) [Ar] \(4s^2 3d^{10}4p^6\). The fourth period, thus, contains in all 18 elements (2+10+6=18).

The fifth period accomodates 18 elements as a result of successive filling of electrons in the 5s, 4d and 5p subshells.

When Mendeleev put forth his periodic table in 1869, the atomic structure was not known. He observed periodicity in the properties of elements on arranging them in an increasing order of atomic mass. Later, with the advent of quantum mechanical model of atom, the properties of elements were correlated to electronic configuration.

You have learnt in the Chapter 4 that the electrons in atom are distributed in shells and subshells in accordance with the aufbau principle which includes increasing order of energy, Pauli exclusion principle and Hund's rule of maximum multiplicity. When elements are arranged in an increasing order of atomic number (Z), periodicity is observed in their electronic configuration and which reflects in the characteristic structure of the modern periodic table. The location of elements in the modern periodic table is correlated to quantum numbers of the last filled orbital. Let us have a deeper look into the electronic configuration of the elements and the structure of the modern periodic table.

7.3.1 Electronic Configuration In Periods

We noted earlier that periods in the modern periodic table are numbered 1 to 7. On inspection of the electronic configurations of elements in various periods we understand that the period number is same as the principal quantum number n of the outermost or valence shell of the elements.

Teacher's Note

Electronic configuration tells us how electrons are arranged in an atom. Think of it like seating students in a classroom - we fill seats row by row, starting from the front, just like electrons fill shells starting from the closest to the nucleus.

Exam Trick

Remember: Period number = valence shell number. If an element is in period 2, its outermost electrons are in shell 2. If it is in period 3, its outermost electrons are in shell 3.

Points to Remember

Each period corresponds to filling of a new electron shell.
Period number tells you which shell is being filled with electrons.
First period has 2 elements (only H and He).
Second and third periods have 8 elements each.
Fourth and fifth periods have 18 elements each.

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