Development of Modern Periodic Table

Introduction

All known living or non-living things are classified and arranged based on their characteristics. Like different plants and animals are classified and organised into groups and species likewise many scientists tried to set the bases of the elements on their atomic number or atomic mass. Many scientists fail to arrange the elements properly. The elements are arranged according to periodic properties and atomic numbers in that table. Several scientists worked to arrange elements, but Mendeleev’s Table worked well. Mendeleev’s periodic table was later modified by atomic numbers by the scientist Henry Moseley.

Brief History of Periodic Table

For a long time, scientists tried to arrange elements in tabular form on a different basis, but most of them failed to arrange all elements in a table. In 1789 Antoine Lavoisier tried to arrange elements as metals and non-metals. Later around 1817 German scientist Johann Wolfgang Dobereiner tried to set elements in Triads of increasing atomic masses which also did not work for all elements.

Later, Scientist John Newland correlated the elements based on their atomic masses but he also failed. After scientists in Newland, many other scientists tried to arrange elements based on atomic masses and elements’ properties. After many scientists, the Russian Chemist Dimitri Mendeleev starts to set elements based on atomic masses and their chemical and physical properties. He arranged 63 elements in his periodic table. Later his theory fails to arrange all elements. In 1913, Scientists Henry Moseley modified the periodic law and arranged the elements according to atomic numbers.

Dobereiner’s Triads

John Wolfgang Dobereiner arranged the elements of similar properties in a group of three elements, that’s why it is known as the ‘Triad’. He arranged three elements according to the atomic masses. He grouped three elements in such a way that the average of their atomic mass is equal to the mass of the middle elements. Like, he grouped Lithium (Li), Sodium (Na), and Potassium (K) in a group with atomic masses of 6.9, 23, and 39. The average of their atomic mass is 23 which is the atomic mass of sodium. But he was able to arrange only three such groups from the known elements, that’s why his theory of triads failed.

Dobenier’s Triad of Atoms
Li	Na	K
7	23	39
Be	Mg	Ca
8	24	40

Newland’s Octaves

Around 1866, the scientist John Newland was inspired by the attempt of arranging elements in a group. He Tried to arrange elements in respect of their atomic masses and their properties. He arranged around 56 known elements in his table. Let’s check;

• He found that every 8th element poses a similar property to the first element. So, he compared the elements to octaves of music. That’s why his theory is known as Newland’s law of octaves.

• Potassium is the eighth element of sodium it has a similar property to sodium. So, they are arranged in the same column. Likewise, he arranged 56 known elements but did not leave any space for newly discovered elements.

• He put some elements in the same group as nickel and cobalt and chlorine and bromine put in the same group as cobalt and nickel which do not pose similar properties.

• Another drawback of his theory was after calcium, not every eights element poses similar properties to the first element. So, his theory of octaves failed.

Newland’s Octave
do	re	mi	pha	sol	la	si
sa	re	ga	ma	pa	dha	ni
Li	Be	B	C	N	O	F
7	9	11	12	14	16	19

Mendeleev's Periodic Table

After, Newland's octave theory many scientists tried to arrange elements based on their atomic masses, one such scientist was Dmitri Ivanovich Mendeleev. He was a Russian scientist, and he arranged the 63 known elements. They arrange them by their chemical and physical properties and atomic masses. He tried to find the relationship between elements and their chemical properties. He was the first who got the credit for creating the periodic table.

• He formed the oxides and hydrides of elements and considered hat as the basis of the arrangement of elements.

• He took cards and wrote their properties on the card, then pinned the cards of similar properties together.

• He also found the elements with similar chemical properties have similar occurrence and physical properties.

• He formulated the periodic law that says the atomic mass plays a vital role in periodic functions.

• He grouped elements in a vertical and horizontal column known as group and period respectively.

Mendeleev’s Periodic Table

Modern Periodic Table Introduction

Henry Moseley, 1913 modified Mendeleev’s periodic law by an atomic number of elements. He said that the atomic numbers of an element play a vital role in the arrangement of elements.

• He says that the properties of an element are periodic functions of their atomic number rather than atomic masses.

• The atomic number of molecules increases as we move from one element to another element. Thus, he arranged the elements based on their increasing atomic numbers.

• His theory fulfilled the drawbacks found in Mendeleev's Periodic Table and developed the Modern Periodic table.

• He grouped elements in 18 vertical and 7 horizontal columns that are known as groups and periods respectively.

Modern Periodic Table

History of Modern Periodic Table

The first periodic table was developed by Mendeleev in 1869, which contains around 17 columns, two complete periods and three incomplete periods. In 1871 Mendeleev revised the periodic table by repositioning 17 elements. Mendeleev with Lothar Mayer revised the table with seven-period and eighth groups. Later after the discovery of noble gases, Mendeleev and other scientists added a zero group to the periodic table. This table was used till 1930. The size of the tables was increased by the increase in the length of the table. The main disadvantage of this table was the larger size of the table. A Werner divides the short periods into two parts and a separate table for lanthanide and actinoid elements. The table was widely used till the mid-20th century.

Despite the correction made in the periodic table the position of some elements was not in sequences like cobalt and nickel and tellurium and iodine, these positions were not correct as the tellurium with high atomic weight was placed earlier than iodine with less atomic weight. Moseley when analysing the frequency of x-rays discovered that the foundation of elements is an atomic number, not atomic mass. He observed that the Argon with higher atomic mass (39.9 amu) was placed before potassium with low atomic mass (39.1 amu). This discovery of Moseley revised the periodic table to the modern periodic table based on the atomic number.

Limitations of Periodic Table

There are a few limitations of the periodic table.

• The position of hydrogen was not fixed, as hydrogen, it has properties both like halogens and alkali metals.

• Isotopes of some elements have the same atomic number but have different atomic masses, so their position is also uncertain.

• A few elements with higher atomic masses are placed earlier than the elements with lower atomic masses. Like cobalt and Nikel, etc.

Conclusion

In this tutorial, we have studied the development of the periodic table. Since 1789 the scientist was trying to arrange the elements in a table. First, the Lavoisier tried to arrange elements in metals and non-metals. Then other scientists like John Wolfgang Doberener, and John Newland tried to arrange elements according to their atomic masses. Lothar and Mendeleev arrange the elements according to their properties of elements and their atomic masses. The first periodic table which was approved was Mendeleev's periodic table, which was modified by him from time to time. He formulated the periodic law that defines the periodic property of an element defined by its properties and atomic masses of an element. Later Moseley modified the periodic law by atomic number.

FAQs

1. Who discovered the noble gases and represented them in a separate group in the periodic table?

William Ramsay discovered the noble gases and suggested adding a separate group for noble gases.

2. What are the pros of Mendeleev's Periodic table?

He left some spaces for the unknown elements in his periodic table. He grouped some elements of higher atomic mass before elements with lower atomic masses. He has given space to inert gases which were discovered later without disturbing the periodic table.

3. What are the cons of Mendeleev's Periodic Table?

He has not given space to Hydrogen in his periodic table that’s the biggest drawback of his table. Isotopes of elements did not get space in his periodic table.

4. Who revised Mendeleev’s Periodic Table by atomic number?

Henry Moseley determined that atomic number plays a more vital role than atomic masses in the arrangement of elements.

5. Name the father of the Modern Periodic table?

Henry Moseley is the father of the modern periodic table.