Hi everyone! You might be wondering why I thought of writing about the periodic table. Well, the answer is “just for fun!”
History of the Periodic Table
The first human classification of elements was done by Antonie Lavoisier back in the 18th century. In his classification, he included certain elements like Oxygen, various compounds and even certain types of energy as light! At that time elements were not defined as the fundamental ‘substances’ of matter. People who lived back then considered elements as the most fundamental things which made up the world. They didn’t know about the difference between compounds and elements and nor did they know that light was an energy.
Time passed and different people came up with different classifications, but none of them could classify all elements according to a proper pattern and that is when the British Chemist John Newlands came into play. In about 1865 he published a research paper, describing his “Law of Octaves”, which said that when the elements are arranged in the ascending order of their atomic weights, after every 7 elements, the physical and chemical properties of every element are repeated periodically. The scientists didn’t want to believe that Chemistry was related to Music. So they rejected it.
You’ll be amazed if I say that the first ‘periodic table’ is a result of Mendeleev’s attempt to make a good text book for his students. Yes. Dmitri Mendeleev wanted a better text book for his university students, but since he could not find any, he started writing one by himself. In it, he started classifying the elements which were discovered by then in the order of their atomic masses. Following in John Newland’s foot steps, in his classification, he followed the law that “characteristics of elements is a periodic function of the ascending order of the atomic masses of the elements”.
When he was arranging the elements, he saw that some elements had to switch their places in order to keep the periodicity of the characteristics. When they were switched, the atomic masses were not in ascending order. Anyway, he arranged the elements so that their characteristics occur periodically, and gave each element an index number. He also predicted the characteristics of some elements that were yet to be discovered and kept empty spaces for them in his periodic table. When time passed, those elements were discovered, had the same characteristics Mendeleev predicted and perfectly fitted in the places which were kept by Mendeleev in his periodic table.
Later it was discovered that the index numbers given by Mendeleev to the elements represented the number of protons and the number of electrons of an atom of that specific element which made that index number the true identity of an element. We call it the true identity of an element, because when we know the number of electrons, we can identify the electron configuration which is responsible for most of the physical and chemical properties of elements. Today that ‘index number’ is called as the ‘Atomic Number’ of the element.
The reason for some elements to switch places in Mendeleev’s periodic table was that some elements had different isotopes found in the environment. When calculating average atomic mass for an element it gets a higher or lower value than what it should get according to the trend. At that time, they had no idea about sub atomic particles. All they knew was the atom as a whole.
When talking about the history of our modern periodic table, Julius Lothar Meyer is another important scientist. He created a periodic table which was a creation of his own work, similar to the periodic table of Dimitri Mendeleev. Mendeleev’s table was based on qualitative properties, while Lothar Meyer’s table was based on quantitative properties. Lothar Meyer didn’t predict the characteristics of the elements which were yet to be found, but he created his table before Mendeleev. But the thing is that, he published it after Mendeleev. Mendeleev’s periodic table was published in 1869 while Meyer’s table was published in 1870.
Order of the elements in the Periodic Table
It is not that ancient periodic table we are using today. The modern periodic table which has about 118 elements is a collective creation of many many scientists, so we cannot give the credit to one specific person. In this table also, the elements are arranged in the order of their atomic numbers. The elements are arranged in the ascending order of their number of protons which is the same as the number of electrons.
The elements are arranged according to the Aufbau principle, which states the order of the building up of electrons one by one in particular sub energy levels.
Rows of the Periodic Table
Rows of the periodic table are known as “periods”, because each row is a specific ‘period’ of characteristics. As a year of four seasons Spring, Summer, Autumn and Winter repeat after another year, when one period of the periodic table is over, another period with similar characteristics to the previous one begins.
Each period is given a positive integral value as 1, 2, 3,…etc. That integral value corresponds to the number of main energy levels an atom of an element has, but it is not the the number of energy levels which are ‘completed’ with electrons. There can be electrons in higher energy levels while the lower ones are incomplete, but the lower ones cannot be empty. Therefore the integral value of the highest energy level of an atom corresponds to the number of energy levels that atom posses. Therefore, the integral value of the period in which the element is, also represents the highest main energy level of it.
So, by looking at the period of an element, we can say how many main energy levels it has and that helps us to identify some features of that elements which are based on the number of energy levels it has.
Columns of the Periodic Table
Columns of the periodic table are known as “groups”, because since characteristics of elements occur periodically, all the elements in each group have similar characteristics. There are 18 groups in the periodic table numbered from 1 to 18 according to the ascending order of atomic numbers.
Each group is numbered in such a way that all elements in a group have the same number of valence electrons. But the group number is not always equal to the number of valance electrons of the elements in that group. The number of valence electrons is equal to the group number in periodic tables where f-block and d-block are not considered. But that relationship drops in the long form of the periodic table. That does not mean that we cannot identify the number of valence electrons of an element by looking at its place in the periodic table. There is a pattern.
Number of valance electrons of the elements in the d-block vary between 2 and 1. From group 13 to 18 the number of valence electrons is the group number – 10. For an element in the group 15, the number of valence electrons is 5. So, if we know the group number of an element we can identify its chemical properties, because most of the chemical properties are based on the reactivity of the element and the reactivity of an element is based on its number of valence electrons.
- Group 1 – Alkali Metals
- Group 2- Alkaline Earth Metals
- Group 16- Chalcogens
- Group 17- Halogens
- Group 18- Noble Gases and etc.
Blocks of the Periodic Table
There are 4 types of sub- energy levels an atom can have. They are known as s, p, d and f. Energys < Energyp < Energyd < Energyf. s- sub energy level can posses a maximum of 2 electrons. p-sub energy level can posses a maximum of 6 electrons. d-sub energy level can posses a maximum of 10 electrons and f-sub energy level can posses a maximum of 14 electrons.
The elements in which the s-sub shell is being filled are together known as the s-block. The elements in which the p-sub shell is being filled are together known as the p- block. The elements in which the d-subshell is being filled are together known as the d-block and the elements in which the f-sub shell is being filled are together known as the f-block.
Number of groups in a Block
There are 2 groups in the s-block, 6 in the p-block, 10 in the d-block and 14 in the f-block. Why is that? If you observe closely, you will see that those numbers are somewhat similar to the maximum numbers of electrons sub shells can posses.
Since it needs 2 electrons for the s-block to be completed, there are 2 consecutive groups in that block. It is the same for all the blocks. For an example, 6 electrons of the p block is filled one by one along the 6 groups of that block across the particular period.
IMPORTANT NOTE: Maximum of only one subshell of a particular type can be there in a main energy level of an atom. For an example there cannot be 2 p-subshells in the 2nd energy level. And also, all the types of subshells cannot be seen in one energy level all the time. [1st energy level has only one subshell which is s. 2nd Energy level has only 2 subshells which are s and p, and so on. If you want to learn more about this, do a google search about ‘Quantum Numbers’]
Number of elements in a row of the Periodic Table
Before going into this sub topic, I have to tell you about the total number of electrons in a main energy level. It is always equal to 2n2 , where ‘n’ is the integral value of the energy level. We can see that the total number of electrons in energy levels are 2, 8, 18, 32,.. Now let’s go into our topic.
If we look at the periodic table, we see that the number of elements in periods are also 2,8,18 and 32! So, the question is, “Is there are relationship between the total number of electrons and the number of elements in a period?” the short answer is, “NO AND YES”.
Actually this happens because of the blocks that can be seen in a period. There is a period with only a row of the s-block and that period has 2 elements, because an s-subshell can have a maximum of 2 electrons. When there are both s and p, there are 8 elements, 2 for s and 6 for p and so on.
The total numbers of electrons in main energy levels has those numbers, due to the subshells they have. If there is only s, the number is 2. If there are s and p, the number is 8 and so on.
Those two are not connected, because a period does not represent a main energy level. The periodic table is arranged according to the Aufbau principle (the order of building up of electrons). For an example, the 4th period has the subshells 4s, 3d and 4p.
The numbers are equal due to a similar fact and that is subshells. But the reasons for them to be equal are not conceptually connected.
We can identify many trends across periods and down groups of a periodic table and there are many more advantages of the periodic table. See how all those things are connected to each other. Isn’t that amazing? Hope this helped. Thanks for reading!