Also Read:-Periodic classification of elements, Part-2
Dobereiner's Triads
In the year 1829, a german chemist Dobereiner observed that certain elements had similar properties and that he could put them together in groups of three elements each. These groups of three elements were called triads.
When elements are arranged in the order of increasing atomic masses, groups of three elements(knowns as triads), having similar chemical properties are obtained. The atomic mass of the middle elements of the triads is equal to the arithmetic mean of the atomic masses of the other two elements.
2:-The Alkaline Earth Metal Group:-The elements calcium, strontium and barium have similar chemical properties and form a triad.
3:-The Halogen Group:-The elements chlorine, bromine and iodine have similar chemical properties and form a triad.
The limitation of Dobereiner's classification:-It is that failed to arrange all the known elements in the form of triads of elements having similar chemical properties. Dobereiner could only identify only three triads from the elements known at the time. So his classification was not much successful.
Newland's Law of Octaves
In 1864 Newlands arranged the then-known elements in the order of increasing atomic masses and found that the properties of every eighth(8th) element are similar to the properties of the first elements.
According to Newlands law of octaves "when elements are arranged in the order of increasing atomic masses than the properties of the eighth elements(starting from given elements) are a repetition of the properties of the 1st element"
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| Classification of elements based on Newland's law of octaves |
All three elements, lithium, sodium and potassium possess similar chemical properties
Limitation of Newland's law of octaves:-
- Newland's law of octaves applied to the classification of elements up to calcium only. After calcium, every eighth element did not possess properties similar to the first elements. Thus Newland's law of octaves worked well with lighter elements only.
- Newland assumed that only 56 elements existed in nature and no more elements would be discovered in nature and no more elements would e discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newland's law of octaves.
- To fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties. For example, the two elements cobalt(Co) and Nickel(Ni) were put together in just one slot, and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.
- Iron elements (Fe) which resemble cobalt and nickel elements in properties, was placed far away from these elements.
Note:-Dobereiner's triads also exist in the columns of Newland's classification of elements based on the law of octaves. for Example:-the second column of Newland's classification has the elements lithium(Li), sodium(Na) and potassium (K) which constitute a Dobereiner's triad.
Question:-What is the periodic table?
Answer:-The periodic table is a chart of elements represented in such a way that the elements having similar properties occur in the same vertical column or group. It is called a periodic table because the elements having similar properties are repeated after certain intervals or periods and it is called tabular form.
"A periodic table consists of a horizontal row of elements called periods and vertical column called groups"
Note:-1:-Mendeleev's periodic table the elements are arranged based on their atomic masses and also on the similarity of chemical properties.
2:-The modern's periodic table is based on the atomic numbers of elements.
Mendeleev's Periodic Table
In 1869 Mendeleev gave periodic law: According to Mendeleev elements are arranged based on their atomic masses and also on the similarity of chemical properties.
The properties of elements are a periodic function of their atomic masses.
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| Mendeleev's Periodic Table |
In his periodic table, Mendeleev arranged all the then known 63 elements in the order of increasing atomic masses in horizontal rows but in such a way that elements having similar properties came directly under one another in the same vertical column or group. The similar properties used by Mendeleev to classify elements into groups were the similar formula of their oxides and hydrides.
- There were seven periods(horizontal rows) and eight groups(vertical columns) in the original periodic table of Mendeleev.
- Out of eight groups, first, seven groups are of normal elements and eight groups are of transition elements and eight groups are of transition elements.
- Noble gases were not known at that time. So, there was no group of noble gases in Mendeleev's original periodic table.
- The undiscovered elements(or unknown elements) at that time far which gaps were named by Mendeleev as eka-boron, eka-aluminium and eka-silicon by prefixing the term "eka" to the name of the preceding element in the same group. The term"eka" is derived from Sanskrit and mean first. So, eka-boron means first come boron and then the unknown elements. When the elements were discovered later on, then eka-boron was named as Scandium(Sc), eka-aluminium was named as Gallium(Ge).
- To make sure that the elements having similar properties fell in the same vertical column(or group), Mendeleev placed a few elements in the wrong order of their atomic masses by keeping the element with higher atomic mass first and the elements with lower atomic mass later.Example:-Mandleev placed cobalt(having a higher atomic mass of 58.9) before nickel(having a lower atomic mass of 58.7) so that cobalt could be in the same colour as rhodium(Rh) which closely resembles it in properties.
Merits of Mendeleev's Classification of Elements:
- Mendeleev's periodic law predicted the existence of some elements that had not been discovered at that time.
- Mendeleev's periodic table could predict the properties of several elements based on their position in the periodic table.
- Mendeleev's periodic table could accommodate noble gases when they were discovered.
Limitations of Mendeleev's classification of elements:
- The position of isotopes could not be explained.
- The wrong order of atomic masses of some elements could not be explained
- A correct position could not be assigned to hydrogen in the periodic table.
Modern Periodic Law(or Long form of Periodic Table)
According to modern periodic law, "The properties of elements are a periodic function of their atomic numbers".The modern periodic law means that if elements are arranged in tabular form in the order of increasing atomic number, then the elements having similar properties will occur after fixed intervals of periods.
Henery Moseley the scientist who said that atomic number of elements is a better basis far the classification of elements
Explanation of Modern Periodic Law:-
When the elements are arranged according to increasing atomic number, then the elements having the same number of valence electrons occurs at regular intervals(or periods).
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| Explanation of modern periodic law |
The modern periodic table was repaired by Neil Bhor. It is also known as the long form of the periodic table. In the modern periodic table, the elements are arranged in the order of increasing atomic numbers in horizontal rows called periods. We place under of valence electron so that all the elements having the same number of valence electrons come in the same vertical column or group.
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| Modern periodic table |
[1] The horizontal rows of elements in a periodic table are called periods
7 periods in the long form of the periodic table, the elements in a period have consecutive(continuous) atomic numbers.
- 1st period contains 2 elements. It is called short periods. (K shell)
- 2nd period contains 8 elements. It is called short periods. (L shell)
- 3rd periods contain 8 elements. It is also a short period.
- 4th period contains 18 elements. It is called a long period
- 5th period contains 18 elements.it is also a long period.
- The 6th period contains 32 elements. It is called very long periods.
- The 7th period contains the rest of the elements. It is incomplete.
[2] The vertical column in a periodic table are called groups
- There are 18 groups in the long form of the periodic table.
- The elements in a group do not have consecutive atomic numbers.
- All the elements in a group have similar electronic configurations and show similar properties.
- The valence shells of all the noble gases are completely filled with electronics.
- The group 3 to group 13 elements are called transition elements. In these elements, the outermost shell as well as next to the outermost shell (penultimate shell).
- The elements with atomic numbers 57 to 71 are called lanthanide series(because their first element is lanthanum).
- The elements with atomic numbers 89 to 103 are called actinide series (because their first member is actinium)
- In the modern periodic table, the elements on the left side of the periodic table are metals whereas those on the right side are non-metals.
- In the periodic table, metals have been separated from non-metals by some elements called metalloids which are placed diagonally in the periodic table.(B,Si Ge,As Sb,Po).
[3] Position of Hydrogen
Hydrogen elements hs been placed at the top of group 1, above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals. Both hydrogen as well alkali metals have 1 valence electronic each. Since hydrogen atom is very small in size, many properties of hydrogen are different from those of alkali metals. Therefore while discussing the alkali metals of group1, hydrogen is never included. In some of the periodic tables, however, hydrogen is not placed in any group. Hydrogen is treated as a very special element and placed alone at the head of the periodic table.