Notes on periodic classification of elements 0th
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In the universe 115 elements have been discovered till today.
Each of these elements possesses different properties.
It is difficult to understand and use the properties of each element at a time.
Hence attempts were made to discover ways to learn the properties of elements in systematic order.
Dobereiner’s triads
In 1829, 30 plus elements were known. Dobereiner, a German scientist made some groups of three elements each and called them triads.
Characteristics:
Atomic mass of the second element of a triad is nearly equal to the arithmetic mean of atomic masses of other two elements.
Elements in triad have similar properties.
Limitations:
Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could arrange only 9 elements in triad form.
Newland’s law of Octaves
Newland an English chemist in 1866 gave Law of Octaves.
Till then 56 elements were known.
Law of Octaves says that “If elements are arranged by the increasing order of their atomic masses, property of every eighth element (starting from first element) repeats”.
Characteristics of Law of octaves:
It contained the elements starting from hydrogen and ends at thorium.
Properties of every eighth element follow of that of first element.
Limitations of Newlands law of octaves:
Similarity in properties of elements as per the law was seen up to calcium only.
Only 56 elements known that time were talked about. At that time around 1 element was discovered every year. The elements to be discovered were not considered.
At many places, 2 elements were placed in a single slot (ex Co &Ni)
Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
Mendeleev’s Periodic Table
Dmitry Mendeleev a Russian chemist in 1869 gave Mendeleev’s Periodic Table.
Till then 63 elements were known.
Mendeleev arranged elements in increasing order of their atomic mass.
He tried to put elements with similar properties in a group.
Due to this we find empty boxes in his table.
Properties of groups studied by Mendeleev:
(a) Formation of Oxides: Oxides are compounds of elements with oxygen.
For example:
Li2O , Na2O and K2O resembles to R2
MgO, CaO, ZnO resembles to RO.
(b) Formation of Hydrides: Hydrides are compounds of elements with hydrogen.
Limitations of Mendeleev’s Periodic Table:
1. Position of Isotopes
2. Anomalous pairs of elements
3. Position of hydrogen
Merits of Mendeleev’s periodic classification:
Earlier 63 elements were known.
Mendeleev discovered Prediction of new elements.
Mendeleev’s periodic table had some blank spaces in it. These vacant spaces were for elements that were yet to be discovered.
For example, he proposed the existence of some unknown elements
1. Eka – boron → Scandium
2. Eka – aluminium → Gallium
3. Eka – silicon → Germanium
Scandium, Gallium and Germanium were discovered later and their properties matched very closely with the predicted properties of Eka - boron, Eka – aluminium and Eka – silicon respectively.
Modern Periodic Table
In 1913, Moseley showed or proved that atomic number is a very important property of a element.
After that, Neil Bohr made the modern periodic table using atomic number.
Basic concept of Modern Periodic Table:
Most of the properties of an element depend on number of valence electrons.
Elements having same number of valance electrons are grouped together.
Thus elements in a group have similar properties.
Exception: In 18th group, element have 8 valence e- except Helium. But still helium is a appropriately placed in 8th group as it also has stable electronic configuration in that group. Also its properties are very similar to other elements of that group.
Characteristics of Modern Periodic Table:
In periodic table, elements have been arranged by increasing atomic number.
Horizontal rows on the periodic chart are called periods.
There are seven rows in the periodic table. Each row is called a period. The periods have been numbered from 1 to 7.
The first period is the shortest period of all and contains only 2 elements, H and He.
The second and third periods are called short periods and contain 8 elements each.
Fourth and fifth periods are long periods and contain 18 elements each.
Sixth period is very long period containing 32 elements.
Vertical columns are called groups. There are 18 groups in the periodic table.
Group 1 on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr).
Group 18 on extreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn).
Revision Notes on Periodic Classification of Elements
Placing similar groups and species together is known as Classification. Classification is needed to easily understand the properties of different elements in a periodic table. Elements with similar properties are placed in one group to understand them easily.
Dobereiner’s triad consider three elements, in which atomic mass of central element is the arithmetic mean to two other elements. For Example, Atomic masses of lithium, sodium and potassium are 7, 23 and 39. The arithmetic mean of 7 and 39 gives 23. But disadvantage is the presence of only few triads.
Dobereiner’s Triad
Newland’s Law of Octave say that elements are arranged in such a way that every eight element has same properties as in the first element. According to him, only 56 elements are found which is a drawback of Newland Law of Octaves. This was not accepted. It is acceptable only upto calcium. He also placed some unlike elements in the same slot.
Newland’s Law of Octave
Mendeleev’s Periodic Table
Mendeleev used atomic masses as the basis of arrangement of elements. According to him, elements were arranged in increasing order of their atomic masses. It believes that there was a periodic reappearance in their physical and chemical properties.
Advantages of Mendeleev Periodic Table
He left gap for some undiscovered elements. For Example, Eka Boron etc.
This table also accommodate the noble gases
Also corrected the atomic masses of certain elements.
Limitations of Mendeleev Periodic Table
Position of isotopes cannot be explained
Position of hydrogen is not fixed. It is placed in group 1A, though its some properties matches with those of halogens.
Modern Periodic Table
D. Mendeleev discovered the modern periodic table in the year 1869. According to modern periodic law, “properties of an element are the periodic function of their increasing atomic number”.
Anomalies of Modern Periodic Table
Isotopes are placed at one place in the same group.
There is no element between hydrogen and helium as atomic masses always comes in whole numbers.
Atomic number is represented by Z, and it is equal to the number of protons in the nucleus of the atom.
It also consists of 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
Elements having same number of valence electrons are placed in the same group.
As we go down in a group, number of shell increases.
Elements having same number of occupied shells are placed in same period.
Each period has a new electronic shell getting filled.
Number of elements placed in a particular period depends on the point how electrons are filled into various shell.
To find out the number of electrons in a shell, 2n2 formula can be used, where is n is shell number.
K Shell n = 1 or 2n2 = 2(1)2 = 2
L shell n = 2 or 2n2 = 2(2)2 = 8
Valence electron also determines the number of bonds which is formed by an element.
Trends in Modern Periodic Table
Valency and Valence Electrons - On moving left to right in a period, valency increases and then it decreases. But it remains same down in a group. As we move from left to right in a period, valence electron increases and remain same as we go down the group.
Atomic Size - It decreases left to right in a period as the nuclear charge increases due to large positive charges on the nucleus. Atomic size increases down in a group due to decrease in nuclear charges and addition of new shell.
Metallic Character - Ability of atom to lose the electron is known as Metallic Character. Metallic character decreases from left to right in a period. This is due to increase in nuclear charge. But non-metallic character increases left to right in a period. And metallic character increases down the group as the size increases it can easily lose electron.
Ionization Energy is the energy required to remove an electron from an isolated gaseous atom. Ionization energy increases as we move left to right in a period. This is due to increase in nuclear charge as we move left to right in a period. But down in a group ionization energy decreases due to decrease in nuclear charge but there are some exceptional cases.
Electropositive Character decreases from left to right in a periodic table and increases down the group. This is due to decrease in metallic character from left to right in a period.
Basic Character of Oxides increases down the group as atomic radius increases and ionization energy decreases. This is due to increase in metallic character or electro positivity of elements. Acidic character of oxides decreases as non-metallic character of elements decreases from top to bottom.