-> There are now 118 known elements known at present. Of these 118 elements,
94 occur naturally on Earth. Six of these occur in extreme trace quantities
i.e. technetium (atomic number 43), promethium (atomic number 61), astatine (atomic number 85), francium (atomic number 87), neptunium (atomic number 93), and plutonium (atomic number 94).
-> The periodic table is a tabular method of displaying the elements in such a way that the elements having similar properties occur in the same group or same vertical column.
e.g. Na & K, both have similar properties such as:-
* They have one valence electron.
* Their valency is also one.
* They react with hydrochloric acid and formed metal chloride and so on.
Dobereiner’s Triads
-> In 1829, a scientist named ‘Dobereiner’ said that when the elements are arranged in increasing order of atomic masses, a group of three elements having similar chemical proportions is obtained.
These elements are known as the triad.
According to this, the atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic mass of the other two elements.
-> But the actual mass of strontium is 88. So, this Doberenier method doesn’t produce actual value, it produces approx value.
-> But the actual mass of Bromine is 81. So,
this Doberenier method doesn’t produce actual value, it produces approx value.
Newland Law of Octaves
-> In 1864, Scientist named ‘Newland’ arrange the elements on the basis of increasing atomic masses and said that the physical and chemical properties of 8th element are similar to properties of the 1st element.
In other words, the properties of the 8th element are a repetition of properties of the 1st element.
e.g. Li, Na, K
Be, Mg, Ca
Note:- It gave a very important conclusion that there is some systematic relationship between the order of atomic masses and the repetition of properties of elements.
Chart of Newland Octaves
|
Sa (do) |
re (re) |
ga (mi) |
ma (fa) |
pa (so) |
da (la) |
ni (ti) |
| H | Li | Be | B | C | N | O |
| F | Na | Mg | Al | Si | P | S |
| Cl | K | Ca | Cr | Ti | Mn | Fe |
| Co & Ni | Cu | Zn | Y | In | As | Se |
| Br | Rb | Sr | Ce & La | Zr | _ | _ |
Limitation of Newland Octaves
* Law of octaves was applicable only up to calcium (only for lighter elements).
* Newland adjusted two elements in the same slot like Co and Ni, having different properties.
For e.g. Co and Ni with Fluorine, Chlorine, Bromine, and Iodine.
* According to Newland, only 56 elements existed in nature and no more elements would be discovered in the future.
Mendeleev’s Periodic table
-> In 1869, scientist named ‘Mendeleev’ said
that the properties of elements are a periodic function of their atomic masses.
In other words, if elements are arranged in increasing order of atomic masses then the properties of elements are repeated after regular interval.
-> In Mendeleev’s periodic table, 8 groups and 7 periods are are present.
* Periodicity of Properties- The repetition of properties of elements after certain regular intervals is known as Periodicity of Properties.
Merits of Mendeleev’s Periodic Table
* In Mendeleev’s periodic table, he left some blank spaces. These vacant spaces were left for including elements that were not discovered at that time.
e.g. Eka-boron, Eka-aluminium and Eka-silicon
* Mendeleev’s periodic table was predicted properties of several undiscovered elements
on the basis of their position in Mendeleev’s
periodic table.
-> He predicted the presence of three elements. They were named as Eka boron, Eka-aluminium, and Eka-silicon. The properties of these three elements were similar to the actual elements discovered later.
* It is useful in correcting the doubtful atomic masses of some elements.
* Noble gases could accommodate in the Mendeleev’s periodic table without disturbing the periodic table after discovery.
Demerits or Anomalies of Mendeleev's Periodic Table
* There was no fixed position for hydrogen in his periodic table, as it resembled alkali metals as well as halogens.
* No regular trend in atomic mass. Elements of higher atomic mass were placed before elements of lower atomic mass.
e.g. Cobalt with atomic mass 58.93 was placed before nickel with atomic mass 58.71.
* Elements having different properties were placed in the same sub-group like manganese (Mn) was placed with halogens which had totally different properties.
* Position of isotopes was not decided.
e.g. Cl-35 and Cl-37.
Mendeleev’s periodic table was guided by two factors:-
i) Increasing atomic mass
ii) Grouping together of elements having similar properties
Modern periodic table
-> In 1913, scientist named ‘Henry Moseley’ showed that the atomic number of an element is a more fundamental property than its atomic mass.
The significance of atomic number in the classification of element is to being equal to the number of electrons in an atom.
It also helps in arranging the elements according to their electronic configuration (EC).
Some important points
* Modern periodic table contains 18 vertical column known as groups and 7 horizontal rows known as periods which start with alkali metals and ends with noble gas elements.
* On moving from left to right in a period, the number of valence electrons increases from 1 to 8 in the elements present.
The number of shells also remains the same.
* All the elements of a group of the periodic table have the same number of valence electrons.
-> Valence shell- The outermost shell of an atom is called a valence shell.
-> Penultimate shell- The second last outermost shell of an atom is called a penultimate shell.
i) The group 3 to 12 are known as transition elements.
ii) The elements 57 to 71 are called lanthanide series because their first element is lanthanum.
iii) The elements 89 to 103 are called actinide series because their first element is actinium.
iv) Metalloids are Boron (B), Silicon (Si), Germanium (Ge), Arsenic (As), Antimony (Sb),
Tellurium (Te) and Polonium (Po).
v)
| One Valence Electron | 1st group |
| Two Valence Electrons | 2nd group |
| Three Valence Electrons | 13th group |
| Four Valence Electrons | 14th group |
| Five Valence Electrons | 15th group |
| Six Valence Electrons | 16th group |
| Seven Valence Electrons | 17th group |
| Eight Valence Electrons | 18th group |
vi) Hydrogen is similar to those of alkali metals. Both hydrogens, as well as alkali metals, have one valence electron. 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 group 1, hydrogen is never included.
Note- In some of the periodic tables, 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.
Vii) Noble gases are also Non-metals.
Trends in Modern Periodic Table:-
* Valency * Atomic size * Metallic characters * Non-metallic characters
* Electronegativity * Chemical reactivity * Nature of oxides
(i) Valency:- The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom i.e. the combining capacity of an element is known as its valency.
Notes
-> If valence electron of an atom is 1, 2, 3 & 4 then the valency is also 1, 2, 3
& 4 respectively.
-> If valence electrons of an atom are 5, 6 and 7 then the valency is calculated
by:- 8-5=3, 8-6=2, 8-7=1
e.g. Valency of 2nd elements are:-
| Element Name | Valence electron | Valency |
| Li | 1 | 1 |
| Be | 2 | 2 |
| B | 3 | 3 |
| C | 4 | 8-4=4 |
| N | 5 | 8-5=3 |
| O | 6 | 8-6=2 |
| F | 7 | 8-7=1 |
| Ne | 8 | 8-8=0 |
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