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 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.
- The horizontal rows present in the periodic table are called periods.
- The vertical columns present in it are called groups. There were total eight groups in Mendeleev’s periodic table, I to VIII.
- Properties of elements in a particular period show regular gradation (i.e. increase or decrease) from left to right.
- Groups I to VII are subdivided into A and B subgroups. Groups VIII don’t have any subgroups.
- All the elements in a particular group have similar properties. They show regular gradation in their physical properties and chemical reactivities.
Limitations of Mendeleev’s Periodic Table:
1. Position of Isotopes
- Isotopes are atoms of same element having different atomic masses but have similar chemical properties.
- Isotopes are placed together by Mendeleev as they have similar properties. But then this violated the arrangement scheme of increasing atomic masses.
Mendeleev could not explain that problem.
2. Anomalous pairs of elements
At some locations, elements were put in order of decreasing atomic mass.
For example; Co, Ni and Te, I.
This was not explained by Mendeleev.
3. Position of hydrogen
Properties of H are similar to group 1 as well as group 7. But Mendeleev placed it in group 1 without any proper explanation.
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.
Atomic Number
- Atomic number is defined as the total number of protons present in the nucleus of an atom. It is denoted by ‘Z’.
- Atoms of two different elements will always have different number of protons.
- Atoms of same element have same number of protons and thus they have same atomic number ‘Z’.
In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus.
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).
