Science [7 marks Questions]

The main postulates of Neil’s Bohr are as follows.

1. In atoms, electrons revolve around the nucleus in special orbits called discrete orbits or shells or energy levels.
2. While the electrons revolve, they do not radiate energy.
3. The circular orbits are numbered as 1,2,3,4 or designated as K, L, M, N shells. These numbers are referred as principal quantum numbers (n).
4. K shell (n = 1) is closer to the nucleus and is associated with lowest energy. L, M, N are the next higher energy levels. As the distance from the nucleus increases, the energy of the shells also increase.
5. The energy of each orbit or shell is a fixed quantity.
6. As the distance from the nucleus increases, the size of the orbits also increases.
7. The maximum number of electrons that can be accommodated in an energy level is 2n² (n = quantum number of its orbit).
8. When an electron absorbs energy, it jumps from lower energy level to higher energy level.
9. When an electron returns from higher energy level to lower energy level, it gives off energy.

Energy levels around the nucleus of an atom: Bohr's model.

1. It has eight vertical columns called ‘groups’ and seven horizontal rows called ‘period’. 2. Each group has two subgroups ‘A’ and ‘B’. All the elements appearing in a group were found to have similar properties. 3. For the first time,-elements were comprehensively classified in such a way that elements of similar properties were placed in the same group. 4. It was noticed that certain elements could not be placed in their proper groups in this manner. The reason for this was wrongly determined atomic masses, and consequently those wrong atomic masses were corrected. Eg: The atomic mass of beryllium was known to be 14. Mendeleev reassessed it as 9 and assigned beryllium a proper place; 5. Columns were left vacant for elements which were not known at that time and their properties also were predicted. This gave me the motivation to experiment in Chemistry. Eg: Mendeleev gave the names Eka Aluminium and Eka Silicon to those elements which were to be placed below aluminium and silicon respectively in the periodic table and predicted their properties. The discovery of Germanium later on, during his lifetime, proved him correct

Property	Mendeleev’s prediction (1871)	Actual property (1886)
Atomic Mass	About 72	72.59
Specific Gravity	5.5	5.47
Colour	Dark grey	Dark grey
Formula of oxide	EsO2	GeO2
Nature of chloride	ESCl4	GeCl4

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Hydrogen is the lightest, smallest and first element of the periodic table. Its electronic configuration (1s¹) is the simplest of all the elements. It occupies a unique position in the periodic table. It behaves like alkali metals as well as halogens in its properties. In the periodic table, it is placed at the top of the alkali metals.

• Hydrogen can lose its one electron to form a hydrogen ion (H⁺) like alkali metals.
• It can also gain one electron to form the hydride ion (H^–) like halogens.
• Alkali metals are solids while hydrogen is a gas.
  Hence the position of hydrogen in the modem periodic table is still under debate
  as the properties of hydrogen are unique.