Structure of Atom Test - 1

• J. J. Thomson, who discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model.
• In Thomson's model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "plums" surrounded by positively charged "pudding".
• The 1904 Thomson model was disproved by Hans Geiger's and Ernest Marsden's 1909 gold foil experiment.

The emission of free electrons from a metal surface when light is shone on it is called the photoemission or the photoelectric effect. This effect led to the conclusion that light is made up of packets or quantum of energy. Einstein already associated the light quantum with momentum. This strongly supported the particle nature of light and these particles were named photons. Thus, the wave-particle duality of light came into picture. Einstein won the Nobel Prize for Physics not for his work on relativity, but for explaining the photoelectric effect.

Azimuthal quantum number l = 1 is for p and l = 2 is for d
Now Cr has configuration.
1s²,2s²,2p⁶,3s²,3p⁶,3d⁵,4s¹
Hence there are 12, p-electrons and 5, d-electrons

Mass number. The mass number (symbol A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.
mass No. = no. of protons + no. of neutrons.

Isoelectronic species refers to the elements that have the same number of electrons.
Mg²⁺ is a 10 electron species (1s²,2s²,2p⁶).Its configuration is like that of Ne.
Thus it is isoelectronic with any element having 10e- or we can say 8e- in its valence shell.
Atomic number of Sodium (Na) is 11 after loosing one electron it became Na+ and have 10 electron.
Thus Mg2⁺ is isoelectronic with Na⁺.

Rutherford's model of an atom: Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil.
On the basis of his experiment, Rutherford put forward the model of an atom, which had the following features:

• There is a positively charged centre in an atom called the nucleus.
• Nearly all the mass of an atom resides in the nucleus. The electrons revolve around the nucleus in well-defined orbits.
• The size of the nucleus is very small as compared to the size of the atom.