Q1. What are the limitations of Rutherford's model of the atom ?
Q2. Describe Bohr's model of the atom ?
Q3. Explain with examples Atomic number, Mass number, Isotopes and Isobars. give any two uses of isotopes ?
Q4. If Z=3, what would be the valency of the element? Also name the element?
Answers
Answer:
1. Limitations
Listed below are the limitations of Rutherford’s experiment
Rutherford’s model was inadequate to explain the stability of an atom.
It did not mention anything about the arrangement of an electron in orbit.
As per Rutherford’s model, electrons revolve around the nucleus in a circular path. But particles that are in motion on a circular path would undergo acceleration, and acceleration causes radiation of energy by charged particles. Eventually, electrons should lose energy and fall into the nucleus.
2. The Bohr model of the atom was proposed by Neil Bohr in 1915. It came into existence with the modification of Rutherford’s model of an atom. Rutherford’s model introduced the nuclear model of an atom, in which he explained that a nucleus which is positively charged is surrounded by negatively charged electrons.
Postulates of Bohr’s model of the atom
According to Bohr’s theory:
The atom consists of a small positively charged nucleus at its centre and surrounded by negatively charged electrons in a definite circular path.
The nucleus contains all the protons and neutrons of the atom.
The electrons of the atom revolve around the nucleus in definite circular paths known as orbits which are designated as K, L, M, N or numbered as n=1,2,3,4 outward from the nucleus. The orbits are also known as stationary orbits.
Each orbit is associated with a fixed amount of energy. The lowest energy level of the electron is called the ground state. Therefore, these orbits are also known as energy levels or energy shells.
3. Atomic number (Z)
The total number of protons in the nucleus of an atom gives us the atomic number of that atom. All the atoms of a particular element have the same number of protons, and hence the same atomic number. Atoms of different elements have different atomic numbers.
Examples:
All carbon atoms have the atomic number of 6,
All atoms of Oxygen have 8 protons in their nucleus.
Mass number (A)
The number of protons and neutrons combined to give us the mass number of an atom. As both protons and neutrons are present in the nucleus of an atom, they are together called nucleons.
Example
An atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is 12.
Nitrogen has a mass number of 14 as it has 7 protons and 7 neutrons in its atom.
Isotopes
Those elements that have the same atomic number, but a different mass number are referred to as isotopes. Isotopes occur due to the presence of a different number of neutrons in elements having the same atomic number as mass number is the sum of the number of neutrons and protons.
Example:
The isotopes of hydrogen are protium (has one proton and no neutrons), deuterium (has one proton and one neutron) and tritium (has one proton and two neutrons).
Isobars
Isobar is that element which differs in the chemical property but has the same physical property. Isobars are those elements which have a different atomic number but the same mass number. Their chemical property is different because there is a difference in the number of electrons. It has the same atomic mass but different atomic number.
Examples:
40S, 40Ar, 40Ca. 40Cl, and 40K.
4. If Z = 3, i.e., atomic number is 3.
Its electronic configuration is 2, 1, i.e.
(Z = 3) = 1s2 2s1
Hence, the valency of the element is 1 (since the outermost shell has only one electron).
The element is Lithium(Li).
Explanation:
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