1.The hydrogen atom is stable in the ground state. Why?

If the hydrogen atom is in the ground state (n = 1), there is no state of lower energy to which a downward transition can occur. Thus, a hydrogen atom in the ground state is stable.

2.The ionisation potential of hydrogen is 13.6 V. What does it mean?

The ionisation energy of hydrogen is 13.6 eV. Therefore, if the electron which has been accelerated from rest through a potential difference of 13.6 V collides with a hydrogen atom, it has exactly the right amount of energy to produce ionisation. This is a common method of producing ionisation and, therefore, the term ionisation potential is often used.

3.What are the main differences between Rutherford's model and Bohr's model of an atom?

There are two principal differences between Rutherford's model and Bohr's model of an atom.

• In Rutherford's atomic model, electrons can revolve around the nucleus in any orbit. However, in Bohr's atomic model, the electrons can revolve in orbits of definite radii. In the latter case, the requirement is that the angular momentum of an electron in the orbit must be an integral multiple of h/2π.
• In Rutherford's model, the electrons lose energy continuously and hence produce continuous spectrum. According to Bohr's model, an atom emits radiation only when an electron makes a sudden change from one energy state(higher) to another energy state (lower). Therefore, radiation of definite wavelengths is emitted that is, line spectrum is obtained.

4.What is the meaning of negative energy of orbiting electron?

The negative total energy means that it is bound to the nucleus. If it acquires enough energy from some external source (a collision, for example) to make its total energy zero, the electron is no longer bound, it is free.

5.The hydrogen spectrum contains a large number of spectral lines although the hydrogen atom contains only one electron. Why?

A hydrogen atom is usually in its lowest energy state(ground state). However, if the atom is bombarded by particles such as electron or proton, collisions can excite it. An excited atom remains excited for only about 10^-8 s ; its electron then falls back to the ground state in one step or in a series of steps and during each step, radiation of definite wavelength is emitted. When there are a large number of atoms, the different transitions take place simultaneously and radiation of many different wavelengths is emitted.

6.Explain, how Rutherford's experiment on scattering of α-particles led to the estimation of the size of the nucleus.

In Rutherford's experiment, most of the α-particles passed straight through the gold foil and a very few were scattered back. It indicated that nucleus (positively charged) is a very small part of the atom. It was found that only 1 in 8000 particles retraced its path. It means that nucleus is about 1/8000th of the size of the atom.

7.Why Thomson's model of the atom is known as plum pudding model?

It is because in Thomson's model, the electrons are assumed to be uniformly embedded in a sphere of positively charged matter like the plums are arranged in pudding.

8.What are stationary orbits?

According to Bohr's theory, only such circular orbits are permissible for the motion of an electron around the nucleus in which the angular momentum of the electron is integral multiple of h/2π. Such orbits are called stationary orbits. They are so called because electrons moving in these orbits do not radiate energy even though they are under constant acceleration.

9.State Bohr's frequency condition.

When an electron from an outer orbit of energy E₂ jumps to an inner orbit of energy E₁, the frequency ƒ of the emitted radiation is given by E₂ - E₁ = hƒ.

10.What are the values of first and second excitation potential of hydrogen atom?

Excitation energy for first excited state = -3, 4 - (-13.6) = 10.2 eV and for second excited state = -1.51 - (-13.6) = 12.1 eV. Therefore, excitation potential for first and second excited states are 10.2V and 12.1V respectively.

11.What is the significance of negative energy of electron in an orbit?

The total energy (P.E + K.E) of an electron in the orbit of an atom is negative. It means that the electron is bound to the nucleus due to electrostatic force of attraction between the nucleus and the electron. Therefore, P.E of the electron is negative and more than its K.E. As the electron is moved to higher orbits, its total energy becomes less negative and hence binding of the nucleus on the electron decreases. It follows that binding of the nucleus decreases as the electron is moved away from it.

12.What is the main conclusion of Rutherford's α-particle scattering experiment?

Most of the mass and the entire positive charge of an atom is concentrated in a very small volume of the atom called nucleus.

13.An electron orbiting in hydrogen atom has energy level of -3.4 eV. What will be its angular momentum?