Test: Structure Of The Atom- 1 - UPSC

• British physicist J.J Thomson measured the ratio of electrical charge (e) to the mass (m) of cathode ray particles by using specially designed cathode ray tubes.
• e/m = 1.758820 x 10¹¹ C/Kg
  Here m is the mass of the particle of cathode rays in Kg and e is its charge in coulomb C.
• The particles are negatively charged and therefore the charge on the cathode ray particle which was later named as an electron is represented as -e.
• The value of e/m remained constant irrespective of the nature of the gas. 

Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of different sizes and mass. 

Dalton's atomic theory explains:

1. Law of conservation of mass
2. Law of constant composition
3. Law of multiple proportions

Rutherford's scattering formula fails for very small scattering angles because due to the strong repulsive force present between the α-particles and the nucleus and also the full nuclear charge of the target atom is partially screened by its electrons.

From the α scattering experiment, Rutherford concluded that:

1. There is a positively charged centre in the atom called the nucleus. Nearby all the mass of an atom resides in the nucleus.
2. Electrons revolve around the nucleus in well-defined orbitals.
3. The size of the nucleus is very small compared to that of an atom.
   (i) Scattering follows Columb's law
   (ii) Radius of the nucleus is less than 10⁻¹⁴ m
   (iii) α particles can approach with a distance of the order of 10⁻¹⁴ m of the nucleus.

• Electronic configuration of ¹²Mg atom = 2, 8, 2 and that of Mg²⁺ ion = 2, 8 
• Number of protons in Mg atom = 2+ 8 + 2 = 12
• Number of neutrons in Mg atom = 24 -12 = 12
  [as mass number of Mg atom = 24 and number of neutrons = mass number – number of protons]

Therefore, option (d) is the correct answer.

When alpha particles are sent through a thin metal foil, most of them go straight through the foil because most part of the atom is empty space. In other words, there is no obstruction in the path of alpha particles.

Weight of 1 electron = 9.1 x 10^-31 kg

Let 'x' electrons combine to weigh 30 kg.

∴ (9.1 x 10^-31).(x) = 30

Hence, taking the value to R.H.S., we get

x = 3.29 x 10³¹ 

Points (ii) and (iv) are correct. An atom consists of a positively charged, dense and very small nucleus which have all the protons and neutrons. Positive charge is due to protons, as neutrons have no charge. Most of the space is empty because most of the alpha particles pass straight through the gold foil without any deflection.
Electrons have negative charge, it was explained by Thomson. The existance of neutron was discovered by Chadwick.

• Rest mass of the electron = 9.1093837015 × 10⁻³¹ kg
• Mass of a neutron = (1.008665 amu)
• Mass of a proton =  (1.007277 amu)

If the charge is negative, add the amount of charge to the atomic number to get the number of electrons. i.e., Here, the atomic mass given is 16, which means atomic no. should be 8 
So, 8+2 = 10  is the required answer.