Democritus coined the term 'atom' in 400 BC. He hypothesised that atoms cannot be destroyed. They differ in size, shape and temperature. They are always moving and are invisible.

Rutherford inferred that nucleus in atom is very small as compared to the total size on observing that 1 in 100000 alpha-particles completely rebound on hitting the foil.

According to Neil Bohr's model, M-shell can accommodate 18 electrons.

 Electrons are stuck in a lump of positively charged material.

 Most of the alpha particles passed through the gold foil without getting deflected.

J.J. Thomson used cathode rays in his experiment. Rays attracted a positively charged plate kept over the cathode ray tube. It made him infer that the rays are made up of negatively charged particles, which were later called electrons.

Number of protons = Atomic number Mass number = Number of protons + Number of neutrons = 48 + 36 = 84

The atomic mass is rounded off to give the mass number, which is 11. Mass number = Number of protons + Number of neutrons  Number of protons = Number of electrons = 5 So, the number of neutrons present in boron = Mass number - Number of protons = 11 - 5 = 6

Atomic number of the element is 7 because it contains 7 electrons, which equals 7 protons. Number of protons = Atomic number

 They have the same atomic mass.

The atomic number of the element is 16. 

Neutron is the heaviest subatomic particle. Neutron is 1842 times heavier than an electron. Mass of neutron is 1.0086654 a.m.u. or 1.6749 x 10^-27 kg.

Rutherford's experiment is also known as Geiger-Marsden experiment. This experiment lead to the discovery of atomic nucleus. 

Br has the tendency to gain electrons. Br has 7 electrons in its outermost shell and by gaining one more electron, it can attain stability.

This was a limitation of Rutherford's model. He said that nucleus and electrons are held together by electrostatic force while revolving around nucleus, but this lead to confusion. According to the electromagnetic theory, an accelerating charged particle must emit radiation and lose energy. Because of this loss of energy, the electron would slow down and will not be able to withstand the attraction of the nucleus. As a result, the electron should follow a spiral path and ultimately, fall into nucleus. But, it does not happen.