Question

Describe the main postulates of the Neils Bohr model of the atom. Draw and explain the atomic structure of Na and K based on it.

Answer 1

Bohr’s model of an atom: 

1. An atom is made up of charged particles, electrons, and protons. Electrons are negatively charged, whereas protons are positively charged. 

2. The electrons revolve around the nucleus in fixed orbits called energy levels or shells which are represented by numbers 1, 2, 3, — or letters K, L, M, N, — counted from the nucleus outwards. 

3. Protons are present within the nucleus. Due to the presence of protons, the nucleus is positively charged. 

4. Each energy level has a fixed amount of energy. The energy of orbits increases with distance from the nucleus. 

5. The number of electrons and protons are equal so that an atom, on the whole, is electrically neutral. 

6. There is no change in the energy of electrons as long as they revolve in the same orbit and remain stable. However, if an electron gains energy, it jumps to a higher energy level and if it loses energy, it falls back to the lower energy level. 

Structure of potassium: The atomic number of potassium is 19 and its mass the number is 39. It is represented by the symbol ³⁹₁₉K

Being a neutral atom, it has: Number of electrons = 19 = Number of protons & Number of neutrons = 39 – 19 = 20 The nucleus of potassium contains 19 protons and 20 neutrons i.e. 39 nucleons. 

According to rule 1, K – shell (n = 1) can not have more than 2 electrons. L-shell according to rule 1 

(n = 2) can not have more than 8 electrons. The new shell i.e. the M-shell begins to fill and may contain a maximum of 18 electrons. However, being the outermost shell, it can not have more than 8 electrons (Rule 2). Thus, no. of electrons in N-shell = 8. 

Now, according to rule 4, the filling of N-shell starts and since only one electron is left after filling K, L, M shell it occupies N-shell. 

Thus, the electronic configuration of potassium would be 2, 8, 8, 1. or K – shell = 2 electrons, L – shell = 8 electrons M – shell = 8 electrons, N – shell = 1 electron Structure of Sodium: Same rule applies for sodium element also. Thus, the electronic configuration of sodium would be 2, 8, 1 K shell = 2 electron L shell = 8 electron M shell = 1 electron.