Everything about Bohr’s Model of An Atom

In this post we will discuss about Bohr’s model of an atom, Bohr-Bury scheme, how to write electronic configuration of an element and nuclear composition.

Rutherford’s nuclear model of atom, although a great contribution to the science, could not explain certain observations. A brilliant description of atomic structure was proposed by Neil Bohr in 1913.

What are the postulates of Bohr’s atomic model?

The main postulate of Bohr’s atomic model are:

• In an atom, electrons revolve around the nucleus in certain definite circular paths called orbits or shells.
• Each shell or orbit corresponds to a definite energy. Therefore, these circular orbits are also known as energy levels or energy shells.
• The orbits or energy levels in an atom are characterized by an integer n, where n can have values 1,2,3,4…. The integer n is called the quantum number of the respective shell (orbit). The orbits are numbered as 1,2,3,4,… starting from the nucleus side. Thus, the orbit for which n=1 is the lowest energy level.

The orbits corresponding to n=1,2,3,4,….are also designated as K, L, M, N,…. shells. When the electron is in lowest energy level, it is said to be in ground state. Since, electrons can be present only in these orbits, hence, these electrons can only have energies corresponding to these energy levels, i.e., electrons in an atom can have only certain permissible energies.

How are the electrons arranged in an atom? (The Bohr-Bury scheme)

According to the Bohr atomic model, the electrons revolve around the nucleus in certain orbits or shells. These electrons are distributed in various orbits or shells according to a scheme proposed by Bohr and Bury. The Bohr and Bury scheme gives the following rule for the distribution of electrons in various orbits or shells.

• The maximum number of electrons which can be accommodated in a given orbit is given by the formula 2n², where n is the quantum number of the orbit. Thus,

n	Designation of orbit	Maximum number of electrons (2n2)
1	K	2
2	L	8
3	M	18
4	N	32

• The outermost orbit can have at most 8 electrons, and the next inner to the outermost (penultimate) can have at the most 18 electrons (if permissible in accordance with the rule).
• It is not always necessary to fill up an orbit completely before starting the next higher orbit.

How to write the electronic configuration of an element?

The electronic configuration of an element is defined as follows:

A stepwise distribution of electrons in different orbits (energy levels) of an atom is called its electronic configuration.

To write the electronic configuration of an element, one should proceed as follows:

• Find out the total number of electrons present in an atom of that element.
• Fill these electrons in the various orbits one by one in the order of increasing energy according to 2n² rule, i.e., the shell with the lowest energy is filled first and then the orbits of the increasing energies are filled.
• Then write the numerals denoting the number of electrons in various shells starting with the lowest. Each numeral is separated from the other by a comma.

For example, lithium, atomic number 3, has 3 electrons in its atom. According to the 2n² rule:

The shell corresponding to n=1 (K-shell) will have 2 electrons and the shell corresponding to n=2 (L-shell) will have 1 electron. Therefore, the electronic configuration of lithium is 2,1.

The distribution of electrons in various orbits, and the electronic configuration in the notation form for the first 18 elements are presented in table below:

Atomic Number	Name and symbol of element	K (n=1)	L (n=2)	M (n=3)	N (n=4)	Electronic configuration notation
1	Hydrogen (H)	1				1
2	Helium (He)	2				2
3	Lithium (Li)	2	1			2,1
4	Beryllium (Be)	2	2			2,2
5	Boron (B)	2	3			2,3
6	Carbon (C)	2	4			2,4
7	Nitrogen (N)	2	5			2,5
8	Oxygen (O)	2	6			2,6
9	Fluorine (F)	2	7			2,7
10	Neon (Ne)	2	8			2,8
11	Sodium (Na)	2	8	1		2,8,1
12	Magnesium (Mg)	2	8	2		2,8,2
13	Aluminium (Al)	2	8	3		2,8,3
14	Silicon (Si)	2	8	4		2,8,4
15	Phosphorous (P)	2	8	5		2,8,5
16	Sulphur (S)	2	8	6		2,8,6
17	Chlorine (Cl)	2	8	7		2,8,7
18	Argon (Ar)	2	8	8		2,8,8
19	Potassium (K)	2	8	8	1	2,8,8,1
20	Calcium (Ca)	2	8	8	2	2,8,8,2

Nuclear Composition : Atomic Nucleus

Nuclear composition of any atom can be expressed in terms of number of neutrons and protons inside its nucleus. Scientifically, the composition of a nucleus is described in terms of atomic number (Z) and mass number (A).

What is atomic number?

In 1931, Moseley introduced an atomic parameter called atomic number. The atomic number of an element is denoted by Z, and can have only one integral value. According to him,

The atomic number is equal to the nuclear charge. The charge on the nucleus of an atom is equal to the number of protons inside its nucleus. So, the atomic number of an element is equal to number of protons present inside the nucleus of an atom, i.e.,

Atomic number (Z) = Number of proton inside the nucleus

An atom has no net charge on it. So, in an atom, the total number of electrons is equal to the number of protons inside the nucleus. Thus, in an atom, the total number of electron is equal to atomic number of that element.

What is mass number?

The mass number of an element is denoted by A. The mass number of an element is equal to the sum of number of protons (P) and number of neutrons (N).

Thus,

Mass Number = Number of protons + Number of neutrons

i.e., A=P+N

Since, the number of protons (P) inside the nucleus of an atom is equal to the atomic number (Z) of that element. So,

Mass Number (A) = Atomic Number (Z) + Number of neutrons (N)

i.e., A=Z+N

The mass number (A) and atomic number (Z) of an element are shown as superscript and subscript respectively, on left side of the symbol of the that element, or the other way as given below, e.g., if A is the atomic mass and Z is the atomic number of an element X, then the atom of X may be denoted as,

How to determine the number of electrons, protons and neutrons in an atom?

From the knowledge of the atomic number and mass number of an element, one can determine the number of electrons, protons and neutrons in an atom of that element as follows:

For an atom, number of electrons = number of protons

and number of protons = Atomic number

Therefore, for an atom,

Number of electrons = Atomic Number

Also, Mass Number (A) = Number of protons (P) + Number of neutrons (N)

or, Mass Number (A) = Atomic Number (Z) + Number of neutrons (N)

This gives,

Number of neutrons (N) = Mass Number (A) – Atomic Number (Z).

The nuclear composition of first 18 elements are represented as given below:

Symbol of element	Name of element	Atomic Number (Z)	Mass Number (A)	No. of electrons	No. of protons	No. of neutrons
1H1	Hydrogen	1	1	1	1	0
2He4	Helium	2	4	2	2	2
3Li7	Lithium	3	7	3	3	4
4Be9	Beryllium	4	9	4	4	5
5B11	Boron	5	11	5	5	6
6C12	Carbon	6	12	6	6	6
7N14	Nitrogen	7	14	7	7	7
8O16	Oxygen	8	16	8	8	8
9F19	Fluorine	9	19	9	9	10
10Ne20	Neon	10	20	10	10	10
11Na23	Sodium	11	23	11	11	12
12Mg24	Magnesium	12	24	12	12	12
13Al27	Aluminium	13	27	13	13	14
14Si28	Silicon	14	28	14	14	14
15P31	Phosphorous	15	31	15	15	16
16S32	Sulphur	16	32	16	16	16
17Cl35	Chlorine	17	35	17	17	18
18Ar40	Argon	18	40	18	18	22
19K39	Potassium	19	39	19	19	20
20Ca40	Calcium	20	40	20	20	20

Read More: Anode rays and discovery of proton & neutron

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