Maharashtra Board Class 11 Chemistry Chapter 4 Structure of Atom PDF Download

Structure of Atom

4.1 Subatomic Particles

Dalton's atomic theory was able to explain the laws of chemical combination successfully. However, it failed to explain some properties of matter. For example, it could not explain why substances like glass or ebonite when rubbed with silk or fur, generate electricity. Discovery of subatomic particles in late nineteenth and early twentieth century set a blow to Dalton's atomic model of hard sphere. Three important subatomic particles which are of concern to Chemistry are proton, electron and neutron. Proton and neutron are present in the atomic nucleus and together are called nucleons. Electrons are present in the extranuclear part of an atom. The properties of electron, proton and neutron are summarised in Table 4.1

4.1.1 Discovery of Electron

In the year 1897, J. J. Thomson investigated the cathode rays and found that the cathode rays are a stream of very small, negatively charged particles which are 1837 times lighter than a hydrogen atom and are present in all atoms. Later these particles were named as electrons.

4.1.2 Discovery of Proton

In the year 1911, Ernest Rutherford found in the experiment of scattering of α-particles by thin gold foil that a few α-particles bounce back. From this he inferred the presence of massive and positively charged nucleus inside the atom. Following the discovery of nucleus in an atom, Rutherford found (1919) that fast moving α-particles transmuted nitrogen into oxygen with simultaneous liberation of hydrogen.

\[{}_{7}^{14}\text{N} + {}_{2}^{4}\text{α} \rightarrow {}_{8}^{17}\text{O} + {}_{1}^{1}\text{H}\]

He further showed that other elements could also be transmuted, but hydrogen was always emitted.

Teacher's Note

When you rub a balloon on your hair, it gets negatively charged. This is because electrons move from your hair to the balloon. Just like this, J. J. Thomson found electrons by studying cathode rays.

Exam Trick

Remember: Electrons are negative and 1837 times lighter than protons. So in exam, if they ask about the lightest particle in the atom, the answer is always electron!

Points to Remember

Atoms have three main particles inside them: proton, neutron, and electron.
Protons and neutrons stay in the nucleus at the center.
Electrons move around the nucleus in the space outside.
Electrons are much lighter than protons and neutrons.

4.1.3 Discovery of Neutron

Existence of an electrically neutral and massive particle in the nucleus was predicted by Ernest Rutherford in 1920 to account for the disparity in atomic number and atomic mass of an element. In the year 1932, James Chadwick measured velocity of protons knocked out from paraffin by an unidentified radiation from beryllium. From that he determined the mass of the particles of the unidentified neutral radiation which came out to be almost the same as that of a proton. He named this particle as 'neutron' which was predicted by Rutherford earlier.

4.2 Atomic Number and Atomic Mass Number

The number of protons in the nucleus is chemical identity of an element. This number is called as atomic number (Z) of the element. The positive charge on the nucleus is due to the protons present in it (+Ze). Atom being electrically neutral, it contains the same number of extranuclear electrons in it as its atomic number. Therefore the total electronic charge on an atom is -Ze. Thus in any atom:

Atomic number (Z) = Number of protons = Number of electrons

As can be seen from Table 4.1, mass of an electron is negligibly small compared to that of the nucleons. As a result the mass of an atom can be considered to be concentrated in its nucleus. The approximate mass of one proton or one neutron is 1u. Therefore approximate atomic mass in daltons is numerically equal to the number of nucleons in the atom. The number of neutrons in the nucleus is designated by the symbol N; and the total number of protons and neutrons, that is nucleons, in an atom is called its atomic mass number (A).

Mass number (A) = Number of protons (Z) + Number of Neutrons (N)

Therefore A = Z + N or N = A - Z

The composition of any atom is represented by element symbol (X) with the atomic mass number (A) as superscript on left and atomic number (Z) as subscript its:

\[{}_{Z}^{A}\text{X}\]

The atom or nucleus having a unique composition as specified by \[{}_{Z}^{A}\text{X}\] is called a nuclide.

Teacher's Note

Think of atomic number like your Aadhaar number. It is unique for each person. Same way, atomic number is unique for each element. Carbon always has 6 protons, oxygen always has 8 protons.

Exam Trick

Remember: A = Z + N. Always use this formula. If they give you A and Z, you can find N. If they give you A and N, you can find Z!

Points to Remember

Atomic number tells us how many protons are in an atom.
Mass number tells us how many protons and neutrons are together in an atom.
An atom is always neutral, so protons = electrons.
Neutrons = Mass number - Atomic number.

4.3 Isotopes, Isobars and Isotones

Similarities in composition of nuclides results in three types of relationships.

i. Isotopes: Some elements exist as single natural nuclide. For example \({}_{9}^{19}\text{F}\). However, many elements exist naturally as mixture of two or more types of atoms or nuclides. These individual nuclides are called isotopes of that element.

All the isotopes of an element have the same number of protons but different number of neutrons in their nuclei. As the proton number is the atomic number, all the isotopes of an element have the same position in the modern periodic table which exhibit similar chemical properties. All the natural isotopes of an element coexist and a definite natural abundance. Table 4.2 shows various features of the three common isotopes of carbon.

Teacher's Note

Carbon has three types. All have 6 protons, but different neutrons. Like three brothers from the same family - they look similar but are different. Carbon-12 and Carbon-14 both are carbon, but different.

Exam Trick

Remember: Isotopes = Same element, different mass numbers. Same number of protons but different neutrons. Like \({}_{17}^{35}\text{Cl}\) and \({}_{17}^{37}\text{Cl}\) - both are chlorine!

Points to Remember

Isotopes are atoms of the same element with different mass numbers.
Isotopes have the same atomic number but different neutron numbers.
Isotopes have similar chemical properties because they have the same number of electrons.
Carbon-12 is the most common form of carbon found in nature.

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