ICSE Class 10 Physics Chapter 12 Radioactivity

Chapter 12: Radioactivity

Modern Physics

Syllabus

(i) Radioactivity and changes in the nucleus; background radiation and safety precautions.

Scope of syllabus: Brief introduction (qualitative only) of the nucleus, nuclear structure, atomic number (Z), mass number (A), radioactivity as spontaneous disintegration. α, β and γ - their nature and properties; changes within the nucleus. One example each of α and β decay with equations showing changes in Z and A. Uses of radioactivity - radio isotopes, Harmful effects, Safety precautions. Background radiation.

Radiation: X-rays, radioactive fall out from nuclear plants and other sources.

Nuclear energy: Working on safe disposal of waste. Safety measures to be strictly reinforced.

(ii) Nuclear fission and fusion; basic introduction and equations.

A. Atomic Structure And Radioactivity

12.1 Structure Of The Atom And Nucleus

The discovery of electrons by Sir J.J. Thomson towards the end of the nineteenth century and the scattering experiments of alpha particles by Rutherford and others led to the following structure of an atom and its nucleus.

Structure of an atom: An atom consists of electrons, protons and neutrons. The protons and neutrons reside inside the nucleus of the atom which is at its centre, while the electrons revolve around the nucleus in some specific orbits in which they do not radiate out energy. Such orbits are called the stationary orbits (or stationary shells). In each stationary shell, the electron has a definite energy. The number of shells varies in atoms of different elements, depending upon the total number of electrons present in the atom of that element. The maximum number of electrons in a shell can be 2n², where n is the number of that shell. The various shells around the nucleus for n = 1, 2, 3, 4, 5, 6, 7, ... are named as K, L, M, N, O, P, Q, ... respectively and these shells can accommodate at the most 2, 8, 18, 32, 50, 72, 98, ... electrons respectively. The electrons in different shells have different energy which increases with the increase in the number n of that shell.

The size of an atom is determined by the radius of the shell of its outermost electron and it is of the order of 10^-10 m. The electron has a negative charge equal to - 1.6 \times 10^-19 C (or - e) and its mass (m_e) is nearly 9.1 \times 10^-31 kg which is approximately 1/1840 times the mass of a proton.

Structure of nucleus: The nucleus is at the centre of atom whose size is of the order of 10^-15 m to 10^-14 m (i.e., 10^-5 to 10^-4 times the size of the atom). It consists of protons and neutrons. The proton has a positive charge equal to + 1.6 \times 10^-19 C (or + e) and its mass (m_p) is nearly 1.67 \times 10^-27 kg. The total number of protons in the nucleus determines the place of the atom in the periodic table and is called the atomic number of the element of that atom. The atomic number of an element is denoted by the symbol Z. The neutron is an electrically neutral particle (i.e., charge = 0) and its mass (m_n) is also nearly 1.67 \times 10^-27 kg which is equal to that of a proton.

The protons and neutrons which are the main constituents of the nucleus, are called the nucleons. The total number of nucleons in the nucleus is called the mass number of the element and it is denoted by the symbol A. The nucleus is thus positively charged and its total charge is + Ze. The mass of nucleus is approximately A times the mass of a proton.

The atom is specified by the symbol \(_Z^A X\) where X is the chemical symbol for the element.

The actual size of an atom is very small (nearly 10^-10 m) which is invisible, but just to understand the distribution of its constituents, we can draw the model of an atom not to the scale.

Examples: (1) The lightest atom is hydrogen whose mass number A is 1 and atomic number Z is also 1. It is represented as \(_1^1 H\) and it has one proton in the nucleus and one electron in the K shell as shown in Fig. 12.1.

(2) The helium atom has the mass number A = 4 and the atomic number Z = 2. It is represented as \(_2^4 He\). It has 2 neutrons and 2 protons inside the nucleus and 2 electrons in the K shell.

(3) The sodium atom has atomic number Z = 11 and mass number A = 23. It will have Z = 11 protons and A - Z = 23 - 11 = 12 neutrons inside the nucleus and Z = 11 electrons distributed in the K, L and M shells (2 in the K shell, 8 in the L shell and 11 in the M shell) as shown in Fig. 12.3. It is represented as \(_{11}^{23} Na\).

Note: (1) If an atom undergoes a chemical change, there is a change in the number of orbital electrons of the atom, whereas if the atom undergoes a nuclear change, there is a change in the number of nucleons inside the nucleus of the atom.

(2) A nuclear change requires much higher energy, of the order of few MeV, which is nearly 10⁶ times as compared to the energy required for a chemical change, which is of the order of few eV only.

(3) In a nuclear reaction, the atomic number (or number of protons) and the mass number (or total number of protons and neutrons) remain conserved. In other words, the total sum of atomic numbers of the reactants is equal to the sum of atomic numbers of the products. Similarly, the total sum of mass numbers of the reactants is equal to the sum of mass numbers of the products.

12.2 Atomic Model

An atom is electrically neutral, therefore the number of protons in the nucleus of an atom are equal to the number of electrons revolving around the nucleus of the atom.

If Z is the atomic number and A is the mass number of an atom, then the atom contains

number of electrons = Z

number of protons = Z

number of neutrons = A - Z

(12.1)

Atomic number: The atomic number of an atom is equal to the number of protons in its nucleus (which is same as the number of electrons in a neutral atom). i.e. Z = number of protons in the nucleusof an atom.

Mass number: The mass number of an atom is equal to the total number of nucleons (i.e., number of protons and neutrons combined) in its nucleus. i.e. A = number of protons + number of neutrons in the nucleus of an atom.

Note: The mass of an electron is negligible as compared to that of a proton (or a neutron). Hence total mass of an atom can be considered to be same as the mass of its nucleus.

Thus we can define the atomic number and man numbers as follows.

Teacher's Note

Understanding atomic structure helps explain why different elements have different properties and how atoms bond together to form the materials we see in everyday life, from the metals in our phones to the oxygen we breathe.

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