Maharashtra Board Class 11 Chemistry Chapter 13 Nuclear chemistry and radioactivity PDF Download

Chapter 13: Nuclear Chemistry And Radioactivity

13.1 Introduction

Nuclear chemistry is a branch of physical chemistry. It is the study of reactions involving changes in atomic nuclei. This branch started with the discovery of natural radioactivity by a physicist Antoine Henri Becquerel (1852-1908). Most of the 20th century research has been directed towards understanding of the forces holding the nucleus together. To understand the changes occurring on the earth, Geologists explore nuclear reactions. Astronomers study nuclear reactions taking place in stars. In biology and medicine, the interactions of radiation emitted from nuclear reactions within the living system are important.

Examples of nuclear reactions are: radioactive decay, artificial transmutation, nuclear fission and nuclear fusion.

We briefly discuss these in this chapter. We have studied the structure of atom in Chapter 4.

13.1.1 Similarity Between The Solar System And Structure Of Atom

Our solar system is made up of the Sun and planets. The Sun is at the centre of solar system and planets moving round it under the force of gravity. Analogous to this in atomic systems, the forces which hold subatomic particles are attractive electrostatic. As studied in Chapter 4, the atom consists of tiny central core called nucleus consisting of protons and neutrons and surrounding region of space being occupied by fast moving electrons.

The radius of nucleus is of the order of 10^-15 m whereas that of the outer sphere is of the order of 10^-10 m. The size of outer sphere, is 10^5 times larger than the nucleus. There is a large space vacant outside the nucleus.

An element is denoted as \(^A_Z X\) where X is the symbol of the element. Z equals number of protons and is called atomic number of the element.

'A' is the mass number or the sum of number of protons and neutrons in an atom. N is the number of neutrons in an atom.

\[A = Z + N\]

The number of neutrons can be obtained by subtracting the atomic number, Z, from the mass number A.

The atomic number, appearing as a subscript to the left of the elemental symbol, gives the number of protons in the nucleus. The mass number written as a superscript to the left of element symbol, gives the total number of nucleons that is the sum of protons (p) and neutrons (n). The most common isotope of carbon, for example, has 12 nucleons: 6 protons and 6 neutrons.

Mass number (A) = 12

6 protons

6 neutrons

12 nucleons

Carbon 12: \(^{12}_6 C\)

Atomic number (Z) = 6

As you know, atoms with identical atomic numbers but different mass numbers are called isotopes. The nucleus of a specific isotope is called nuclide.

The charge on the nucleus is +Ze and that of outer sphere is −Ze, ('e' is the magnitude of electronic charge). The atom as a whole is, thus, electrically neutral. The mass of an electron is negligible (1/1837th of the mass of proton) in comparison to proton or neutron. The entire mass of atom is concentrated in its nucleus. The density of nucleus is considerably higher, typically 10^5 times the density of ordinary matter.

The radius of nucleus is given by \(R = R_o A^{1/3}\) where \(R_o\) is constant, common to all nuclei and its value is 1.33 x 10^-15 m. The volume of nucleus, \(V \propto R^3\) and hence, \(V \propto A\).

How small is the nucleus in comparison to the rest of atom? If the atom were of the size of football stadium, the nucleus at the centre spot would be the size of a pea.

Teacher's Note

Nuclear chemistry helps us understand how atoms work inside. It is like learning how the heart works inside our body. In India, BARC uses nuclear science to make mangoes stay fresh longer.

Exam Trick

Remember: A = Z + N. A is the total. Z is protons. N is neutrons. Think of A as the All nucleons, Z as the proton charge (+), and N as Neutrons. Just like your total marks = maths + science + English.

Points to Remember

Nucleus is very very small compared to the whole atom.
Mass number A = number of protons + number of neutrons.
Atomic number Z = number of protons only.
All the mass of atom is in the nucleus.
Electrons move around the nucleus very fast.

13.2 Classification Of Nuclides

13.2.1 Classification Based On Number Of Neutrons And Protons

On the basis of the number of neutrons and protons constituting the nucleus, the nuclides (which refer to atomic nucleus without relation to the outer sphere) are classified as:

i. Isotopes: These are nuclides which have the same number of protons but different number of neutrons. For example, \(^{22}_{11}Na\), \(^{23}_{11}Na\), \(^{24}_{11}Na\). The number of neutrons in each being 11, 12 and 13, respectively.

ii. Isobars: These are nuclides which have the same mass number and different number of protons and neutrons. For example, \(^{14}_6 C\), \(^{14}_7 N\). The number of neutrons in each are 8 and 7, respectively. Likewise \(^3_1 H\) and \(^3_2 He\), or \(^{14}_6 C\) and \(^{14}_7 N\) are the pairs of isobars.

iii. Mirror nuclei: These are isobars in which the number of protons and neutrons differ by 1 and are interchanged. Examples: \(^3_1 H\) and \(^3_2 He\), \(^{13}_6 C\) and \(^{13}_7 N\)

iv. Isotones: Isotones are nuclides having the same number of neutrons but different number of protons and hence, different mass numbers. For example, carbon, nitrogen, sodium, magnesium. \(^{13}_6 C\) and \(^{14}_7 N\), \(^{23}_{11} Na\) and \(^{24}_{12} Mg\)

v. Nuclear isomers: The nuclides with the same number of protons (Z) and neutrons (N) or the same mass number (A) which differ in energy states are called nuclear isomers. For example, \(^{60m}Co\) and \(^{60}Co\). An isomer of higher energy is said to be in the meta stable state. It is indicated by writing m after the mass number.

Teacher's Note

Isotopes are like brothers of the same family. They have the same father (same protons) but different mothers (different neutrons). Carbon-12 and Carbon-14 are isotopes used in India for dating old things in museums.

Exam Trick

Remember: Isotopes = same Z (atomic number), Isobars = same A (mass number), Isotones = same N (neutrons). Think ISO = SAME. So Iso-topes have same protons, Iso-bars have same total, Iso-tones have same neutrons.

Points to Remember

Isotopes have same number of protons but different neutrons.
Isobars have same mass number but different protons and neutrons.
Isotones have same number of neutrons but different protons.
Mirror nuclei are special isobars where protons and neutrons swap.
Nuclear isomers have same protons and neutrons but different energy.

13.2.2 Classification On The Basis Of Nuclear Stability

i. Stable nuclides: The number of electrons and the location of nuclei may change in outer sphere but the number of protons and neutrons therein is unchanged.

ii. Unstable or radioactive nuclides: These nuclides undergo spontaneous change forming new nuclides.

13.3 Nuclear Stability

Why some nuclei are stable while others are radioactive and undergo spontaneous change? To answer this question we need to know the factors governing stability of the nucleus.

13.3.1 Even-Odd Nature Of Number Of Protons And Neutrons

The conclusions drawn from the above data are:

a. Nuclides with even Z and even N are stable. They tend to form proton-proton and neutron-neutron pairs and impart stability to the nucleus. The nuclides with the even Z and even N constitute 85% of earth crust.

b. The number of stable nuclides with either Z or N odd is about one third of those where both are even. These nuclides are less stable than those having even number of protons and neutrons. In these nuclides one nucleon has no partner. This indicates the separate pairing of neutrons and protons. The nucleon pairing does not take place between proton and neutron. Further the nuclides with odd A are nearly the same irrespective of Z or N is odd. This indicates that protons and neutrons behave similarly in the respect of stability.

c. The stable nuclides with odd Z and odd N are only four. These are unstable; which can be attributed to the presence of two unpaired nucleons. Only 2% of the earth's crust consists of such nuclides.

Teacher's Note

Even-odd numbers in nuclei are like pairing in socks. Even protons and even neutrons stay together nicely, just like matching socks. Odd numbers create unpaired ones, making the nucleus unstable. In India's nuclear labs, scientists use this idea to find stable elements.

Exam Trick

Remember: Even-Even is most stable (like a happy pair of socks in a drawer). Odd-Odd is least stable (like two lonely unpaired socks). Just think of pairing = stability. Most stable nuclides have even protons AND even neutrons.

Points to Remember

Nuclides with even protons and even neutrons are most stable.
Nuclides with odd protons and odd neutrons are least stable (only 4 exist).
85% of earth's crust is made of even-even nuclides.
Pairing of same nucleons makes nucleus stable.
Protons and neutrons do not pair with each other.

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