The mechanism by which free electrons are released from the surface of metal while additional heat energy is used is known as thermionic emission. Thermionic emission happens in metals that are heated to a high temperature. In other words, it happens when a sufficient amount of additional energy in the form of heat is provided to free electrons in metals.

As a negligible amount of heat energy is given to the metal, the valence electrons accumulate sufficient energy and sever the bond with the parent atom. The valence electron, which splits the bond with the parent atom, gets freed. This electron, which splits the bond with the parent atom, is considered a free electron.

Due to nuclear instability, the nucleus of the atom exhibits the phenomenon of radioactivity. Energy is lost due to radiation released by the unstable nucleus of the atom. Two forces, namely the electrostatic repulsion force and the strong attraction forces of the nucleus, hold the nucleus together. These two powers are thought to be highly powerful in the natural world. The risk of witnessing instability increases as the nucleus’s size increases since the nucleus’ mass becomes even denser. That is why the atoms in plutonium, uranium are highly unstable and undergo the phenomena of radioactivity.

Nuclear structure studies the characteristics of the nucleus in isolation, such as nuclear mass, characteristic energy levels and modes of radioactive decay.

The overall number of protons in the atom’s nucleus gives us the atomic number of the atom. It is represented by the letter ‘Z’. The atoms of a given element have the same number of protons and thus the same atomic number. Atoms with different components have different atomic numbers. For example, all carbon atoms have an atomic number of 6, and all oxygen atoms have 8 protons in their nucleus.

The sum of protons and neutrons together givesnucleus’s us the mass of the atom. It is represented by the letter ‘A’. As both protons and neutrons are found in the nucleus of the atom, they are called nucleons. For example, the carbon atom has 6 protons and 6 neutrons, and therefore, its mass number is 12. Although the number of protons remains the same in all the atoms of the element, the number of neutrons can differ. As a result, atoms of the same element can have different mass numbers, and these are called isotopes. The electron’s weight is almost negligible. Thus, the atomic mass of an atom is exactly the same as its mass number.

Radioactivity is exhibited naturally by certain kinds of radiation-emitting matter and subatomic particles. It is, in fact, the attribute of a single atomic nucleus.

An unstable nucleus will decompose spontaneously or decay into a more stable state, but only in a few particular ways by releasing certain particles or certain sources of electromagnetic radiation. Radioactive decay is the property of a number of naturally occurring elements and chemically formed isotopes of the elements. The rate at which the radioactive element decays is calculated in terms of its half-life—the time taken for one-half of some given amount of the isotope to decay.

Some uses of radioactivity are listed below. These properties depict the phenomenon of radioactivity and changes in the nucleus as well.

• Americium-241 is an alpha emitter used by domestic smoke alarms in the United States.
• The alpha particles produced by the Americium sample ionise the air in the smoke detector chamber, contributing to a slight current in the chamber.
• When smoke reaches the chamber, a decrease in the current causes the alarm to go off. While Alpha particles have a very short range, they are lethal when in near contact with each other.
• Alpha emitters, when ingested, come in direct contact with tissue and are lethal in certain situations and are often used in assassination attempts via radiation poisoning.

This chapter shed light on the concept of modern physics, where we learnt about some concepts like radioactivity, thermionic emission, nuclear structure, atomic number and mass number.

1. tate the difference between an isotope and a radioactive isotope?
   Isotopes are atoms of the same element with different neutron numbers but the same number of protons and electrons. Radioactive (unstable) isotopes have nuclei that decay randomly over time to form other isotopes.
2. What does the half-life of an isotope mean?
   Half the life of an isotope is the total time it takes for half of the atoms to decay.
3. Give some uses of radioactivity.
   A. Used for indoor smoke detectors.
   B. Used to sterilise surgical devices.
   C. Used to detect and cure illness.
   D. Used to generate electrical power.
4. Who found the atomic number?
   The number of protons (positive charges) in an atom’s nucleus is determined by its atomic number. Henry Gwyn-Jefferies Moseley first introduced the concept.
5. Define mass number.
   The total number of protons and neutrons in the nucleus of the atom. For example, nitrogen has seven protons in its nucleus and seven neutrons to provide 14 masses.