CHAPTER- 4 STRUCTURE OF THE ATOM
NOTES
2. Substances are composed of charged particles- these particles are divisible atoms. By the 19th century, it was discovered that atoms are not simple and indivisible particles, but rather contain at least one subatomic particle, the electron, which was discovered by J.J. Thomson. In 1886, E. Goldstein discovered a new type of radiation, which was named canal rays.
- Electrons are represented by e- and protons by p+.
- An atom is made up of protons and electrons, which balance each other's charges. Protons are located in the innermost part of the atom.
Structure of an Atom-
- Thomson's Atomic Model- According to Thomson, an atom is a positively charged sphere in which electrons are embedded like dried fruits in a Christmas cake or seeds in the red part of a watermelon.
- J.J. Thomson was awarded the Nobel Prize in Physics in 1906 for his discovery of the electron.
- Thomson proposed-
1. An atom is made up of a positively charged sphere, and electrons are embedded within it.
2. Negative and positive charges are equal in magnitude. Therefore, an atom is electrically neutral.
A. Rutherford's Atomic Model- E. Rutherford is considered the father of nuclear physics. He became famous for his contributions to radioactivity and the discovery of the nucleus through the gold foil experiment. He was awarded the Nobel Prize in 1908.
- In his experiment, Rutherford used alpha particles (doubly charged helium particles) with a mass of 4u.
Characteristics of Rutherford's Atomic Model-
1. The center of an atom is positively charged and is called the nucleus. Almost the entire mass of an atom is concentrated in the nucleus.
2. Electrons revolve around the nucleus in circular paths.
3. The size of the nucleus is much smaller compared to the size of the atom.
Limitations of Rutherford's Atomic Model- It is unlikely for an electron to remain stable while revolving in a circular path. In this way, the atom would become unstable, whereas we know that atoms are stable.
B. Bohr's Atomic Model- To address the objections raised against Rutherford's model, Niels Bohr proposed the following concepts about the structure of the atom-
1. Electrons can only revolve in certain fixed orbits, which are called the electron's permissible orbits.
2. When electrons revolve in these permissible orbits, they do not emit radiation. These orbits (or shells) are called energy levels.
- Neutron- In 1932, J. Chadwick discovered another subatomic particle, which was uncharged and had a mass equal to that of a proton. Eventually, it was named the neutron.
- Except for hydrogen, these are present in the nuclei of all atoms. Neutrons are commonly represented by 'n'. The mass of an atom is expressed by the sum of the masses of protons and neutrons present in the nucleus.
3. Distribution of Electrons in Different Shells- Bohr and Bury proposed certain rules for the distribution of electrons in the different shells of atoms, which is known as the Bohr-Bury scheme.
1. The maximum number of electrons present in a shell is represented by the formula 2n², where n is the shell number or energy level.
2. The maximum number of electrons in the outermost shell can be 8.
3. Electrons do not occupy a given shell of an atom until the inner shells preceding it are completely filled.
4. Valency- The electrons present in the outermost shell of an atom are called valence electrons.
- The number of electrons shared or transferred to form an octet in the outermost shell of atoms determines the element's valency, i.e., its bonding capacity.
- Therefore, each element's atom has a specific bonding capacity, which is called its valency.
5. Atomic Number and Mass Number-
Atomic Number- Protons are present in the nucleus of an atom. The number of protons in an atom indicates its atomic number, which is represented by 'z'. All atoms of a given element have the same atomic number.
- The total number of protons present in the nucleus of an atom is called the atomic number.
Mass Number- The sum of the total number of protons and neutrons present in the nucleus of an atom is called the mass number.
- These particles are present in the nucleus of the atom, and therefore they are also called nucleons. Example- the mass number of carbon is 12u = 6 protons + 6 neutrons.
6. Isotopes- Atoms of the same element that have the same atomic number but different mass numbers are called isotopes.
- Many elements have a mixture of isotopes. Each isotope of an element is a pure substance. The chemical properties of isotopes are the same, but their physical properties are different.
Applications of Isotopes-
1) One isotope of uranium is used as fuel in nuclear reactors.
2) Cobalt isotopes are used in the treatment of cancer.
3) Iodine isotopes are used in the treatment of goiter.
7. Isobars- Elements with different atomic numbers but the same mass number are called isobars.
