Chapter 4.1 – Structure of Atom | Chapter Solution Class 9

No, this statement is not correct at present. It has been established that atoms are composed of smaller particles such as electrons, protons, and neutrons, which are called subatomic particles.

Subatomic particles are particles that are smaller than the atom. Protons, neutrons, and electrons are the three main subatomic particles found in an atom.

Subatomic particles are the smaller constituents of an atom. The three fundamental subatomic particles are electrons, protons, and neutrons.

The existence of neutrons was first imagined by Rutherford.

The nucleus of hydrogen (¹H) does not contain a neutron.

• Radioactivity was discovered by Henri Becquerel.
• The term radioactivity was given by Madame Curie.

Becquerel rays are the invisible radiation emitted spontaneously by radioactive substances, such as uranium salts. These rays were first discovered by Henri Becquerel.

• Charge of α-particle: +2 units
• Mass of α-particle: 4 atomic mass units (amu)

Cathode rays are negatively charged, so they are attracted towards the positive pole of the electric and magnetic field.

Atomic number (Z): The number of protons present in the nucleus of an atom.

Mass number (A): The sum of the protons and neutrons present in the nucleus of an atom.

(a) Mass number:

Mass number A = Protons + Neutrons = 29 + 34 = 63

(b) Atomic number:

Atomic number Z = Number of protons = 29

(c) Number of electrons:

Since the atom is neutral, number of electrons = 29

(d) Write the symbol of its nuclide:

The nuclide symbol is ₆₃X²⁹.

According to Rutherford’s α-particle scattering experiment, most of the space inside the atom is empty, but when an α-particle collides directly with the nucleus, it is deflected or rebounds due to the small, dense, and positively charged nucleus.

Nucleons are the particles present in the nucleus of an atom, which include protons and neutrons.

The protons inside the nucleus do not repel each other because they are held together by a strong nuclear force, which is much stronger than the electrostatic force of repulsion.

Nuclear forces arise due to the strong attraction between protons and neutrons within the nucleus. These forces act over a short range and hold the nucleons together.

The atomic spectrum is discontinuous in nature, consisting of distinct lines corresponding to specific wavelengths of light emitted or absorbed when electrons transition between energy levels.

In Bohr’s model, electrons revolve around the nucleus in fixed circular orbits. These orbits are called energy levels or shells.

Stationary orbits are the fixed energy levels around the nucleus where electrons revolve without radiating energy.

An electron jumps from one orbit to another when it absorbs or emits a definite quantum of energy corresponding to the difference between the energy levels.

The letters K, L, M, N represent the electron shells (energy levels) in an atom, where electrons revolve around the nucleus.

When an electron from the outermost orbit is completely detached, the atom becomes a positively charged ion (cation) due to the loss of an electron.

(b) Anion

Explanation:

A chlorine atom has 7 electrons in its outermost shell (electronic configuration: 2, 8, 7). When it gains one electron, it forms a negatively charged ion called an anion (Cl⁻).

(c) Protons and neutrons

Explanation:

Nucleons are the particles present inside the nucleus of an atom. These include protons (positively charged) and neutrons (neutral particles). Electrons do not belong to the nucleus.

(d) Atomic nucleus

Explanation:

Rutherford’s gold foil experiment showed that most of the atom is empty space, but a small, dense, positively charged nucleus exists at the center, where most of the atomic mass is concentrated.

(c) 2, 8, 1

Explanation:

Sodium (Na) has 11 electrons. The electronic configuration follows the rule:

• First shell (K) can hold 2 electrons
• Second shell (L) can hold 8 electrons
• Third shell (M) gets the remaining 1 electron
  So, the electronic configuration is 2, 8, 1.

(a) 1

Explanation:

The element with electronic configuration 2, 8, 8, 1 belongs to group 1 (alkali metals). Valency is determined by the number of electrons lost or gained to achieve a stable configuration. Since this element has one electron in its outermost shell, it loses one electron to become stable, making its valency = 1.

Electrons

Stream of negatively charged particles

Henri Becquerel

Hydrogen (¹H)

¹²C, ¹³C, ¹⁴C

True

False (due to isotopic abundances)

False (isotopes have the same protons but different neutrons)

False (they are isobars, not isotopes)

Isotopes have the same number of protons and electrons, which determine chemical properties. The difference in neutrons does not affect chemical behavior.

The strong nuclear force acts between protons and neutrons, overcoming the electrostatic repulsion and holding the nucleus together.

Isobars have the same mass number but different atomic numbers, meaning they have different numbers of protons and electrons, leading to different chemical properties.

Inert gases have completely filled outermost electron shells, making them stable and unreactive under normal conditions.

Experiment:

Rutherford conducted an α-particle scattering experiment to determine the structure of an atom. He directed fast-moving α-particles (Helium nuclei) toward a thin gold foil (0.0004 mm thick) and observed how they scattered. A fluorescent screen was placed around the foil to detect deflected particles.

Observations:

1. Most of the α-particles passed through the foil undeflected → Suggesting that most of the atom is empty space.
2. Some α-particles were deflected at small angles → Indicating that positive charge is concentrated in a small region.
3. Very few α-particles (1 out of 12,000) rebounded back → Suggesting the presence of a dense, positively charged nucleus at the center of the atom.

Inferences Drawn:

1. Most of the atom is empty space since most α-particles passed through the foil without deflection.
2. The nucleus is positively charged as it repelled positively charged α-particles.
3. The nucleus is very small but contains most of the atom’s mass, making it extremely dense.
4. Electrons revolve around the nucleus, preventing the collapse of the atom due to electrostatic attraction.

Drawback of Rutherford’s Atomic Model:

According to Rutherford, electrons revolve around the nucleus in circular orbits. However, this model had a major defect:

• Electrodynamics states that a moving charged particle emits radiation. This means the revolving electron should lose energy, spiral inward, and eventually fall into the nucleus, making the atom unstable. This does not happen in reality.

Bohr’s Postulates of Atomic Model to Overcome the Defects:

1. Electrons move in fixed orbits (stationary energy levels) around the nucleus and do not lose energy.
2. Only specific energy levels are allowed for electrons, and each orbit has a fixed energy.
3. Electrons can jump between orbits by absorbing or emitting discrete energy (quantized) in the form of photons.
4. The angular momentum of electrons is quantized and follows the formula: mvr = nh/2π

1. where n is an integer, h is Planck’s constant, m is mass, v is velocity, and r is the radius of orbit.

Bohr’s model successfully explained the stability of atoms and spectral lines.

(a) Fundamental Particles of an Atom:

(b) Structure of Three Isotopes of Oxygen:

• All three have same atomic number (8) but different mass numbers (16, 17, 18).
• Their chemical properties are identical, but physical properties differ.

Radioactivity is the spontaneous emission of radiation (alpha, beta, or gamma rays) due to the disintegration of unstable atomic nuclei.

Examples of Radioactive Elements:

1. Uranium (U)
2. Radium (Ra)