What is The Structure of an Atom? CBSE NCERT Notes for Class 9 Science Chapter 4 Structure Of The Atom

CBSE NCERT Notes for Class 9 Science Chapter 4 Structure Of The Atom​

Chapter 4 Structure of NCERT Solutions for Class 9 Science At skillyogi, the Atom gives students with answers to all of the questions in the NCERT Class 9 textbook, which is aligned with the CBSE board.

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CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

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The Structure of an Atom

The atom, according to Dalton’s atomic theory, was indivisible and unbreakable. Now that two basic particles (electrons and protons) have been discovered within the atom, this feature of Dalton’s theory has been disproved. Numerous scientists proposed numerous atomic models in order to determine the arrangement of electrons and protons inside an atom.

Thomson’s Model of an Atom

J.J. Thomson was the first scientist to suggest an atomic model. Thomson’s atomic model is like Christmas pudding. The electrons in a positively charged sphere were analogous to the currants (dried fruits) in a spherical Christmas pudding.

Additionally, it may be likened to a watermelon, in which the positive charge in an atom is distributed evenly through, similar to the red edible part, while the electrons are contained in the positively charged sphere, similar to the seeds in a watermelon.

Electrons are encased in a positively charged sphere.

  1. The magnitudes of the negative and positive charges are equal. As a result, the atom is electrically neutral.
  2. It is thought that an atom’s mass is equally distributed across the atom.

Limitations of Thomson’s Model of an Atom

  1. Thomson’s model could not account for the experimental findings of other scientists, such as Rutherford, since Thomson’s atomic model lacked a nucleus.
  2. It cannot explain an atom’s stability, i.e. how positive and negative charges may exist so close together.

‘Rutherford’s Model of an Atom

Ernest Rutherford conducted an experiment to determine the arrangement of electrons inside an atom. He blasted a thin sheet of gold foil with rapidly moving a-particles (these are doubly charged helium ions with a mass of (4 u). He chose gold foil because he desired a covering that was as thin as possible. The thickness of this gold foil was around 1000 atoms.

Rutherford made the following observations:

  1. The majority of the high-speed a-particles passed directly through the gold foil.
  2. The foil deflected a few of the a-particles at tiny angles.
  3. A very small number of a-particles (one in 12000) seemed to rebound.

Rutherford determined the following based on his experiment:

  1. The majority of space within the atom is unfilled since the majority of a-particles went through the gold foil undeflected.
  2. Only a few particles were diverted off their course, showing that the atom’s positive charge occupies a little space.
  3. An extremely tiny percentage of a-particles deflected 180° (i.e. rebounded), showing that the atom’s entire positive charge and mass were concentrated in a very small volume.

Rutherford created the nuclear model of an atom based on his experiment, which has

the following characteristics:

  1. In an atom, there is a positively charged, very dense centre called the nucleus. The nucleus contains almost all of the atom’s mass.
  2. The electrons follow a circular route around the nucleus.
  3. The nucleus is very tiny (10-15 m) in comparison to the atom (10-1°m).

Limitations of Rutherford’s Model of an Atom

Rutherford’s model of an atom has the following limitations:

  1. When accelerated, every charged particle is predicted to radiate energy. The electron would have to accelerate in order to maintain a circular orbit. 

As a result, it would radiate.Thus, the spinning electron would lose energy and finally fall into the nucleus. If this is the case, then the atom should be very unstable.

As a result, matter would not exist, yet we know it does. This indicates that atoms are very stable. Thus, it cannot account for an atom’s stability when charged electrons are traveling in the direction of the positively charged nucleus.

  1. Rutherford’s model could not account for the distribution of electrons in the atom’s extranuclear region.

Bohr’s Model of an Atom

To address the criticisms levelled against Rutherford’s atomic model, Neils Bohr advanced the following postulates concerning the atomic model:

  1. The atom is made up of a positively charged nucleus around which electrons orbit in defined orbits, that is, electrons orbit in particular allowable orbits and not in any orbit.
  2. Each of these orbits is connected with a certain energy value. As a result, these orbits are referred to as energy shells or levels of energy. Because an orbit’s energy is constant (stationary), it is sometimes referred to as a stationary state.
  3. The lowest energy electrons are found in the first energy level (E1). Energy levels grow as they approach the outer energy levels.
  4. Beginning with the nucleus, energy levels (orbits) are represented numerically (1, 2, 3, 4, etc.) or alphabetically (K, L, M, N etc.).
  5. An electron’s energy stays constant as long as it remains in a defined orbit and does not radiate energy while rotating.
  6. When an electron receives energy, it may go to higher energy levels. While an electron descends When it radiates energy, it has a lower energy level.

Neutrons (n)

  1. Chadwick discovered the neutron, another subatomic particle, in 1932. It is denoted by n. Neutrons are electrically neutral particles that weigh the same as protons (1.67493 x 10-27 kg), the same as a proton.

All atoms except hydrogen have neutrons in their nucleus. The mass of an atom is calculated by adding the masses of the protons and neutrons contained inside the nucleus.

Distribution of Electrons in Different Orbits (Shells)

Bohr and Bury proposed that electrons are distributed across the many orbits of an atom. Certain principles are followed when expressing the amount of electrons in various energy levels or shells. These include the following:

  1. The maximum number of electrons in a shell is determined by the formula 2n2, where n denotes the orbit number or energy level, which may be 1, 2, 3, or 4.

As a result, the maximum number of electrons allowed in each shell is as follows:

First orbit or K-shell = 2X(1)2=2

Second orbit or L-shell = 2x(2)2=8

Third orbit or M-shell = 2x(3)3= 18

Fourth orbit or N-shell = 2x(4)4= 32 and so on.

  1. The outermost orbit may hold a maximum of eight electrons.
  2. Electrons cannot be contained inside a shell until the inner shells are completely filled (i.e. the shells are filled in a stepwise manner).


  • The electrons in an atom’s outermost shell are referred to as valence electrons. They are in charge of atoms’ chemical characteristics. Atoms of elements with a totally filled outermost shell have little chemical activity, i.e. they are very stable. These are referred to as inert elements.
  • This indicates that their valency is zero. Helium has two electrons in its outermost shell, whereas the other inert elements have atoms with eight electrons in their outermost shell.
    • The ability of atoms of the same or different elements to react and form molecules is an effort to fill the outermost shell completely. This implies that atoms react with one another in order to form a completely filled outermost shell. An octet is a phrase that refers to the outermost shell that contains eight electrons.
    • Thus, atoms would react to form an octet in the outermost shell. This was achieved by the sharing, gaining, or loss of electrons. The amount of electrons lost or gained or shared by an atom in order to attain stability or an octet in the outermost shell is referred to as the element’s valency.
    • In other words, it is the ability of an element’s atom to combine with the atom(s) of another element(s) to complete its octet.

The following table describes the valencies of certain groups’ elements:

  1. One electron is included in the hydrogen (H), lithium (Li), sodium (Na), and potassium (K) atoms.

Each of them is in their outermost shell, and so each of them may lose one electron in order to become stable. As a result, their valency is 1.

  1. Mg, Ca, and Be all have two valence electrons and may lose these two electrons to form an octet of electrons in the outermost shell or to become stable.
  2. Boron and aluminium both have a valency of three due to their three valence electrons.
  3. Carbon and silicon both have a valency of four due to their four valence electrons.
  4. Nitrogen and phosphorus both have five valence electrons and hence have a valency of three since they may acquire three electrons (rather than losing five electrons) to become stable. As a result, their valency is calculated by deducting five electrons from the octet, i.e. 8 — 5 = 3. However, P may also share five electrons, giving it a valency of five in addition to three.
  5. Each of oxygen and sulphur has six valence electrons; hence, their valency is two, since they may either gain or share two electrons to complete their octet.
  6. Similarly, fluorine and chlorine each have seven valence electrons; their valency is one due to their ability to obtain or share one electron to complete their octet.
  7. All inert elements, such as He, Ne, and Ar, have entirely occupied their outermost shells. As a result, their valency is equal to zero.

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