NCERT 10th class solution :Structure of the molecule

Updated Tue, 17 May 2022 09:23 PM IST


Do you know what is the molecule from which all the things in the world are made, if not then read this article completely to know.

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Structure of the molecule  introduction

The atomic structure of the molecule is the study of neutrons and protons residing in its nucleus and the positive charge electrons revolving around the nucleus.
The history of atomic structure and quantum mechanics dates back to the time of Democritus, the man who first suggested that matter is composed of atoms. Studying the structure of an atom provides insight into a whole class of chemical reactions, bonds, and physical properties. The first scientific theory of atomic structure was proposed by John Dalton in the 1800s.

The period from the 18th to the 19th century is considered to be the peak for scientists. In the 5th century BC, Greek philosophers Leucippus and Democritus first developed the concept of the atom. The Greeks called these particles atoms, meaning indivisible, and the modern word “atom” is derived from this term.

Source: Amar ujala

Democritus proposed that different types and combinations of these particles were responsible for the various forms of matter. However, these ideas were largely ignored. In the 6th century, Indian philosopher Maharishi Kanada suggested that matter is made up of very small particles. He called the smallest particle of matter and. Kanada also believed that and may be made of sub-particles, which he called para.

Structure of the molecule Dalton's theory

John Dalton was the first to explain the unchanging makeup of matter. The main points, called postulates, proposed by John Dalton are:
The matter is made up of very small particles called atoms. All atoms of an element are chemically alike.

The postulates of the theory

• Atoms of one element differ from the atoms of other elements in all respects.
• I think cannot be broken down by ordinary chemical means
• Item of the same of different elements combine in simple whole-number ratios
• Atoms can neither be created nor destroyed

Structure of the molecule:Thompson's theory

In the early 19th century, a scientist named Thompson did many experiments related to atomic structure, for his cathode ray experiments, he was later given a noble prize for this. he invented election.

Thompson experiment

 In 1897, J. J. Thomson experimented to study the properties of cathode rays by applying an electric field. He observed that when a positively charged plate and negatively charged plate are kept near the cathode ray discharge tube, the cathode ray bends towards the positively charged plate. Thus, he concluded that cathode rays are negatively charged. These negatively charged particles are electrons. He also measured the ratio of the charge (e) to mass (m) of electrons.
• An atom consists of a sphere of positive charge with a negatively charged electron embedded in it.
• The positive and negative charges in an atom are equal in magnitude due to which an atom is electrically neutral is has no overall negative or positive charge. It is commonly called the "plum pudding pattern" because it can be visualized as a plum pudding dish where the pudding describes the positively charged atom and the plum pieces describe the electrons.
• Thomson's atomic structure described atoms as electrically neutral, meaning that the positive and negative charges were of equal magnitude.
• Limitations of Thomson's Atomic Structure: Thomson's atomic model does not explain the stability of an atom clearly. Additionally, other discoveries of other subatomic particles could not be placed in his atomic model.

Structure of the molecule: Rutherford model

The nucleus was discovered by Rutherford in 1911 through the scattering experiment. The experiment was conducted by Hans Geiger and Ernest Marsden under the guidance of Rutherford. In this experiment, a thin gold foil was bombarded with very small positively charged particles called alpha (a) particles. Most of the particles were undeflected and passed right through the foil, but a few of them were deflected by small angles.

The following are the conclusions drawn by Rutherford:

  • • Most of the a-particles passed straight through the gold foil, indicating that there is a lot of space in the atom.
• Some of the a-particles were deflected at small angles to their path, indicating that a positively charged body is present at the centre of the atom.
• Few of the a-particles rebounded, indicating that the centrally located body or the nucleus is dense and hard.


• When the electrons have to rotate around the nucleus they consume energy and that too against the strong gravitational pull of the nucleus, a lot of energy is consumed by the electrons and finally they lose all their energy and fall into the nucleus therefore the stability of the atom is not explained.
• If the electrons orbit the nucleus continuously, the expected type of spectrum is a continuous spectrum. But in reality, ty we see a line spectrum.
Particles of atom

Structure of the molecule: Proton

An atom is electrically neutral its an atom contains negatively charged particles electrons then it must contain positively charged particles in 1886 German scientist yugonGoldsteinperformedrannd experiment using discharge to feated with for rated cathode with small holes to allow the passage of positive rays is rays thaw areas just opposite to the cathode rays was emitted from the ray was called the annual day consist of positively charged particle which is called Proton.

Structure of the molecule: Protons.

• The mass of a proton is 1837 times that of an electron the mass of a proton is 1.67 * 10 - ²⁷ kg.
• The charge of a proton is equal and opposite to the charge of an electron there for a portion has a unit positive charge of magnitude 1.602 into 10 -¹⁹ coulomb.
• Proton on spins around it's like a spinning ball.
• The relative mass of a proton is equal to the mass of a hydrogen AMU atomic mass unit

Structure of the molecule:Neutrons

 Neutron is neutral in 1932 British scientist JamesChadwickk experimented and demonstrated that the rays emerging from beryllium were composed of a particle with a charge but with awaswasilar to that of the proton This particle was called neutrons.
• The mass of a neutron is almost the same as that of a proton, 1.674×10-24
• Neutrons are electrically neutral particles and carry no charge.
• Different isotopes of an element have the same number of protons, but differ in the number of neutrons that are present in their respective nuclei.

Structure of the molecule: Electron

• Any electron has about 1/ 1837 mass of a hydrogen atom or 9.1 08 ×10 -³⁷ kg
• An electron has a unit negative charge of magnitude 1.602 × 10 -¹⁹ coulombs. The electron is very small with a radius of 10-⁷ m. Electrons from all the sources are alike having identical mass and negative electric charge.

Structure of the molecule: Isotopes

  • Each element contains a unique number of protons, described by its unique atomic number. However several atomic structures of an element can exist, differing in the total number of nucleons.
  • These variants of elements with different numbers of nucleons (also called mass numbers) are called isotopes of the element. Therefore, isotopes of an element have the same number of protons but differ in the number of neutrons.
  • The atomic structure of an isotope is described using the chemical symbol of the element, the atomic number of the element, and the mass number of the isotope. For example, there are three known natural isotopes of hydrogen, namely protium, deuterium and tritium. The atomic structures of these hydrogen isotopes are shown below.
  • Isotopes of an element have different stability. The half-lives of isotopes also differ. However, they generally have similar chemical behaviour due to sharing the same electronic structures.

Q1-what is the molecular structure?

Answer-Molecular geometry, also known as the molecular structure, is the three-dimensional structure or arrangement of atoms in a molecule. Understanding the molecular structure of a compound can help determine the polarity, reactivity, phase of matter, color, magnetism, as well as the biological activity.

Q-2 what is the lone pair of electron?

Answer- Lone pair (nonbonded pair, nonbonded electron pair): A valence shell electron pair associated with one atom, and not part of a covalent bond.

Q3-what are the 5 basic molecular structure?

Answer-Molecular Geometries. The VSEPR theory describes five main shapes of simple molecules: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

Q4-Why do molecules have different shapes?

Answer -These shapes are decided by the arrangement of electrons around the central atom in the molecule. VSEPR works on the assumption that the shape adopted is that which minimises repulsions between the electron pairs in the molecule.

Q5-What are the electron group?

Answer-A “group” of electrons can be a single bond, double bond, triple bond, or a lone pair of electrons. The table below indicates the “Molecular Geometry” of the central atom depending on whether the groups of electrons around it are covalent bonds to other atoms or simply lone pairs of electrons.

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