1. Home
  2. /
  3. Basic study
  4. /
  5. Atoms and Molecules-

Atoms and Molecules-


Molecule is the smallest part of a substance that has all the properties of the substance. It can remain in free state. These are of two types-

  • Homato Atomic: These molecules are made up of elements, such as -H2

The total number of atoms present in a molecule is called atomicity.

  • heteroatomic (Hetero Atomic): These molecules are made up of different types of atoms, such as HCl, CO.2H2SO4 e.t.c.

2-Atomic Atom

An atom is that microscopic part of an element, which is chemical Can participate in change, but cannot remain in a free state. Atoms of all elements are highly reactive, but they do not exist in the free state, but they exist in the combined state with another or similar element of their own compound. Only ideal gases the atom itself inactive and can live in a free state.

Dalton’s atomism

The Indian sage Kanad (800 AD) first gave the atomic theory, which was carried forward by the Greek philosophers Lucius and Democrites and in 1808 AD John Dalton confirmed it by experiments.

Dalton’s atomism is as follows:

  • Every substance is made up of very fine particles called atoms. The atom is indivisible.
  • Atom can neither be created nor destroyed.
  • All atoms of the same element are similar in size, mass and chemical properties, but are different from atoms of another element.
  • Atoms retain their individual entity in chemical changes.
  • All the compound atoms (molecules) of any compound are similar to each other and the combination mass of the element is the combinational mass of the atoms.

Dalton removed the shortcomings of atomism and gave the theory of modern atomism. It could explain the divisibility, isotope and isotope of an atom.

Atoms and Molecules-

3-Structure of Atom

Before the 20th century, atoms were believed to be indivisible, but J.J. Thomson, Rutherford, Chadwick etc. proved that the atom can be divided. In an atom, electrons, protons and neutrons are permanent and positrons, neutrinos, antineutrinos and mesons are temporary particles.

electron Electron: discovery of electrons JJ thomson had done. Its unit is negative charge. Its rest mass is 9.1 × 10-19 Coulomb charge. They revolve around the nucleus of the atom in their fixed orbits.

protron Protron: its discovery rutherford had done. It has a positive charge equal to the charge of the electron. Its charge 1.6 × 10-19 Coulomb happens. It is found in the nucleus of an atom along with neutrons.

neutron Neutron: its discovery chadwick had done. It is an electrically neutral particle. Its mass is the mass of the proton (1.6748 × 10-24) is equal to. The neutron is stable in the nucleus along with the proton but is temporary in the free state outside the nucleus.

nuclear model Atomic Model

Thomson’s model: In 1903, Thomson first presented the atomic model, according to which the atom is like a solid spherical shape, in which positively charged and negatively charged particles are equally distributed. The mass of an atom is spread unevenly around the atom. Thomson’s atomic model explained the electrical neutrality of the atom. But the alpha particle (±) could not explain the experiment to Rutherford.

Atoms and Molecules-

Rutherford’s model, Rutherford in 1911 AD. Presented the atomic model from the findings obtained from the scattering experiment of alpha particles (±). according to this-

  1. Atoms are tiny, spherical, electrically neutral particles made up of a positively charged nucleus and its outer part, which contains electrons.
  2. The total positive charge and almost all the mass of an atom is stored in the center, which is called the nucleus.
  3. The electrons in an atom keep moving around the nucleus.
  4. The number of electrons in an atom is equal to the number of positive charges in the atomic nucleus, so atoms are neutral.
  5. The nucleus exerts a force of attraction on the electrons. The centrifugal force produced by the rotation of electrons balances the force of attraction of the nucleus. This does not allow the electron to fall into the nucleus.

Niels Bohr in 1913 AD by removing the defects of Rutherford, the new model quantum theory model given.

Niels Bohr’s model

Regarding Niels Bohr’s model, a quantum mechanical model was presented by interpreting the hydrogen spectrum, according to which-

At the center of the atom is a nucleus, where protons and neutrons are located. The size of the nucleus is very small.

The electrons revolve around the nucleus in a definite circular path called energy level. An attraction force acts between the nucleus and the electron, which is equal to the centripetal force of the electron.

Each energy level has a certain energy.

The energy levels are called K, L, M, N (1. 2. 3.4) respectively.

When an electron moves from a higher energy level to a lower energy level or from a lower energy level to a higher energy level, it undergoes an energy change. Energy is absorbed when going from a lower orbit to a higher, and energy is released when going from a higher to a lower orbit.

The electron can move around the nucleus only in those orbits in which its angular momentum (mvr) has a simple coefficient of n/2A. ie

Atoms and Molecules-

1-Important Question Answer-

Question . In a reaction 5.3 g sodium carbonate and 6.0 g ethanoic acid are reacted. 2.2 g of carbon dioxide, 8.2 g of sodium ethanoate and 0.9 g of water are obtained as products. Show by this reaction that this test is consistent with conservation of mass.
sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Answer- Mass of reactants = mass of sodium carbonate + mass of ethanoic acid solution
= (4.2 + 10)G = 14.2 G
mass of products = sodium ethanoate + carbon dioxide
= (12 + 2.2)G = 14.2 G
Thus, mass of reactants = mass of products
Hence, it agrees with the law of conservation of mass. Question. Hydrogen and oxygen combine in the ratio 1:8 by mass to form water. How much mass of oxygen gas would be required to completely combine with 3 g of hydrogen gas?

Answer- Mass ratio of hydrogen and oxygen = 1 : 8
Oxygen required for the complete reaction with 1g of hydrogen = 8g
Oxygen required for the complete reaction with 3g of hydrogen = 8 X 3 = 24gQuestion. Which axiom of Dalton’s atomic theory is a consequence of the law of conservation of mass?

Answer- Dalton’s atomic theory, ‘Atoms are the smallest indivisible particles that are neither created nor destroyed in a chemical reaction’ is the result of the law of conservation of mass. Which postulate of Dalton’s atomic theory explains the law of definite proportions?

Answer- The relative number and type of atoms of any compound are fixed. This explains the law of definite proportions of Dalton’s atomic theory. Question Define atomic mass unit.

Answer- An atomic unit mass is that unit mass which is equal to 1/12th of the mass of one atom of carbon-12. Why is it not possible to see an atom with the naked eye?

Answer- The size of an atom is so tiny that it is not possible to see it with the naked eye. Also, the atoms of an element do not exist independently. write the following formula
(I) sodium oxide
(ii) aluminum chloride
(iii) sodium sulfide
(iv) Magnesium hydroxide.

Answer- (I) sodium oxide = Na2O
(ii) Aluminum chloride = AlCl3
(iii) sodium sulfide = Na2S
(iv) Magnesium Oxide = Mg (OH)2Question . Write the names of the compounds represented by the following formula-
(I) Al2 ( SO4 ,3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(V) CaCO3

Answer- (I) Al2 ( SO4 ,3 → aluminum sulfate
(ii) CaCl2 → calcium chloride
(iii) K2SO4 → potassium sulfate
(iv) KNO3 → potassium nitrate
(V) CaCO3 → Calcium CarbonateQuestion. What is meant by chemical formula?

Answer- The representation of the molecule of a substance in a symbolic form is called a chemical formula. For example, the molecular formula H of water2O is. The chemical formula is also called molecular formula. Question. How many atoms are present in the following?
(I) H2S molecules and
(ii) (PO)4,3- ion?

Answer- (I) H2In S molecule, 3 atoms are present, 2 of hydrogen and 1 of sulfur.
(ii) (PO)4,3- In an ion, five atoms are present, 1 of phosphorus and 4 of oxygen.

Question . By analysis in a sample of a compound containing 0.24 G of oxygen and boron, it was found that it contains 0.096 G of boron and 0.144 G of oxygen. Calculate the percentage composition of that compound in weight form.

Answer – Boron in 0.24g compound = 0.096g

Boron in 100g compound =

0.24 Oxygen in compound = 0.144g

Oxygen in 100g compound =

Q.3.0 G carbon burns in 8.00 G oxygen to form 11.00 G carbon dioxide. How many grams of carbon dioxide will be formed when 3.00 G of carbon is burned in 50.00 G of oxygen? On which law of chemical combination will your answer be based?

Answer- Because 3.0g of carbon burns 8.00g of oxygen to form 11.00g (3.00 + 8.00 ) of carbon dioxide, so by burning 3.0g of carbon to 50.00g of oxygen, (3.0 + 50.00 ) = 53g of carbon dioxide will be obtained.
It is based on the law of constant proportion. What are polyatomic ions? Give an example.

Answer- Groups of atoms which have net charge are called polyatomic ions. For example, nitrate (NO3,), hydroxide ion (OH),Question. Write the chemical formula of the following:
(A) magnesium chloride
(B) calcium chloride
(C) Copper nitrate
(D) aluminum chloride
(E) calcium carbonate

Answer- (A) Magnesium chloride – MgCl2
(B) Calcium chloride – CaCl2
(C) Copper Nitrate – Cu(NO)3,2
(D) Aluminum chloride – AlCl3
(E) Calcium Carbonate – CaCO3Question . Name the elements present in the following compounds:
(A) slaked lime
(B) Hydrogen bromide
(C) Baking Powder (Eating Soda)
(D) potassium sulfate

Answer- (A) Slaked lime (CaO) – calcium and oxygen
(B) Hydrogen bromide (HBr) – hydrogen and bromine
(C) Baking Powder (Eating Soda) (NaHCO3)- Sodium, Hydrogen, Carbon and Oxygen
(D) Potassium sulphate (K)2SO4) )- Potassium, Sulfur and Oxygenl Question 6. Calculate the molar mass of the following substances-
(A) ethene, C2H2
(B) Sulfur molecule, S8
(C) Phosphorus molecule, P4 (Atomic mass of Phosphorus = 31)
(D) Hydrochloric acid, HCl
(E) nitric acid, HNO3

Answer-(A) ethene, C2H2 = 2 X 12 + 2 X 1 = 26 U
(B) Sulfur molecule, S8 = 8 X 32 = 256 U
(C) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31 ) = 4 X 31 = 124 U
(D) Hydrochloric acid, HCl = 1 X 1 + 1 X 35.5 = 36.5 U
(E) nitric acid, HNO3 = 1 X 1 + 1 X 14 + 3 X 16 = 63 U

Read in Hindi: Atoms and Molecules-
in Hindi

Leave a Reply

Your email address will not be published.