Content Curator
Atoms are the smallest and most elementary unit of any ordinary matter. Every atom has a nucleus in the center and one or more electrons revolving around it in energy shells.
- One or more protons and several neutrons make up the nucleus.
- The protons have a positive charge, while the electrons have a negative charge and the neutrons have none.
- The atom is electrically neutral if the number of protons and electrons is equal and thus nullifies each other.
Structure of Atom
When an atom contains more or fewer electrons than protons, it has a negative or positive overall charge accordingly and is called an ion. The electromagnetic force attracts electrons to its protons in the nucleus. The number of protons in the nucleus is the atomic number and it determines the chemical properties the atom belongs to.
The following are the five atomic models:
- The atomic model of John Dalton -Solid Sphere model
- The model of J.J. Thomson - Model of Plum Pudding
- The model of Ernest Rutherford
- The model of Niels Bohr
- The model of Erwin Schrödinger - Quantum Model/Electron Cloud Model
Multiple Choice Questions
Ques. Which of the following correctly represents the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2, 2
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Ans. (b) 2, 8, 2
Explanation: Atomic number and the number of electrons in a magnesium atom is 12.
So, the electronic configuration is 2, 8, 2 (as 12 = 2 + 8 + 2).
The maximum electrons on the K shell are 2 and the maximum electrons in the L shell are 8.
Ques. A sample of NH3 molecules irrespective of source contains 82.35% Nitrogen and 17.65% of Hydrogen by mass. This data supports:
(a) Law of Conservation of Mass
(b) Law of Multiple Proportions
(c) Law of Definite Proportions
(d) Avogadro’s Law
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Ans. (c) Law of Definite Proportions
Explanation: According to the law of definite proportions, or ratio, the mass proportion of components in a compound's samples will always be the same.
The law of proportions, also known as the law of fixed constant, holds that the proportions of the constituent components that make up a chemical compound or composition will always be the same.
Ques. What is the molecular formula of potassium nitrate?
(a) KNO3
(b) KNO
(c) KNO2
(d) KON
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Ans. (a) KNO3
Explanation: Potassium Nitrate is a crystalline salt with strong oxidizing powers. They are used in the manufacture of gunpowder, fertilizer, and in medicine. Potassium nitrate is the inorganic nitrate salt of potassium. K is the chemical representation of potassium and NO3 is the chemical representation of nitrate.
Ques. Molecular mass is defined as the:
(a) Mass of one molecule of any substance compared with the mass of one atom of C – 12
(b) Mass of one atom in comparison with the mass of one atom of hydrogen
(c) Mass of one atom in comparison with the mass of one molecule
(d) None of the above
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Ans. (a) Mass of one molecule of any substance compared with the mass of one atom of C – 12
Explanation: The sum of all atoms' weight in a molecule is calculated using a scale with hydrogen, carbon, nitrogen, and oxygen atomic masses of 1, 12, 14, and 16, respectively. Water, for example, has a molecular mass of 18 (i.e. 2 + 16) since it contains two hydrogen atoms and one oxygen atom. Also known as molecular mass.
Ques. The number of moles in 46g of sodium with atomic number 23 is equal to.
(a) 4
(b) 2
(c) 0
(d) ½
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Ans. (b) 2
Explanation: It is given that,
Atomic Mass of Sodium or molar mass = 23
The number of moles in 46 g of Na is.
No. of moles = Given Mass/ Molar Mass
by substituting the value we get
No. of Moles = 46/23
No. of Moles = 2
Ques. Which of the following represents a correct chemical formula?
(a) CaCl
(b) BiPO4
(c) NaSO4
(d) NaS
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Ans. (b) BiPO4
Explanation: BiPO4 is the only one of these compounds with the exact chemical formula. Bismuth orthophosphate is a compound containing bismuth. The valencies of all the elements are not satisfied in the other compounds. Calcium chloride (CaCl₂), sodium sulfate (Na₂SO₄), and sodium sulphide (Na₂S) are the correct chemical composition representations.
Ques. What is the formula mass unit of ZnO?
(a) 18 u
(b) 81 u
(c) 88 u
(d) 188 u
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Ans. (b) 81 u
Explanation: Atomic mass of Zn = 65u
The atomic mass of O = 16u
Calculating the formula unit masses of them:
ZnO = Zn + O
ZnO = (65 + 16)u = 81u
Ques. Atomic models have been improved over the years. Arrange the following atomic models in their chronological order
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(ii) Bohr’s atomic model
(a) (i), (ii), and (iii)
(b) (ii), (iii), and (i)
(c) (ii), (i), and (iii)
(d) (iii), (ii), and (i)
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Ans. (c) (ii), (i) and (iii)
Explanation: The first atomic model was given by J.J. Thomson in 1904. A model of the atom with electrons revolving around the nucleus was proposed by Rutherford in 1911. The final atomic model with improvement came in 1915 by Bohr. The chronological order of the atomic models is (ii) Thomson’s atomic model (i) Rutherford’s atomic model (iii) Bohr’s atomic model.
Ques. How many atoms of oxygen are present in 300 grams of CaCO3?
(a) 54.207 × 1023
(b) 6.207 × 1023
(c) 12.207 × 1023
(d) 22.2 × 1023
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Ans. (a) 54.207 × 1023
Explanation: Given the mass of CaCO3 = 300g
Molar mass of CaCO3 = 40 + 12 + (3 x 16) =100 g mol⁻¹
Number of moles of CaCO3 = mass / molar mass
= 300g/ 100 g mol⁻¹
= 3 moles
Number of oxygen atoms in CaCO3 = 3
Number of moles of oxygen atoms = 3 x 3 moles = 9 moles
1 mole of oxygen contains 6.022 x 1023 atoms of oxygen.
Therefore , 9 moles of oxygen contains = 9 x 6.022 x 1023 atoms
=5.4 x 1024 oxygen atoms.
Ques. Which of the following represents the correct relation between Avogadro's number (No), number of particles (N), and moles (n)?
(a) n = N / No
(b) n = No / N
(c) n = N No
(d) all are correct
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Ans. (a) n = N / No
Explanation: The correct relation between Avogadro’s number and the number of particles and moles is given by the formula, n = N / No Where, No is Avogadro’s number, n is the number of miles and N is the number of particles.
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