LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 9 Chemistry Chapter 4 - Structure Of The Atom
Chapter 4 - Structure Of The Atom Exercise 191
(a) innermost shell of an atom ?
(b) outermost shell of an atom ?
(b). Maximum of 8 e- can be accommodated in outermost shell of an atom.
Chapter 4 - Structure Of The Atom Exercise 192
(a) Thomson proposed that the nucleus of an atom contains protons and neutrons.
(b) The cathode ravs obtained from all the gases consist of negatively charged particles called electrons.
(c) The anode rays obtained from all the gases consist of positively charged particles called protons.
(a) Neon (b) Chlorine
(a). 8 electrons are present in outermost shell of Neon.(b). 7 electrons are present in outermost shell of Chlorine.
(a) 8 electrons? (b) 32 electrons?
(b). N shell can accommodate maximum of 32 e-.
(a) 2 electrons (b) 18 electrons
(b). M shell can accommodate maximum of 18 e-.
(i) Chadwick? (ii) Thomson? (iii) Goldstein?
(ii). Thomson discovered 'electron'.
(iii). Goldstein discovered 'proton'.
(a) +1 (b) -1 (c) 0
(b). Electron has relative charge of -1.
(c). Neutron has relative charge of 0.
Fill in the blanks in the following statements:
(a) The number of protons in the nucleus of an atom is called its ______.
(b) The total number of protons and neutrons in the nucleus of an atom is called ______.
(c) An atom has atomic mass number 23 and atomic number 11. The atom has ______ electrons.
(d) An atom of an element has 11 protons, 11 electrons and 12 neutrons. The atomic mass of the atom is ______.
(e) If the nucleus of an atom has atomic number 17 , mass number 37 and there are 17 electrons outside the nucleus, the number of neutrons in it is ______.
(f) Almost all the mass of an atom is concentrated in a small region of space called the ______.
(g) Cathode rays are a beam of fast moving ______.
(h) The anode rays obtained from hydrogen gas consist of particles called ______.
(i) The maximum number of electrons that can be accommodated in L shell are ______.
(j) The maximum number of electrons that can go into the M shell is ______.
(k) The subatomic particle not present in a hydrogen atom is ______.
(l) The electron has ______ charge, the proton has ______ charge, and the neutron has ______ charge.
(b) Mass number
(l) Negative; Positive; No
The relative mass of an electron is 1/1840 u.
A charge of -1 is carried by an electron.
Absolute mass of electron is 9 x 10-28 Kg.
Absolute charge on electron is 1.6 x 10-19 C.
Important information furnished about nucleus in Rutherford's alpha- particle scattering experiment is:
(i). Nucleus of an atom is positively charged.
(ii). Nucleus of an atom is very hard and dense.
(iii). Nucleus of an atom is very small as compared to the size of an atom as a whole.
(b) What do we call this number which is characteristic of a particular element?
(b). Atomic no. is characteristic for any particular element.
Chapter 4 - Structure Of The Atom Exercise 193
Relative mass of proton is 1u.
Relative charge of proton is +1 C.
Absolute charge of proton - 1.6 x 10-19 C
Difference between proton and neutron-
(1). Proton is positively charged while electron is negatively charged.
(2). Proton is much heavier than electron.
Two observations which shows that atom is not indivisible are-
(1). In J. J. Thomson's experiment, the stream of cathode rays in the gas discharge tube shows the presence of negatively charged subatomic particles called electrons.
(2). In Goldstein's experiment, the faint red glow in the gas discharge tube shows the presence of positively charged subatomic particles called protons.
(i) What does the formation of cathode rays tell us about the atoms?
(ii) What does the formations of anode rays tell us about the atoms?
(ii). Formation of anode rays tells about the presence of positively charged protons in all the atoms.
Electronic configuration of oxygen (atomic no. = 8) is (2,6)
So, K-2 ; L-8 ; M-2
(a) What is the nucleus of an atom and what is the nature of charge on it?
(b) Name the scientist who discovered the nucleus of atom.
(a). Nucleus is a small positively charged part at the center of an atom. Nucleus is positively charged.(b). Rutherford discovered nucleus of an atom.
Alpha - particles have +2 units of charge.
(a) How many electrons are there in each atom of the element?
(b) How are these electrons distributed in the various energy levels?
(b). Electronic configuration of given element- (2,8,3)
K-2 ; L-8 ; M-3
Electronic configuration - (2,8,8)
The special thing about the outermost shell is that it is completely filled with the electrons.
Electron has relative charge of -1u, proton has +1u and neutron has 0 relative charge.
Electron has relative charge of -1u while proton has +1u of relative charge.
Proton has relative charge of +1u and neutron has no relative charge.
Also, electron has relative mass of 1/1840 u and neutron has relative mass of 1 u.
Fill in the following blanks :
Fill in the following blanks in respect of an atom of an element:
(a) What are cathode rays? What is the nature of charge on cathode rays ?
(b) Explain how, cathode rays are formed from the gas taken in the discharge tube.
(c) What conclusion is obtained from the fact that all gases form cathode rays ?
(b). When electricity at high voltage is passed through a gas at very low pressure taken in discharge tube, stream of minute particles are given out by the cathode. These stream of particles are called cathode rays.
(c). The conclusion is that all the atoms contain negatively charged particles called electrons.
(b) The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?
(a). According to Thomson model of atom- An atom consists of a sphere of positive charge with negatively charged electrons embedded in it. The positive and negative charges in an atom are equal in magnitude.
Neutron was not present in the Thomson model of atom.
(b). When mass no. is 18 and no. of electrons is 7 then
(i). No. of protons = 7
(ii). No. of neutrons = 18 - 7 = 11
(iii). Atomic no. = 7
(b)The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it ? What is the the atommic number of the element ?
(a).Rutherford's model of atom-
1. An atom consists of positively charged, dense and very small nucleus containing all the protons and neutrons. Almost all the mass of atom is concentrated in the nucleus.
2. The nucleus is surrounded by negatively charged electrons. The electrons are revolving at very high speed round the nucleus in fixed circular orbits.
3. The electrostatic attraction between the positively charged nucleus and negatively charged electrons keep the atom held together.
4. An atom is electrically neutral.
5. Most of the space in an atom is empty.
The major drawback of Rutherford model of atom is that it does not explain the stability of the atom.
(b).Given: Mass no. = 23
No. of electrons = 11
Then, no. of protons = 11
No. of neutrons = 23 - 11 = 12
Atomic no. = 11
(a) Describe Bohr's model of the atom. How did Neils Bohr explain the stability of atom ?
(b) An element has an atomic number of 11 and its mass number is 23. What is the arrangement of electrons in the shells? State nuclear composition of an atom of the element.
(a). Bohr's model of atom-
1. An atom is made up of three particles, namely electrons, protons and neutrons.
2.The protons and neutrons are located in the small nucleus at the center of atom.
3. Electrons revolve round the nucleus in fixed circular orbits.
4. Maximum no. of electrons for any given shell is fixed. Any shell cannot exceed that maximum value.
5. Each given shell is associated with fixed amount of energy.
6. There is no change in energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable.
(b).Given: Atomic no. = 11
Mass no. = 23
Then, electronic configuration - (2,8,1)
Nuclear composition is - 11 protons and 12 neutrons
Chapter 4 - Structure Of The Atom Exercise 194
(b) What is the relation between the atomic number and mass number of an element?
(c) If an element M has mass number 24 and atomic number 12, how many neutrons does its atom contain?
(a). (i). Atomic no. is the number of protons in one atom of an element.
(ii). Mass no. is the total number of protons and neutrons present in one atom of the element.
Example- The total no. of protons in a carbon atom is 6, so its atomic no. is 6.
Also, one atom of Na contains 11 protons and 12 neutrons, so its mass no. is 23.
(b). Mass No. = Atomic no. + No. of neutrons
(c). No. of neutrons = Mass No. - Atomic no.
= 24 - 12 = 12
(i) mass number of phosphorus,
(ii) atomic number of phosphorus, and
(iii) electron configuration of phosphorus
(ii). Atomic no. = 15
(iii). E.C. = (2,8,5)
(a) Write the electronic configuration of element X.
(b) What is the atomic number of element X ?
(c) Is it a metal or a non-metal ?
(d) What type of ion will be formed by an atom of element X ? Write the symbol of the ion formed.
(e) What could element X be ?
(a). E.C. - (2,8,7)
(b). Atomic No. = 17
(d). Anion; X-(e). X must be Chlorine
(a) What is its atomic number?
(b) What is its mass number?
(c) Write the electronic configuration of the element E.
(d) State whether element E is a metal or non-metal. Why?
(e) What type of ion, cation or anion, will be formed by an atom of element E? Why?
(f) Write the symbol of the ion formed by an atom of element E.
(g) What could element E be?
(b). Mass no. = 3 + 4 = 7
(c). E.C. - (2, 1)
(e). Cation will be formed; because outermost single electron can be easily donated.
Chapter 4 - Structure Of The Atom Exercise 195
(a) What does the figure 9 indicate ?
(b) What does the figure 4 indicate ?
(c) What is the number of protons in atom X ?
(d) What is the number of neutrons in atom X ?
(e) What is the number of electrons in atom X ?
(f) How many electrons are there in the outermost shell of an atom of element X ?
(g) Write the symbol of ion formed by an atom of element X .
(b). Atomic number
(c). No. of protons = 4
(d). No. of neutrons = 9 - 4 = 5
(e). No. of electrons = 4
(f). Electrons in outermost orbit = 2
(a) What is the atomic number of the element ?
(b) State whether element Z is a metal or a non-metal.
(c) What type of ion (if any) will be formed by an atom of element Z ? Why ?
(d) What is special about the outermost electron shell of the atom of this element ?
(e) Give the name and symbol of element Z.
(f) Name the group of element to which Z belongs.
(b). Element Z is non-metal
(c). As the outermost shell of element Z is completely filled so, it will not form any ion.
(d). Outermost electronic shell is completely filled with electrons.
(e). Name of element 'Z' = Argon
Symbol is Ar
(f). Z belongs to the group 'Noble gases'.
Chapter 4 - Structure Of The Atom Exercise 210
E.C of Nitrogen = 2, 5
So, no. of valence electrons in Nitrogen atom = 5
One such isotope is Cobalt-60.
Radioactive isotope of iodine is used for making the medicine called tincture iodine.
Fill in the following blanks in respect of an atom of an element :
Complete the following statements:
(a) Magnesium has 2 valence electrons in the ..........shell.
(b) The valency of nitrogen in N2 molecule is ..........
(c) Isotopes have different mass number because their nuclei contain different number of .........
(d) Some boron atoms have mass number 10 and some have mass number 11. These boron atoms with
different mass numbers are called.........
(b). Mass no. = 6 + 5 = 11
(c). No. of electrons = 5
(d). No. of valence electrons, per atom = 3
E.C. = (2, 8, 7)
Valency = 8 - no. of valence electrons = 8 - 7 = 1
(a) Write down the electronic configuration of X.
(b) What will be the valency of X ?
Atomic No. of X = 16
E.C. of X = (2, 8, 6)Valency of X = 8 - no. of valence electrons = 8 - 6 = 2
Chapter 4 - Structure Of The Atom Exercise 211
Valency shown by B (atomic no. 4) - 2
Valency shown by C (atomic no. 8) - 2
Valency shown by D (atomic no. 10) - 0
Valency shown by E (atomic no. 13) - 3
Give one use of each of the following radioactive isotopes :
(a) Uranium-235 (b) Cobalt-60
Cobalt-60 : This is used in the treatment of cancer.
For example- Cl-35 and Cl-37, show identical chemical properties as they have same no. of 7 valence electrons.
Which of the following are isotopes and which are isobars?
Argon, Deuterium, Calcium, Tritium, Protium
Deuterium, Protium and Tritium are isotopes.
Argon and Calcium are isobars.
Hydrogen has three isotopes written as :
(i) these isotopes have almost identical chemical properties.
(ii) they are electrically neutral.
(ii). All of them have 1 electron and 1 proton, so, they are electrically neutral.
D - 1 proton, 1 electron and 1 neutron.
T - 1 proton, 1 electron and 2 neutrons.
Atomic No. = 7
E.C = 2, 5
Valency of given element = 3
Given element is NITROGEN.
(b) What is the number of valence electrons in the atoms of an element having atomic number 13 ? Name the valence shell of this atom.
Valence electrons are situated in the outermost shell.
(b). There are 3 valence electrons present in the element with atomic no. 13.
Valence shell of this atom is M.
(a) What are isotopes ? Explain by giving an example.
(b) Give one similarity and one difference between a pair of isotopes.
(c) Give the number of protons, neutrons and electrons per atom in the two isotopes of chlorine 35Cl17 and 37Cl17 .
(a). Isotopes are the atoms of the same element having the same atomic number but different mass numbers.
For example - 35Cl17 and 37Cl17 are isotopes of chlorine.
(b). Similarity - A pair of isotopes have same atomic number.
Difference - A pair of isotopes have different mass numbers.
(c). In 35Cl17 - 17 protons, 17 electrons and 18 neutrons.
In 37Cl17 - 17 protons, 17 electrons and 20 neutrons.
(a) What are radioactive isotopes ? Give two examples of radioactive isotopes.
(b) Give any two uses of radioactive isotopes.
(c) An element Z contains two naturally occuring isotopes 35Z17 and 37Z17. If the average atomic mass of this element be 35.5 u, calculate the percentage of two isotopes.
(a).The isotopes which are unstable due to presence of extra neutrons in their nuclei and emit various types of radiations, are called radioactive isotopes or radioisotopes.
For example: Carbon - 14 , Arsenic - 74
(b). Uses of isotopes-
(i). They are used in the treatment of cancer.
(ii). Radioactive isotopes are used as 'tracers' in medicine to detect the presence of tumors and blood clots in human body.
(c). Average atomic mass = 35.5 u
Let % amount of 35Z17 be y, then amount of 37Z17 is (100 - y).
(b) What is the relation between the valency of an element and number of valence electrons in its atoms? Explain with examples.
(a). The capacity of an atom of an element to form chemical bonds is known as its valency.
Valency of an atom with atomic no. 14 is 4.
(b). The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.
Valency of metal = No. of valence electron in its atom
Valency of a non-metal = 8 - No. of valence electron in its atom
For example- Valency of sodium (metal) is 1 and that of chlorine (non-metal) is also 1.
Chapter 4 - Structure Of The Atom Exercise 212
(a) Sodium ion, Na+ (b) oxide ion, O2-
(a). E.C of Na+ = 2, 8
So, no. of valence electrons in sodium ion, Na+ = 8
(b). E.C of O2- = 2,8
So, no. of valence electrons in oxide ion, O-2 = 8
Atom A has a mass number 209 and atomic number 82.
Atom B has a mass number 209 and atomic number 83.
(i) How many protons atom A has ?
(ii) How many protons atom B has ?
(iii) Are atoms A and B isotopes of the same element ?
Atom A - 209A82
Atom B - 209B83
(i). A has 82 protons.
(ii). B has 83 protons.
(iii). No, A and B are not isotopes.
Chapter 4 - Structure Of The Atom Exercise 213
Which of the following pairs are isotopes ? Give reasons for your choice:
(ii). 79X35 and 80Y35 - these are isotopes as they have same atomic number.
Three different atoms of oxygen are represented as :
(i) What do the subscripts (lower figures) and superscripts (upper figures) represent?
(ii) What factor is responible for the change in the superscripts 16, 17 and 18, though the element is the same?
(iii) What is the usual name for such atoms of an element?
(iv) Give the nuclear composition of 18O8
(i). Subscripts represent atomic number whereas superscripts represent atomic mass.
(ii). Number of neutrons is responsible for the change in the superscripts.
(iii). Isotopes is the usual name for the given atoms of the element.
(iv). Nuclear composition of 18O8 is:-
No. of protons = 8
No. of neutrons = 18 - 8 = 10
The atomic species A and B have different number of protons but the same number of nucleons. On the other hand, the atomic species X and Y have the same number of protons but different number of nucleons. Which pair is an example of isobars? Why?
A and B are the example of isobars. This is because they have same number of nucleons.
Protons : 18 20
Neutrons: 22 20
Give the mass number of A and B. What is the relation between the two species and which element or elements they represent ?
The two species are isobars.
A represents Argon (Atomic no. = 18) while B represents Calcium (Atomic no. = 20).
Which of the following pairs are isobars?
Give reasons for your choice.
Particle Protons Neutrons Electrons
A 17 18 17
B 3 4 2
C 18 22 18
D 17 20 17
E 9 10 10
Giving reasons, find a pair of isotopes from the abave particles.
The composition of two atomic particles is given below:
Protons 8 8
Neutrons 8 9
Electrons 8 8
(i) Which is the mass number of X?
(ii) Which is the mass number of Y?
(iii) What is the relation between X and Y?
(iv) Which element/elements do they represent?
(ii). Mass number of Y = 8 + 9 = 17
(iii). X and Y are isotopes.
(iv). X and Y represent Oxygen.
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