LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 9 Chemistry Chapter 4 - Structure Of The Atom
Chapter 4 - Structure Of The Atom 191
(b). Maximum of 8 e- can be accommodated in outermost shell of an atom.
Chapter 4 - Structure Of The Atom 192
(a). 8 electrons are present in outermost shell of Neon.(b). 7 electrons are present in outermost shell of Chlorine.
(b). N shell can accommodate maximum of 32 e-.
(b). M shell can accommodate maximum of 18 e-.
(ii). Thomson discovered 'electron'.
(iii). Goldstein discovered 'proton'.
(b). Electron has relative charge of -1.
(c). Neutron has relative charge of 0.
(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). Atomic no. is characteristic for any particular element.
Chapter 4 - Structure Of The Atom 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.
(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). 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.
(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.
(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.
(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
(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). 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 194
(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
(ii). Atomic no. = 15
(iii). E.C. = (2,8,5)
(a). E.C. - (2,8,7)
(b). Atomic No. = 17
(d). Anion; X-(e). X must be Chlorine
(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 195
(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
(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 210
E.C of Nitrogen = 2, 5
So, no. of valence electrons in Nitrogen atom = 5
One such isotope is Cobalt-60.
(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
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 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
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.
Deuterium, Protium and Tritium are isotopes.
Argon and Calcium are isobars.
(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.
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). 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).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).
(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 212
(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 - 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 213
(ii). 79X35 and 80Y35 - these are isotopes as they have same atomic number.
(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
A and B are the example of isobars. This is because they have same number of nucleons.
The two species are isobars.
A represents Argon (Atomic no. = 18) while B represents Calcium (Atomic no. = 20).
(ii). Mass number of Y = 8 + 9 = 17
(iii). X and Y are isotopes.
(iv). X and Y represent Oxygen.
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