New Simplified Chemistry Class 8 ICSE Solutions – Atomic Structure
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EXERCISE
Question 1.
State the main postulates of – Dalton’s atomic theory. Explain how the modern atomic theory contradicted Dalton’s atomic theory.
Answer:
(a) Dalton’s Atomic Theory :
The main postulates of theory are :
- Matter consists of small indivisible particles called – atoms i. e. Atom is the basic unit of matter.
- Atoms of the same element are – alike in all respects i.e. atoms of hydrogen have same properties like mass, density and atoms of oxygen are alike in all respects.
- Atoms of different elements are different from each other. i.e. atoms of hydrogen are different from atoms of oxygen.
- Atom can neither be created nor destroyed.
- Atoms combine with other atoms in simple whole number ratio forming compounds or molecules.
(b) Modern atomic theory contradicted Dalton Atomic Theory as :
- Atom is no longer indivisible as atom has been divided and has sub – atomic particles
(a) Protons
(b) Neutrons
(c) Electrons - Atoms of same element may have different properties
- Atoms of different elements may have same properties i.e. isobars
- Atoms combine with other atoms may not be in simple whole number ratio i.e. C12H22O11 (Sugar).
- Atom can be destroyed and converted into energy.
Question 2.
With reference to the discovery of the structure of an atom, explain in brief – William Crookes experiment for the discovery of cathode rays, followed by – J.J. Thomsons experiment pertaining to the constituents of the cathode rays. State which sub-atomic particle was discovered from his experiment.
Answer:
Discovery of the three subatomic particles – electrons, protons and neutrons
Atom are built up of three sub-atomic particles – electrons, protons and neutrons.
Discovery of cathode rays leading to the discover of ‘electrons’
- Scientist – William Crookes [1878]
- Discovery – The cathode rays
Experiment :
- An electric discharge was passed through a tube containing a gas at low pressure.
- Blue rays were emitted from the cathode [negative plate] – which were called cathode rays.
Question 3.
Explain in brief – Goldstein’s experiment which led to the discovery of the proton and – Lod Rutherford’s experiment which led to the discovery of the atomic nucleus.
Answer:
Discovery of – Protons
- Discovery – Constituent of positive rays i.e. particles that contain – protons.
Experiment :
- Goldstein used a modified cathode ray tube with a perforated cathode.
- He observed a new type of rays produced from the anode passing through the holes of the perforated cathode. These rays were called anode rays.
Conclusion :
- Anode rays or positive rays consist of positively charged particles now called – protons.
- The positive rays were affected by electric & magnetic fields but – in a direction opposite to that of cathode rays.
- Thus with the discovery of the positive particles – proton was initiated.
Discovery of – Atomic nucleus :
- Discovery – Study of the atomic model leading to the discovery of – atomic nucleus.
Experiment :
- Rutherford projected alpha particles towards a thin gold foil, – in the path of the rays.
- He saw that most of the alpha particles went straight through the foil, – but some were deflected slightly & some by large angles.
Conclusion :
- An atom on the whole is relatively empty but consists of a – concentrated positive mass in the centre, which lead to the deflection of the alpha particles.
- Thus the discovery of a central positive region – atomic nucleus was initiated.
Question 4.
‘Electrons revolve around the nucleus in fixed orbits or shells called energy levels’. State how these energy levels are represented.
Answer:
(a) Electrons revolve around the nucleus in – fixed ‘orbits’ called energy levels
(b) The energy levels 1, 2, 3… are represented by – integer ‘n’ or as K, L, M, N…
(c) Electrons rotate around the nucleus, in one or .more of the energy levels.
Question 5.
Draw a neat labelled diagram representing an atom. Name the three sub-atomic particles in the atom & represent them symbolically showing the mass & charge of each. State where the sub-atomic particles are present in the atom.
Answer:
Question 6.
Define the term – ‘atomic number’ of an atom. If an atom ‘A’ has an atomic number of – eleven, state the number of protons & electrons it contains.
Answer:
Atomic number is the number of protons in the atom of an element. Since atom is electrically neutral i.e. is charge less, therefore
number of electrons = number of protons.
∴ It has 11 P and 11 electrons.
Atomic number z = p = e
Question 7.
Define the term – ‘mass number ’ of an atom. If an atom t ‘B’ has mass number 35 & atomic number 17, state the number of protons, electrons & neutrons it contains.
Answer:
Mass number of an element is equal to the sum of protons and neutrons in the nucleus of atom.
Mass number = Number of protons + Number of neutrons
A = p + n A = 35 atomic number p = 17
∴ 35 = 17 + n
∴ n = number of neutrons = 35 – 17 = 18
n = 18
But p = e
∴ e = p = 17
Number of electrons = e = 17
Question 8.
State why the atomic weight of an element is also termed – relative atomic mass.
Answer:
- Atomic weight : is mass of an atom, the number times it is heavier than an atom of hydrogen.
Since carbon atom is 12 times heavier than an atom of hydrogen. - Relative mass : is equal to the number of times an atom of an element is heavier than 1/12 the mass of an atom of carbon.
Hence atomic weight of an element is also termed relative atomic mass as it is in comparison with mass of 1/12 th mass of a carbon atom.
Question 9.
State how electrons are distributed in an atom. Explain in brief the rules which govern their distribution.
Answer:
(a) Electrons revolve around the nucleus in imaginary paths called shells or orbits. Shells start from nucleus to outwards.
Rules : Maximum number of electrons in a shell is given by 2n2. Where n is the number of shell i.e. 1st shell can have maximum of 2 electrons.
2n2 = 2(1)2 = 2 × 1 = 2
2nd shell can have maximum of 8 electrons
2n2 = 2(2)2 = 2 × 4 = 8
3rd shell can have maximum of 18 electrons
2n2 = 2 (3)2 = 2 × 9 = 18 and so on….
(b) Outer most orbit cannot have more than 8 electrons and 18 in penultimate orbit.
(c) A new shell cannot start until previous is filled completely.
Question 10.
If an atom ‘A’ has atomic number 19 & mass number 39, state –
- Its electronic configuration.
- The number of valence electrons it possesses.
Answer:
Atom ‘A’ has mass number A = 39 and atomic number Z = 19 = p
∴ A = Z + n
A = p + n
39 = 19 + n
n = 39 – 19 = 20
But e = p = 19
- A (K, L, M, N)
19 = 2, 8, 8, 1
There will be 2 electrons in K-shell or 1st shell
8 electrons in 2nd shell or L-shell
8 electrons in 3rd shell or M-shell
1 electron in 4th shell or nth-shell - The number of valence electrons i.e. in outer most shell = 1 electron.
Question 11.
Draw the atomic diagrams of the following elements showing the distribution of – protons, neutrons & the electrons in the various shells of the atoms.
[The upper number represents the – mass number & the lower number the – atomic number e.g. calcium – mass number = 40, atomic number = 20]
Answer:
Question 12.
‘ Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] forming a compound’. With reference to the above definition of valency, state the valency of chlorine in hydrogen chloride, giving reasons.
Answer:
Hydrogen chloride [HCl], one atom of chlorine has combined with one atom of hydrogen and also 1 atom of hydrogen can be replaced by metals like potassium, sodium. Hence valency of chlorine in one.
Question 13.
‘ Valency is also the number of electrons – donated or accepted by an atom so as to achieve stable electronic configuration of the nearest noble gas’. With reference to this definition –
(a) State what is meant by ‘stable electronic configuration’.
(b) State why the valency of –
- sodium, magnesium & aluminium is : +1, +2 & +3 respectively.
- chlorine, oxygen & nitrogen is : -1, -2 & -3 respectively.
Answer:
(a) Stable electronic configuration means to have 2 electrons in the 1st [or K] outer most shell like He – [Duplet].
OR
8 electrons in outer most orbit like other nearst noble gas – [Octet].
(b)
- Valency is the number of electrons donated or lost from the valence shell. Since sodium donates 1 valence electron its valency is +1.
Magnesium loses 2 electrons and aluminium loses 3 electrons from their valence shell their valency is
+2 – magnesium
+3 – Aluminium - Valency of an element is the number of electrons accepted to achieve stable configuration of nearest noble gas.
Chlorine accepts 1 electron and has valency -1 where as oxygen accepts 2 electrons the valency of oxygen is -2 and nitrogen accepts 3 electrons, valency of nitrogen is -3.
Question 14.
With reference to formation of compounds from atoms by electron transfer – electro valency, state the basic steps in the conversion of sodium & chlorine atoms to sodium & chloride ions leading to the formation of the compound – sodium chloride.
[electronic configuration of : Na = 2, 8, 1 & Cl = 2, 8, 7]
Answer:
Electronic configuration
OBJECTIVE TYPE QUESTIONS
Q.1. Match the statements in List I with the correct answer from List II.
Answer:
Q.2. Select the correct answer from the choice in bracket to complete each sentence :
Question 1.
An element ‘X has six electrons in its outer or valence shell. Its valency is __ [+2/-2/-1].
Answer:
An element ‘X has six electrons in its outer or valence shell. Its valency is -2.
Question 2.
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a __ [metal/non-metal/noble gas].
Answer:
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a non-metal.
Question 3.
A __ [proton/neutron] is a sub-atomic particle with no charge and unit mass.
Answer:
A neutron is a sub-atomic particle with no charge and unit mass.
Question 4.
An element Z with zero valency is a __ [metal/noble gas/non-metal].
Answer:
An element Z with zero valency is a noble gas.
Question 5.
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas __ [neon/argon].
Answer:
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas neon.
Q.3. The diagram represents an isotope of hydrogen [H]. Answer the following :
Question 1.
Are isotopes atoms of the same element or different elements.
Answer:
Isotopes atoms are of the same element.
Question 2.
Do isotopes have the same atomic number or the same mass number.
Answer:
Same atomic number.
Question 3.
If an isotope of ‘H’ has mass no. = 2, how many electrons does it have.
Answer:
One electron.
Question 4.
If an isotope of ‘H’ has mass no. = 3, how many neutrons does it have.
Answer:
Two neutrons. [∵ A = P + n]
Question 5.
Which sub-atomic particles in the 3 isotopes of ‘H’ are the same.
Answer:
Protons and electrons in each isotope are same.
Q.4. State the electronic configuration for each of the following :
Answer:
Electronic configuration of :
Q.5. Draw the structure of the following atoms showing the nucleus containing – protons, neutrons and the orbits with the respective electrons :
- Lithium [At. no. = 3, Mass no. = 7]
- Carbon [At. no. = 6, Mass no. = 12]
- Silicon [At. no. = 14, Mass no. = 28]
- Sodium [At. no. = 11, Mass no. = 23]
- Isotopes of hydrogen [11H, 21H , 31H]
Answer:
Structure of atoms :
Z is Atomic Number A is mass number