Structure of Atoms Cha 2 Chemistry 9 class notes

long question, short question, and numericla problems Structure of Atoms Cha 2 Chemistry 9 class notes.

Short Questions Structure of Atoms Cha 2

Q.1) Aluminum is represented as 13AL27. Draw the structures of Aluminum. Write its electronic configuration.

Structure of aluminum

Electronic configuration
1s2, 2s2, 2p6, 3s2, 3p1

Q.2) The energy of an electron in K and L shells is the same or different. Explain. 

The energy of an electron in K and L shells is different. Each electron shell has a different energy level, with those shells close to the nucleus being lower in energy than those far from the nucleus. K shell is closer to the nucleus than L shell and therefore has less energy. An electron must absorb energy to move to a higher-energy shell i.e L shell in this case. Thus the energy of an electron in L shell is higher than the electron in K shell.

Q.3) Draw the structures of hydrogen isotopes. 





Q.4) How many electrons are present in each of the following atoms? Assuming that each is a neutral atom, identify the element.

a. 1s2 2s2 2p6 3s1
b. 1s2 2s2p6 3s2 3p5
c. 1s2 2s2p6 3s2 3p6 4s2

a. 1s2 2s2 2p6 3s1
Number of electrons: 11
Element: Sodium
b. 1s2 2s2p6 3s2 3p5
Number of electrons: 17
Element: Chlorine
c. 1s2 2s2p6 3s2 3p6 4s2
Number of electrons: 20
Element: Calcium

Q.5) Why an atom is considered as neutral particle? Give reason(s).

An atom is made up of protons, electrons, and neutrons. Electrons and protons are charged particles. The electrons have a negative charge, while protons have a positive charge. An atom as a whole is always neutral because the number of protons and electrons is equal and therefore the charges balance each other. For example, lithium atom is neutral with 3 protons and 3 electrons.

Q.6) The mass of an atom is present in its nucleus. Can you explain it?

The nucleus is made up of neutrons and protons. Outside the nucleus is where the electrons are present. Protons and neutrons have nearly the same mass while electrons are much less massive. One neutron or proton is about 1840 times heavier than an electron; therefore most of the mass of an atom is present in the nucleus.

Q.7) What is the reason that physical properties of the isotopes are different but their chemical properties are the same?

Isotopes are the atoms of the same element having same number of electrons and protons but different number of neutrons. Since the chemical properties of elements depend upon the number of electrons in the outermost shell (which is same in isotopes), the isotopes have same chemical properties. However they have different numbers of neutrons, which results in different mass number. This attributes to different physical properties.

Read more: Class 9 Chemistry Cha 2 Periodic Table and Periodicity of Properties

Q.8) Draw the structures of carbon isotopes. Then write down the number of proton, neutron and electron.


Q.9) How many electrons could be contained in the K, L, M and N energy levels.

K shell can accommodate 2 electrons
L shell can accommodate 8 electrons
M shell can accommodate 18 electrons
N shell can accommodate 32 electrons

Q.10) Write detailed electronic configuration for 3Li76C12, and 12Mg24.

Electronic configuration of:
1s2    ;    2s1
K            L
1s2    ;    2s2, 2p2
K                L
1s2    ;   2s2, 2p6    ,    3s2
K                L                M

Q.11) Write the symbol for an isotope:

a) Containing one proton and two neutrons
b) For which the atomic number is one and there is one neutron.
c) For which atomic number is one and the mass number is also one.

a) 1H3
b) 1H2
c) 1H1

Long Questions Chemistry Cha 2 Class 9th Notes

Q.1) Why Dalton’s atomic theory is considered as a base for modern atomic concepts.

Dalton’s atomic theory is considered as a base for modern atomic concepts because no work on the study of the atom was done until the 19th century. It was Dalton who after a series of experiments concluded that all matter must be composed of tiny particles, which are like solid balls and cannot be further divided. He called them atoms.
The main postulates of Dalton atomic theory are as follows:

  • Matter is composed of very small particles called “Atoms”.
  • Atom is an indivisible particle.
  • Atom can neither be created nor destroyed.
  • Atoms of particular element are identical in size, shape, mass and also in other properties.
  • Atoms of different elements are different in their properties.
  • Atoms combine with each other in small whole numbers ratio.
  • All chemical reactions are due to combination or separation of atoms

His theory was the first complete attempt to describe all matter in terms of atoms and their properties.

Q.2) Summarize Rutherford’s model of an atom and explain how he developed this model based on the results of his famous gold-foil experiment.

Rutherford’s model of atoms
Rutherford was the first scientist who proposed the first atomic model of an atom. In 1911, Rutherford fired alpha-particles, given out by polonium, at a 0.00004 cm thick gold foil, and measured the deflection of the particles as they came out from the other side.
To detect the particles emitting from the radioactive material he placed a zinc sulphide plate detector behind the gold foil. He observed the following:
(i) Most of the alpha particles were not deflected at all. They passed through the foil.
(ii) Few, however, were scattered at huge angles.
(iii) Even fewer of them bounced back directly.

Read more: Class 9 Chemistry Notes Short Q, Long Q Chapter 1
The result led Rutherford to conclude that most of the foil was made of empty space, but had extremely small, dense lumps of matter inside. The bouncing back of particles would have occurred when the particles made a direct collision with the nucleus. And deflection of few particles would have occurred when particles came close to the nucleus and there was a force of repulsion between the positively charged particle and the nucleus.
Rutherford proposed the planetary model of an atom and concluded that most of the volume occupied by an atom is empty. The electrons revolve around the nucleus. The number of electrons in an atom is equal to the number of protons.

Q.3) State the postulates which Bohr suggested to overcome the shortcomings of the Rutherford’s atomic model. 

According to Rutherford’s atomic model the electron revolving around the nucleus will emit energy continuously. Because the electron would lose energy, it would rapidly spiral inwards, collapsing into the nucleus. This atomic model thus predicts that all atoms are unstable.
To overcome this difficulty Neil Bohr proposed an atomic model in 1913 considering hydrogen atom as a model. According to Bohr’s model an electron has a specific quantity of energy which is called quantum. Electrons can only gain and lose energy by jumping from one orbit to another. The energy of revolving electron is quantized as it revolves only in orbits of fixed energy.
The Bohr’s atomic model was based upon the following postulates:

  • Electrons revolve around the nucleus in circular paths, which are known as orbits or energy levels.
  • Energy of an orbit is fixed that is quantized.
  • As long as an electron stays in a particular orbit/energy level, its energy remains constant. An electron radiates or absorbs energy when it jumps from one orbit to another orbit.
  • If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of energy. And if it jumps from higher energy level to a lower energy level, it radiates a definite amount of energy. The energy change of electron is given by the following relationship:
    ΔE = E2 – E= hv
    h is Planks constant 
    v is frequency of radiation.
    E1 Lower energy orbit 
    E2 Higher energy orbit
    ΔE Difference in energy
  • Electron can revolve only in orbits of a fixed angular momentum mvr, given as:
    mvr = nh/2π
    Where ‘n’ is the quantum number or orbit number having values 1, 2, 3 and so on.

Q.5 a) Define energy level and sub-energy level.

Energy level
The circular path around the nucleus where the electrons revolve is called energy level or shell. It makes up the electron configuration of an atom. The energy levels are designated by letters K, L, M, N and so on. An energy level closer to nucleus is of minimum energy. Since K shell is closest to the nucleus, the energy of shells increases from K shell onwards.
The maximum capacity of these shells to accommodate electrons is according to 2n2 rule, where n is number of shells. For example:
K shell can accommodate 2(1)2 = 2 electrons
L shell can accommodate 2(2)2 = 8 electrons
M shell can accommodate 2(3)2 = 18 electrons
N shell can accommodate 2(4)2 = 32 electrons
Energy sub level
An energy level also consists of energy sub levels or sub-shells. Energy sub levels are designated by small letters s, p, d, f etc. The number of energy sub levels is equal to its n value. Such as:
If                     n = 1: K shell has only one sub-shell i.e. 1s
                        n = 2: L shell has two sub-shells i.e. 2s, 2p
                        n = 3: M shell has three sub-shells i.e. 3s, 3p, 3d
                        n = 4: N shell has four sub-shells i.e. 4s, 4p, 4d, 4f
Hence the maximum number of electrons in s, p, d and f sub-shells is given below:
s sub-shell can accommodate maximum 2 electrons.
p sub-shell can accommodate maximum 6 electrons.
d sub-shell can accommodate maximum 10 electrons.
f sub-shell can accommodate maximum 14 electrons.

Q.5 b) Explain the distribution of electrons in various energy levels and sub energy levels for first four elements of the periodic table.

Distribution of electrons in energy levels and energy sub levels for first four elements of the periodic table is as follows:
(i) Hydrogen
Number of electrons = 1
1 electron on s sub-shell of K shell
(ii) Helium
Number of electrons = 2
2 electrons in s sub-shell of K shell
(iii) Lithium
Number of electrons = 3
2 electrons in s sub-shell of K shell
And 1 electron in s sub-shell of L shell
1s2       2s1
K            L
(iv) Beryllium
Number of electrons = 4
2 electrons in s sub-shell of K shell
And 2 electrons in s sub-shell of L shell
1s2       2s2
K           L

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