|Negatively charged||Positively charged||No charge.|
|Located outside the nucleus||Located within the nucleus||Located inside the nucleus of an atom|
|Mass is negligible||1 a.m.u||1 a.m.u|
|Attracted towards positively charged||Attracted towards negatively charged||Do not get attracted to any charged particle|
Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.
The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable. This Rutherford’s model of the atom could not explain.
Neil Bohr put forward the following postulates about the model of an atom:
(i) Atoms has nucleus in the centre.
(ii) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(iii) While revolving in discrete orbits the electrons do not radiate energy.
(iv) These orbits or shells are called energy levels. Energy levels in an atom are shown in Fig.These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….
| 1. Sphere is positively charged
2.Electrons are negatively charged and scattered all through the inside of the sphere.
3. Positively charged = negatively charged
4. The net charge in the atom is zero.
|1. The nucleus is at the centre and is positively charged holding the entire mass.
2.Electrons are negatively charged revolving in a well-defined path
3. In comparison with the nucleus, the size of the atom is very large.
| 1. Nucleus is present at the centre and is positively charged
2. Electrons are negatively charged, revolving around but do not radiate energy.
3. The distinct orbits are labelled as K, L, M, N
The following rules are followed for writing the number of electrons in different energy levels or shells:
(i) The maximum number of electrons present in a shell is given by the formula \( 2n^2 \) , where ‘n’ is the orbit number or energy level index, 1,2,3,….
Hence the maximum number of electrons in different shells are as follows:first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit or N-shell will be = 2 × 42= 32, and so on.
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.
The definite combining capacity of the atoms of each element is valency.
Example : To find the valency of silicon:
The atomic number of silicon is 14
Number of electrons is equal to the number of protons in silicon i.e., 14
The distribution of electrons in silicon atom is K – 2, L – 8, M – 4
Hence, from the distribution of silicon it is clearly evident that to fill the M shell 4 electrons are required. Therefore its valency is 8-4=4.
To find the valency of oxygen:
The atomic number of oxygen is 8
Number of electrons is equal to the number of protons in oxygen i.e., 8
The distribution of electrons in oxygen atom is K – 2, L – 6
Hence, from the distribution of oxygen it is clearly evident that to fill the M shell 6 more electrons are required. Therefore its valency is 8-6=2.
8. Explain with examples
(i) Atomic number,
(ii) Mass number,
(iii) Isotopes and
Give any two uses of isotopes.
(i) The number of positively charged protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z. Example: Hydrogen has one proton in its nucleus, hence its atomic number is one.
(ii) The total number of protons and neutrons present in the nucleus of an atom is known as the mass number. It is denoted by A. 20Ca40 . Mass number is 40. Atomic number is 20.
(iii) The atoms which have the same number of protons but different number of neutrons are referred to as isotopes. Hence the mass number varies.
Example: The most simple example is the Carbon molecule which exists as 6C12 and 6C14
(iv) Isobars: Isobars are atoms which have the same mass number but differ in the atomic number.
Examples are, 20Ca40and 18Ar40
Uses of isotopes:
1. The isotope of Iodine atom is used to treat goitre and iodine deficient disease.
2. In the treatment of cancer, an isotope of cobalt is used.
3. Fuel for nuclear reactors is derived from the isotopes of the Uranium atom.
The atomic number of sodium is 11. It has 11 electrons in its orbitals wherein the number of protons is equal to the number of electrons. Hence, its electronic configuration is K-2 ; L-8 ; M-1 ; The one electron in the M shell is lost and it obtains a positive charge since it has one more proton than electrons, and obtains a positive charge, Na+ . The new electronic configuration is K-1 ; L-8 which is the filled state. Hence it is very difficult to eliminate the electron from a filled state as it is very stable.
The atomic masses of two isotopic atoms are 79 (49.7%) and 81 (50.3%).
Thus, total mass = (79 * 49.7 / 100) + (81 * 50.3 / 100) = 39.263 + 40.743 = 80.006 u
Let the percentage of 8X16 be ‘a’ and that of 8X18 be ‘100-a’.
As per given data,
16.2u = 16 a / 100 + 18 (100-a) /100
1620 = 16a + 1800 – 18a
1620 = 1800 – 2a
a = 90%
Hence, the percentage of isotope in the sample 8X16 is 90% and that of
8X18 = 100-a = 100- 90=10%
Given: Atomic number, Z = 3
The electronic configuration of the element = K-2; L-1, hence its valency = 1
The element with atomic number 3 is Lithium.
Mass number of X: Protons + neutrons = 6+6 = 12
Mass number of Y: Protons + neutrons = 6+8 = 14
They are the same element as their atomic numbers are the same.
They are isotopes as they differ in the number of neutrons and hence their mass numbers.
14. For the following statements, write T for true and F for false.
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore it is neutral.
(c) The mass of an electron is about 1/2000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
15. Rutherford’s alpha – particle scattering experiment was responsible for the discovery of
(a) Atomic nucleus
(a) Atomic nucleus
16. Isotopes of an element have
(a) The same physical properties
(b) Different chemical properties
(c) Different number of neutrons
(d) Different atomic numbers.
(c) Different number of neutrons
17. Number of valence electrons in Cl– ion are:
18. Which one of the following is a correct electronic configuration of Sodium?
(a) 2, 8
(b) 8, 2, 1
(c) 2, 1, 8
(d) 2, 8, 1
(d) 2, 8, 1
Canal rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.
No, the atom will be neutral as a proton is a positively charged particle and an electron is a negatively charged particle, the net charge becomes neutral as both the particles neutralizes each other.
(i) An atom contains a positively charged sphere in which the negatively charged electrons are implanted.
(ii) Electrons and protons are equal in magnitude hence an atom on the whole is electrically neutral.
According to Rutherford’s model of an atom, the positively charged protons are present in the atom.
The observation would remain the same even when a metal other than gold was used in the ?– particle scattering experiment as the structure of an atom when considered individually remains the same.
The three subatomic particles of an atom are:
1.Protons which are positively charged
2. Electrons which are negatively charged
3. Neutrons which are neutral in nature ( no charge )
Given, atomic mass of helium atom = 4 u and 2 protons in helium nucleus
We know that, atomic mass = number of protons + number of neutrons
So, 4 = 2 + number of neutrons
=> Number of neutrons = 4 – 2 = 2
Hence, Helium has 2 neutrons.
The atomic number of Carbon is 6
Number of electrons is equal to the number of protons in carbon atom i.e., 6
The distribution of electrons in carbon atom is K-2, L-4
The atomic number of Sodium is 11
Number of electrons is equal to the number of protons in sodium atom i.e., 11
The distribution of electrons in sodium atom is K-2, L-8, M-1
K shell can hold 2 electrons
L shell can hold 8 electrons
Hence, when both the shells are full, the total number of electrons present in the atom = 2+8 = 10 electrons.
To find the valency of chlorine:
The atomic number of chlorine is 17
Number of electrons is equal to the number of protons in chlorine i.e., 17
The distribution of electrons in chlorine atom is K-2, L-8, M-7
Hence, from the distribution of chlorine it is clear that to fill the M shell only one electron is required. Therefore its valency is -1.
To find the valency of sulphur:
The atomic number of sulphur is 16
Number of electrons is equal to the number of protons in sulphur i.e., 16
The distribution of electrons in sulphur atom is K-2, L-8, M-6
Hence, from the distribution of sulphur it is clear that to fill the M shell two more electrons are required. Therefore its valency is -2.
To find the valency of magnesium:
The atomic number of magnesium is 12
Number of electrons is equal to the number of protons in magnesium i.e., 12
The distribution of electrons in magnesium atom is K-2, L-8, M-2
Hence, from the distribution of magnesium it is clearly evident that to fill the M shell six more electrons are required. But it will be difficult to gain 6 electron. So, it will loss two electrons instead.
Thus, its valency is +2.
(i) What is the atomic number of the atom? and
(ii) What is the charge on the atom?
Given: Number of electrons = 8
Number of protons = 8
(a) The atomic number of an atom is the same as the number of protons in that atom, hence its atomic number is 8.
(b) In an atom, the number of protons is equal to the number of electrons. Hence both the charges – positive and negative neutralize each other. Therefore, the atom does not possess any charge.
(a) To find the mass number of Oxygen:
Number of protons = 8
Number of neutrons = 8
Atomic number = 8
Atomic mass number = Number of protons + number of neutrons = 8 + 8 = 16
Therefore, mass number of oxygen = 16
(b) To find the mass number of Sulphur:
Number of protons = 16
Number of neutrons = 16
Atomic number = 16
Atomic mass number = Number of protons + number of neutrons = 16 + 16 = 32
|Isotope||Symbol||Mass no.||Atomic no.||No. of electrons||No. of protons||No. of neutrons|
(a) Isotopes: Isotopes are atoms which have the same number of protons but different number of neutrons. Example: Carbon molecule exists as 6C12 and 6C14 but when their electronic configuration is K-2; L-4
(b) Isobars: Isobars are atoms which have the same mass number but different atomic number. Example: Electronic configuration of 20Ca40is – K-2; L-8; M-8; N- 2 and of 18Ar40 is – K-2; L-8; M-8