8
Isotopes of an element have different number of neutrons.
Atomic nucleus
2,8,1
(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)
(c) The mass of an electron is about 1/2000 times that of proton. (T)
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)
X | Y | |
Protons | 6 | 6 |
Neutrons | 6 | 8 |
We know that
Mass number = Number of protons + Number of neutrons
Mass number of X = 6 + 6 = 12
Mass number of Y = 6 + 8 = 14
X and Y have the same atomic number but different mass numbers therefore; X and Y are the Isotopes.
Z represents the atomic number of an element. Atomic number 3 is of the element Lithium.
Electronic configuration of Lithium is 2,1. In order to complete its duplet it will lose its valence electron therefore, its valency is 1.
Atomic number of Sodium (Na) is 11.
Electronic configuration of Sodium (Na) is 2,8,1.
In order to complete its octet it will lose its valence electron and form Sodium ion (Na+).
Electronic configuration of sodium ion (Na+) is 2,8.
In the electronic configuration of sodium ion both K and L shell are completely filled.
According to J.J. Thomson’s model of an atom, an atom consists of a positively charged sphere and the electrons are embedded in it. However, later it was found that the positively charged particles are present at the centre of the atom (nucleus) and electrons revolve around the nucleus.
Valency is the number of electrons gained, lost or shared by an atom so as to make the octet of the electrons in the outermost shell.
For example,
Electronic configuration of Silicon is 2,8,4.
Since the number of valence electrons is 4 therefore, valency of Silicon is 4.
Electronic configuration of Oxygen is 2,6.
Since the number of valence electrons is more than 4 therefore, valency of Oxygen is 8 – 6 i.e., 2.
The limitations of Rutherford’s model of the atom are:
Electron |
Proton |
Neutron |
They are present outside the nucleus. |
They are present in the nucleus. |
They are present in the nucleus. |
They are negatively charged. |
They are positively charged. |
They are neutral. |
They are represented as e–. |
They are represented as p+. |
They are represented as n0. |
Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the atomic species |
9 | – | 10 | – | – | – |
16 | 32 | – | – | – | Sulphur |
– | 24 | – | 12 | – | – |
– | 2 | – | 1 | – | – |
– | 1 | 0 | 1 | 0 | – |
Atomic number |
Mass number |
Number of neutrons |
Number of protons |
Number of electrons |
Name of the atomic species |
9 |
19 |
10 |
9 |
9 |
Fluorine |
16 |
32 |
16 |
16 |
16 |
Sulphur |
12 |
24 |
12 |
12 |
12 |
Magnesium |
1 |
2 |
1 |
1 |
1 |
Deuterium |
1 |
1 |
0 |
1 |
0 |
Protium |
(i) Atomic number:
For example,
Atomic number of Boron is 5
Atomic number of Aluminium is 13
Atomic number of Argon is 18
(ii) Mass number:
For example,
Mass number of Boron is 11
Mass number of Aluminium is 27
Mass number of Argon is 40
(iii) Isotopes:
For example,
Carbon has three isotopes 12C, 13C, 14C
Boron has two isotopes 10B, 11B
(iv) Isobars:
For example,
Ar40, 19K40, 20Ca40
C14, 7N14
Uses of Isotopes
The rules for distributing the electrons in various shells are given below.
1.The maximum number of electrons present in a shell (n) is given by the formula 2n2, where ‘n’ is an integer.
For example,
Maximum number of electrons in K shell is 2(1)2 = 2 electrons
Maximum number of electrons in L shell is 2(2)2 = 8 electrons
2.The maximum number of electrons in the outermost orbit of an atom is 8.
For example,
Total number of electrons 13
Expected distribution is 2,11 but actual distribution is 2,8,3
3.Electrons are not accommodated in a given shell, unless the inner shells are filled, i.e., shells are filled in a step-wise manner.
For example,
In order to accommodate 13 electrons in K, L and M shell, the correct distribution is 2,8,3 and not 2,7,4.
Thomson’s Model |
Rutherford’s Model |
Bohr’s Model |
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