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प्रश्न
(a) What is meant by (i) atomic number, and (ii) mass number, of an element? Explain with the help of an example.
(b) What is the relation between the atomic number and mass number of an element?
(c) If an element M has mass number 24 and atomic number 12, how many neutrons does its atom contain?
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उत्तर
(a)(i) Atomic number is the number of protons in the nucleus of an atom. In other words, each element has a unique number that identifies how many protons or electrons are present in one atom of that element. For example, all hydrogen atoms contain one proton and have an atomic number of 1. All carbon atoms contain six protons or electrons and have an atomic number of 6.
(ii) The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each isotope of a chemical element. This is not the same as the atomic number (Z), which denotes the number of protons in a nucleus. Thus, it uniquely identifies an element. Therefore, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N = A − Z.
Example- One atom of Na contains 11 protons and 12 neutrons; so its mass no. is 23.
(b) Relation between mass number and atomic number:
Mass No. = Atomic no. + No. of neutron
(c) No. of neutrons = Mass No. - Atomic no.
= 24 – 12 = 12
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संबंधित प्रश्न
Give the following a suitable word/phrase.
Atoms of same element with same atomic number but a different mass number.
WRITE SHORT ANSWER
Define the atomic mass number of an atom.
Fill in the following blanks in respect of an atom of an element
| No. of protons | No. of neutron | Mass number | Atomic number | No. of electrons | Symbol |
| 11 | 12 | ___________ | ___________ | ___________ | ________ |
Name or state the following:
Elements having same mass number but different atomic number
Define the term ‘atomic number’ of an atom.
If an atom ‘A’ has mass number 35 and atomic number 17, state the number of protons, electrons and neutrons it contains.
Draw the atomic diagram of the following element showing the distribution of – protons, neutrons and the electrons in the various shell of the atom.
Oxygen – \[\ce{^16_8O}\]
[The upper number represent the – mass number and the lower number represent the – atomic number e.g. calcium – mass number = 40, atomic number = 20]
Draw the atomic diagram of the following element showing the distribution of – protons, neutrons and the electrons in the various shell of the atom.
Calcium – \[\ce{^40_20Ca}\]
[The upper number represent the – mass number and the lower number represent the – atomic number e.g. calcium – mass number = 40, atomic number = 20]
Draw the structure of the following atoms showing the nucleus containing – protons, neutrons and the orbits with the respective electron:
Isotopes of hydrogen [11H, 21H, 31H]
Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.
