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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
Complete the following table.
| 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 | - |
Tick the most appropriate answer.
Which of these have similar chemical properties but different set of physical properties and mass number?
- isobars
- elements
- isotopes
- none of these
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What is the atomic number of element X?
If an atom ‘A’ has atomic number 19 and mass number 39, state –
- Its electronic configuration.
- The number of valence electrons it possesses.
Draw the atomic diagram of the following element showing the distribution of – protons, neutrons and the electrons in the various shell of the atom.
Argon – \[\ce{^40_18Ar}\]
[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:
Sodium [At. no. = 11, Mass no. = 23]
The No. of electrons in the outermost shell of the atom of an inert gas ______.
‘Z’ is denoted by ______.
If M is the atomic mass and A is the mass number, then packing fraction is given by ______.
In which one of the following, the number of protons is greater than the number of neutrons but number of protons is less than the number of electrons?
