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  • Complete Physics Course - Class 11
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The Nucleus


Table of Content

Electron, Proton and Neutron in an Atom
The atom consists of a central core called ‘nucleus’ and the electrons revolve around it in nearly circular orbits. It is vital to clearly understand the composition and the structure of an atom.The dense region at the center of the atom is called the nucleus. It was discovered in 1911 as a result of Rutherford’s interpretation. Almost all of the mass of an atom is located at the nucleus. Electrons are outside the nucleus and keep moving in orbits around the nucleus. Protons are positively charged, neutrons as the name suggests are neutral i.e. they don’t have any charge and electrons are negatively charged.
The number of protons and electrons is always the same in a neutral atom. But the number of neutrons within an atom can vary. There may exist atoms of same element having different number of neutrons. Such atoms are called isotopes.

Electron Revolves Around the Orbit

An atomic nucleus is much smaller than an atom. The cloud of electrons that orbit the nucleus define the size of an atom and this cloud is almost 100000 times as large as the atom’s nucleus. At times, the electrons may get removed from an atom. In such a case, if an atom loses all its electrons, then the naked atomic nucleus is termed as an ion. Usually, the smaller nuclei constitute these ions like the nucleus of a hydrogen atom (which has a single proton) or a helium atom which has two protons and two neutrons. This does not necessarily imply that ions cannot be formed with heavy atoms. Some cosmic rays are very heavy ions from more massive atoms. 

Components of Nucleus:-

Components of Nucleus

As discussed above both protons and neutrons are present inside the nucleus. Protons are the positively charged particles while neutrons do not have any charge.
The figure given below describes an atom. It shows hoe electrons keep on orbiting around the nucleus.

The number of protons in the nucleus decides the nature of the atom. In fact, the atomic number of an atom equals the number of protons present in the atom. Since, the carbon atom has 6 protons so its atomic number is 6. Similarly, for oxygen the number is 8.

Atomic Number = Number of protons in the nucleus

The number of protons added to the number of electrons gives the atomic mass of the atom called the mass number.  

Mass Number = Number of protons + Number of Neutrons
Refer this video to know more about, “Nucleus”.

Features of Nucleus:-

  • The positive charge of the proton is equal in magnitude to that of an electron which is +1.6 x10–19 C.
  • The mass of the proton is equal to 1840 times that of an electron

  • A neutron has no charge and mass is approximately equal to that of proton.

  • The atomic number of the atom is generally denoted by the letter Z and the mass number by the letter A.

  • A particular set of nucleons forming an atom is called as nuclide. It is represented as ZXA

  • The nuclides having same number of protons (Z), but different number of nucleons (A) are called as isotopes.

  • The nuclides which have the same number of nucleons but different number of protons (Z) are called as isobars.

  • The nuclide having same number of neutrons i.e. the value (A – Z) is same are called isotones.

  • Atoms of different substances possess different mass. Sometimes we come across different atoms of same element which possess different masses.

  • Certain atoms, generally belonging to the category of high atomic weight, are found to emit radiations consisting of high speed charged particles and electromagnetic radiations.

  • Generally nucleus is a highly stable entity. It is very difficult to pull the constituents of a nucleus apart.

  • If we are able to break a nucleus a lot of energy is liberated.

  • The nucleus possesses a resultant magnetic moment.

Question 1

The graph of ln(R/R0) vessus ln A (R=radius of nucleus and A = its mass number) is,

(a) parabola             (b) a straight line

(c) an ellipse            (d) a circle.

Question 2

In a neutral atom,

(a) number of electrons and protons are same

(b)  number of electrons and protons are different

(c) number of electrons are more than number of protons

(d)  number of protons are more than number of electrons

Question 3

Mass number of an atom is equal to the,

(a) number of protons

(b) number of electrons

(c) sum of number of proptons and electrons

(d) sum of number of proptons and neutrons

Question 4

A charge of neutron is,

(a) positive           (b) negative

(c) chargeless       (d) none of the above

Q.1 Q.2 Q.3 Q.4





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