NUCLEI/ATOMIC MASS/MASS NUMBER

 Get Clarity About Nuclei in one read

Introduction:

   In section we are discuss about atomic model. These are played a vital role to understand about the structure of atom.

In this section we learn about Nuclei structure and properties.

Composition of nuclei:

1.   Atoms consists of nucleus surrounded by electrons
2. Nucleus contain proton and neutron in combined form
3. The positively charged particle is called proton
4. The electrically neutral particle is called neutron
5. The two constituents of nucleus namely protons and neutrons are collectively called nucleons.     

   

To specify the nucleus of any element. We use this general notation

                  ᴬX

                  ᶻ

             

   A- MASS NUMBER

   Z- ATOMIC NUMBER

    X- Chemical symbol of the

Mass number:

                              The total number of proton and neutron in the nucleus is called mass number. It is denoted as'A'.

              A=Z+N

Atomic number:

                                The total number of proton in the nucleus is called atomic number. It is denoted as 'Z'.

Neutron number:

                                    The number of neutron in the nucleus is called neutron number. It is denoted as 'N'.

Example:

Carbon nucleus is represented by

                   ₁₂

                      C

                     ⁶

It is implies that carbon contain 12 nucleons of which 6 are proton and 6 are neutron.

Nucleus is made up of positively charged proton and neutron. The overall charge of nucleus is +Ze, but the atom is electrically neutral, it is implies that the number of proton in nucleus is equal to number of electron in atom.

Before we learn about atomic mass. We need understand about mass.

MASS:

Mass is the property that measures the amount of matter in the body . SI unit of mass is kg- kilogram or resistance to change its state of motion( acceleration).

Atomic mass

1. The mass of atom is very small when expressed in SI unit(kg). Therefore ,it is more convenient to express it in terms of another unit namely, the atomic mass unit. 
2. The mass of an atom is expressed in unified atomic mass/ atomic mass unit/ Dalton
3. One atomic mass unit is approximately equal to mass of proton don't also the mass of neutron  

       1amu ~ mass of proton                                          1amu ~ mass of neutron

  Atomic mass unit:

                                         It is defined as 1/12 of the the mass of the the isotope of carbon-12.

          

     1amu= mass of carbon-12 atom

                   --------------------------------------

                                  12

1 mole of carbon-12 atom=12g/mole

6.023×10²³ of carbon-12  atom= 12g/mole

Mass of one carbon-12 atom=  12/6.023×10²³

            

                 1amu = 1 ×  12           

                                12 × 6.023×10²³

                  1amu=         1                                                                        6.023×10²³

                  1amu=         1         

                               Avogadro number                                

                   1amu= 1.660×10⁻²⁴g

Relative atomic mass:

                                       It is is defined as the ratio of the the average atomic mass factor to the the unified atomic mass unit.

Relative atomic mass

    (Aᵣ)                           = average mass of atom

                                          Unified atomic mass

EXAMPLE: relative atomic mass of the hydrogen

 

= average mass of hydrogen

   Unified atomic mass

  =  1.6735×10⁻²⁷kg

      1.660×10⁻²⁷kg

   =1.00782amu

Atomic model

1. Dalton atomic model:

• Extremely small particles called atom
• Atoms combine to form a molecule.
• Atoms are indivisible.

KEYPOINT:

• Mass of electron is1/1837 times the mass of 
•  Mass of proton=1.0072amu.                                              

2. Thomas atomic model:

• In this model ,the atoms are visualised as homogeneous sphere which contain uniform distribution of positively charged particles.
• The negatively charged particles known as electrons are embedded in it like seeds in watermelon.
• The atoms are electrically neutral this implies that the total positive charge in atom is equal to total negative charge.
• According to this mo⁶del ,all the charges are assumed to be rest

DRAWBACK:

From classical electrodynamics,no stable equilibrium points exist in electrostatic configuration and hence such atom cannot to stable. 

3.Rutherford model:

EXPERIMENTAL ARRANGEMENT:

• The source of Alpha particle is kept inside the thick lead Box with the fine hole.

• The Alpha particles coming through the fine hole of lead box pass through another fine hole made on the lead screen.

• These particles are allowed to fall on a thin gold foil and it is observed that the Alpha particles passing through gold foil are scattered through different angles.

• The Movable screen which is made up of zinc sulfide is kept on the other side of the gold foil to collect the Alpha particles.
• Whenever Alpha particles strike the screen ,flash of light is observed which can be seen through a microscope.

RESULT:

• Most of the Alpha particles are undeflected through the gold foil and went straight. 
• Some of the Alpha particles are deflected through a small angle.
• A few Alpha particles are deflected through the angle more than 90 degree
• Very few Alpha particle returned back that is deflected back by180 degree.

On the basis of Rutherford experiment,

Rutherford proposed,

• The atom is most empty space.
• The nucleus is positively charged.
• The nucleus carries most of the atomic mass.
• The radius of nucleus is directly proportional to the number of nucleons.
• The volume of atom is 10^15 times the volume of nucleus.

DRAWBACK:

• Could not explain the stability of atom.
• The electron losses its energy continuously and fall into the nucleus.
• The observed spectrum should be continuous but the spectrum obtained is discrete.