ATOMIC MASS AND MASS NUMBER

ATOMIC MASS AND MASS NUMBER

 ATOMIC NUMBER

     - number of protons in the nucleus of an atom.

  eg.  oxygen - 8
         sodium - 11

MASS NUMBER

     sum of protons and neutrons in an atom.

eg. 
sodium-  23

number of protons=11
number of neutrons = 12
thus mass number=23


All atoms of one element have the same number of protons
        but may have different number of neutrons
        thus different mass numbers.

MASS NUMBER IS WRITTEN ON THE TOP LEFT SIDE OF THE ELEMENT

eg.  23
          Na  
   

ISOTOPES

       atoms of an element having different number of neutrons and thus different mass numbers

Most isotopes are stable which means that their nuclear structure doesnot change over.

eg.

    The stable isotopes of oxygen are designated as 16    17    18
                                                                                   O,     O,      O
 or O-16, O-17, 0-18

Although the isotopes of an element have different number of neutrons, they have identical chemical properties because they have the same number of electrons.


RADIO ACTIVE ISOTOPES


Isotopes that are unstable

        their nuclei decay or spontaneously charge into a stable  configuration

eg.

    H-3
    C-14
    O-15
    O-19

     As they decay
              they emit radiation
                                either subatomic particles or packets of  energy

            and in this process often transforms into a different element.

eg.
      C-14 ,the radio active isotope of carbon, decays into N-14

    The decay of a radio isotope may be as fast as a fraction of a  second
        OR as slow as millions of years.

HALF LIFE OF AN ISOTOPE

          Time required for half of the radioactive atoms in a sample of that isotope to decay into a stable form

eg.  The half life of C-14 = 5600 years

       The half life of I-131=8 days.

Some uses of radio active isotopes

a. C-14 - 
       used to determine the age of organic samples.


RADIO ISOTOPES AS TRACERS

1. Thalium- 201

               used to monitor blood flow through the heart during an   exercise stress test.

2. I-131

             used to detect cancer of the thyroid gland 
             to assess its size nad activity.

            to destroy  part of   na overactive thyroid gland.

3. Cesium-n 137

            used to treat advanced cervical cancer

4. Iridium

            used to treat prostate cancer.  


ATOMIC MASS / ATOMIC WEIGHT OF AN ELEMENT

IS THE AVERAGE MASS OF ALL ITS NATURALLY OCCURING ISOTOPES

Typically the atomic mass of an element is close to the mass number of its abundant isotope.

UNIT

The standard unit for measuring the mass of atoms and their subatomic particles is a   dalton or atomic mass unit. (amu)

Mas of a neutron = 1.008 daltons.
Mass of a proton = 1.007 daltons. 
Mass of an electron = 0.0005 daltons.
                                = 2000 times smaller than the mass of a neutron or proton.

eg.

sodium

number of neutrons= 12
number of protons= 11

atomic number= 11

mass number= 23

atomic mass= 22. 99