Particulate Nature of Matter Continuation

ATOMIC NUMBER AND MASS NUMBER

            The number of protons in the nucleus of an atom is called the atomic number denoted by letter Z. it increases by one as one move from one element to another on the periodic table the mass number of an element is denoted by letter A, which is the sum of the protons and the neutrons in the element. E.g. sodium with mass numbers 23. Have 11 protons and 12 neutrons. However, given the mass number and atomic number of any element, one can easily calculate the number of neutron present in that element by deducting the atomic number from the mass number. For example the neutron number of sodium having a mass number of 23 and atomic number of 11 is calculated as follows. N ꞊ A – Z, 23 - 11꞊ 12 neutrons.

                                             A                           

                                             Z

The diagram above shows the position of atomic number and mass number of an atom where “X” is used to represent the symbol of the element in question.

ISOTOPY AND RELATIVE ATOMIC MASS      

Isotopy is the occurrence of two or more atoms of the same element having the same atomic number but different mass number. While isotopes are atoms of the same element having the same atomic number but different mass number. This phenomenon was discovered by Aston  in 1920  using Aston Mass Spectrometer. The mass spectrometer shows the relative abundance of the isotopes e.g.  ₁ ³H                                               ²  ₁H                                          ¹ ₁H  

                            TRITIUM                       DEUTRIUM                                PROTIUM

¹⁶₈O                                     ¹⁷₈ O                               ¹⁸ ₈O

³⁵₁₇Cl                                  ³⁷₁₇Cl   e.t.c .

Other examples of element that exhibit isotopes are: Carbon (C -12, C-13, C -14), Radium, Radon, Thorium, Palladium and so on.

            The relative atomic masses of element are the mean weight of the masses of the isotopes of the atoms of the elements. Given the mass numbers and the relative abundance of the isotopes of an element, the relative atomic mass can be calculated.

Example 1: Chlorine exists in two isotopic forms ³⁵Cl and ³⁷Cl respectively. The relative abundance of ³⁵Cl is 75% and ³⁷Cl is 25%. Calculate the relative atomic mass of chlorine.

Solution

³⁵Cl꞊75%

³⁷Cl꞊25%

R.A.M꞊ (75/100×35) + (25/100×37)꞊ 26.25+9.25꞊35.5

Example 2: the relative atomic mass of oxygen is 16.2, oxygen has two isotopes which are ¹⁶O and ¹⁸O respectively. Calculate the relative abundance of oxygen 16.

Solution: let the relative abundance of ¹⁶O be “X” and relative abundance of  ¹⁸O be (100 - X).

Mass of ¹⁶O will be x X 16 ꞊16x

Mass of ¹⁸O will be (100 - X)18 ꞊ 1800 – 18x

Adding the two result together i.e. 1800 – 18x + 16x

1800 -2x.

Now going back to the main method that says R.A.M ꞊ mean mass of the isotopes, just like the case of the first example where we divide the relative abundance of Chlorine by 100 and multiply it with the mass number.

1800 – 2x ꞊16.2

100      cross multiplying will give us, 1620 ꞊ 1800 -2x

Collecting like terms, we will have 2x ꞊ 1800 -1620

2x ꞊ 180

Therefore x ꞊ 180/2

x꞊ 90%

Therefore the relative abundance of ¹⁶O ꞊ 90%.

By God’s grace our next topic will be symbol, formula and equation.