Chemistry, SS 1, Week 6
Topic: The Constituents of Atoms
Atoms are found to contain three types of particles: protons, neutrons and electrons.
It consists of centrally placed nucleus which is surrounded by a cloud of electrons. The nucleus is made of protons and neutrons. The proton and neutron each has a mass of one. Proton carries a positive charge while neutron has no charged. The electrons are very light and negatively charged and revolve round the nucleus in an orbital manner in order to neutralize the positive charge in the nucleus.
The atom is electrically neutral because the number of electrons revolving round the nucleus is equal to the number of protons in the nucleus.
The summary of the properties of sub-atomic particles are shown in the table below
|
Particle |
Mass |
Charge |
|
Proton |
1 |
+1 |
|
Neutron |
1 |
No charge |
|
Electron |
1/1840 |
-1 |
The arrangement of electrons in atoms
The ways in which the electrons are arranged outside and around the nucleus is called electronic configuration. The electrons are arranged in the orbits known as shells. The electrons revolve round the centrally placed nucleus in these shells.
There are seven shells in all and these are denoted by symbols K, L, M, N, O, P and Q. The nearest shell to the nucleus is K shell. It has the lowest energy and contains the smallest number of electrons which is two. It cannot hold more than two electrons.
To calculate the maximum number of electrons in a shell, the formula 2n2 is used.
N – Is the number of energy level of the shell.
When n = 2 for L shell, 2(2)2 = 8. The maximum number of electrons on the L shell is 8.
When n = 3 for M shell, 2(3)2 = 18. The maximum number of electrons on the M shell is 18 etc.
Atomic number and mass number
The atomic number of an element is the number of protons in one atom of that element. It is the number of protons in the nucleus of an atom. It is denoted by a letter Z.
The mass number of an element is denoted by A, which is the sum of the protons and the neutrons in the element. For example, sodium has a mass number of 23. The number of protons is 11 while neutrons is 12.
We can describe an atom of an element by writing its symbol together with its atomic number and mass number.
Using carbon as an example
Isotopy
Isotopy is a phenomenon whereby atoms of an element exhibit different mass numbers but have the same atomic number. This is due to differences in the number of neutrons present in these atoms. Such atoms are known as isotopes. Hence isotopes are atoms of the same elements with the same atomic numbers but different mass numbers. Isotopes of an element have slightly different physical properties but exhibit the same chemical properties. This is because neutrons contribute only to the mass of an atom, not its chemical behaviour.
Examples are as follow:
The mass spectrometer shows the relative abundance of the isotopes. The relative atomic masses of elements are the mean weight of the masses of the isotopes of the atoms of the elements. This is why they are not usually whole numbers. Given the mass numbers and relative abundance of the isotopes of an element, the relative atomic mass can be calculated.
Example 1: chlorine exists in two isotopic form, 35Cl and 37Cl respectively. The relative abundance of 35Cl is 75% and 37Cl is 25%. Calculate the relative atomic mass of chlorine.
Solution:
Example 2: A naturally occurring sample of Lithium contains 7.42% of 6Li and 92.58% of 7Li. The relative mass of 6Li is 6.015 and that of 7Li is 7.016. Calculate the relative atomic mass of a naturally occurring sample of lithium.
Example 3: The relative atomic mass of an element is 21.5. The two isotopes of the element are 20E occurring 25% in nature and xE occurring 75% in nature. Calculate x.
Solution:
Atomic mass = 20 x 25 + X x 75
100
Since relative atomic mass = 21.5, therefore
21.5 = 500 + 75x
100
X = 2150 – 500
75
X = 22
