We have studied how electrons are arranged in an atom across different shells or orbits.
Valence electrons:
The electrons that are found in an atom's outermost orbit are called valence electrons. These electrons determine the chemical reactivity and bonding behavior of an element.
Atoms are continually trying to achieve a stable state.
Stable state:
If an atom has only one shell, the stable state is achieved when two electrons are present in it. Similarly, when an atom has two shells or more than two shella, the stable state is achieved when the outermost shell has eight electrons.
A few elements such as Helium (He), Neon (Ne), and Argon (Ar) already have this configuration.
This is known as the duplet rule (for one shell) or the octet rule (for more than one shell).
Electron distribution in Helium and Neon
The atoms that do not have this octet are involved in bond formation by gaining, losing, or sharing electrons.
i. Example for an atom losing an electron: Sodium (\(Na\))
- Atomic number of \(Na\) = \(11\)
- Number of electrons in \(Na\) = \(11\)
- Electronic configuration = \((2, 8, 1)\)
Sodium atom
To attain a stable state, sodium loses one electron, after losing one electron, its configuration becomes (\(2\), \(8\)).
ii. Example for an atom gaining of an electron: Chlorine (\(Cl\))
Atomic number of \(Cl\) = \(17\)
Number of electrons in \(Cl\) = \(17\)
Electronic configuration = \((2, 8, 7)\)
To attain a stable state, chlorine gains one electron, after gaining one electron, its electronic configuration is (\(2\), \(8\), \(8\)).
Valency:
Valency is the number of electrons an atom can gain, lose, or share to achieve a stable electronic configuration.
The valency of noble gases or inert gases is zero since there are no free electrons in the valence shell, and the elements are already in a stable state.
Example:
Oxygen (O):
- Atomic number = \(8\) → Electronic configuration = \((2, 6)\)
- Needs 2 more electrons to complete its octet.
- Hence, valency = \(2\).
Magnesium (Mg):
Atomic number = \(12\) → Electronic configuration = \((2, 8, 2)\)
Has \(2\) valence electrons; it can either gain \(6\) or lose \(2\).
Losing \(2\) is easier, so valency = \(2\).
Electron distribution in oxygen and magnesium
Isotopes:
We observed that some elements have the same atomic numbers but different mass numbers. These are known as isotopes.
Example:
A hydrogen atom with the same atomic number but a different mass number
Most of the elements consist of a mixture of isotopes. They are pure substances. Their chemical properties are similar, but their physical properties are not.
In nature, chlorine exists in two isotopic forms (, ), with masses of \(35\) u and \(37\) u in a \(3\):\(1\) ratio. Obviously, the question that arises here is what mass of chlorine atoms we can use.
The atomic mass of a given element is a weighted average of its isotopes. Each isotope's mass is divided by its abundance.
Application of Isotopes:
- The age of fossils, fuels, and dead organisms is determined using carbon-\(14\) isotopes.
- Cobalt-\(60\) is a radioactive isotope. It decays by emitting gamma rays, which are used to destroy cancer cells.
- In the treatment of goitre, an isotope of iodine is used.
- Blood flow is traced with sodium-\(24\) to identify whether there is an obstruction.
- Uranium-\(235\) is used as fuel in nuclear reactors.
Isobars:
Isobars are atoms with different atomic numbers but the same mass number. In other words, the nucleon count is the same, but the number of protons is different.
Example for isobars
Isobars are different substances, so their chemical properties are different, but their physical properties may be similar because they have the same mass.
Isotones:
Isotones are atoms of different elements with different atomic numbers and mass numbers but the same number of neutrons.
Example: and
Number of neutrons in boron = \(11\) − \(5\) = \(6\)
Number of neutrons in carbon = \(12\) − \(6\) = \(6\)
The above pair of elements, boron and carbon, has the same number of neutrons but different numbers of protons, resulting in different atomic numbers.