Learn about Valence Electrons and Valency

In this post you will learn about valence electrons and valency, isotopes, isobars and fractional atomic masses of certain elements.

What are valance electrons?

The electrons in the outermost shell of an atom take part in all the chemical reactions given by that element. Thus, these electrons describe the combining capacity or valency of an element. That is why the electrons in the outermost shell of an atom are called its valence electrons. For example, the sodium atom has an electronic configuration of 2,8,1.

The outermost shell, i.e., M-shell of sodium atom contains only one electron. So, the number of valence electrons in sodium is 1.

Similarly, in case of chlorine, there are 7 electrons in its outermost shell (M-shell). So, the number of valence electrons in chlorine is 7.

What is meant by the valency of an element?

Each element has a definite electronic configuration. The electrons in the outermost shell of an atom are termed its valence electrons. The valence electrons take part in chemical reactions. The combining capacity of an element is given by the number of valence electrons which take part in chemical reactions.

The number of valence electrons of an element which actually take part in chemical reaction is called the valency of that element.

The valency of an element may also be defined as follows:

The number of hydrogen atoms or chlorine atoms or double the number of oxygen atom which combine with one atom of the element is called its valency.

For example, the valency of nitrogen (N) in ammonia (NH₃) is three because one atom of nitrogen combines with three atoms of hydrogen.

The valency of metal M in its oxide M₂O₃ can be obtained as follows:

Number of oxygen atoms combining with two atoms of M = 3

So, number of oxygen atoms combining with one atom of M = ³/₂

Hence, valency of metal M = 2*(³/₂)= 3

Generally, the valency of an element is equal to the number of valence electrons, or equal to 8 minus the number of valence electrons. The valency of an element depends upon the nature of the reaction in which that element is involved.

Valency of an element = Number of valence electrons

or

Valency of an element = 8 – Number of valence electrons

The modern approach to the concept of valency is based on the electronic concept of the atom.

How can the electronic configuration of an atom explain its chemical reactivity?

The electronic configuration of an element give an idea about its reactivity. The elements having a completely filled outermost orbit will be chemically inert, i.e., they are non reactive. For example, the outermost shell in case of helium, neon and argon are completely filled. As a result, helium, neon and argon are chemically inactive or inert. These element do not form compound with other elements.

• Because of this chemical inactivity, these gases are called as noble gases or inert gases.
• The elements containing only one or seven electrons in their outermost shell show very high chemical reactivity, i.e., such elements react very fast with other elements, e.g., sodium and chlorine have the following electronic configuration are highly reactive.

Sodium – 2,8,1. Here the outermost shell has only one electron.

Chlorine – 2,8,7. Here the outermost shell has seven electrons.

Isotopes

What are isotopes?

Isotopes may be defined as follows:

Atoms of same element, having same atomic number, but different mass number are called as isotopes of that element.

Since all isotopes of an element have the same atomic number, all the isotopes should contain the same number of protons inside their nucleus. Also, since different isotopes of an element have different mass number, the number of neutrons in the nucleus of isotopes of an element should be different. So, isotope may also be defined as:

The atoms of the same element which have the same number of protons but different number of neutrons inside their nucleus are called isotope of that element.

Isotopes are described by writing the mass number of that isotope has a superscript on the top-left or top-right side of the symbol of the element with atomic number is written as a subscript on the bottom-left side of the symbol.

e.g., _ZX^A

Isotopes of hydrogen

Hydrogen has three isotopes having mass number 1, 2 and 3 but all having same atomic number equal to1. These three isotopes of hydrogen are given below:

• Protium : ₁H¹
• Deuterium : ₁H²
• Tritium (Radioactive isotope) : ₁H³

Isotopes of carbon

Carbon has three isotopes having mass number 12, 13 and 14. These are carbon-12, carbon-13 and carbon-14 respectively. These isotopes have the same atomic number equal to 6. The three isotopes of carbon are given below:

• Carbon-12 : ₆C¹²
• Carbon-13 : ₆C¹³
• Carbon-14 : ₆C¹⁴

What are the characteristics of isotopes?

• The isotope of an element have the same number of protons inside their nuclei. As a result, all the isotopes of an element contain the same number of electrons.
• Different isotopes of an element have different mass numbers. So, isotopes should show the following characteristics:

Since, the isotopes of an element have the same number of protons and electrons, all the isotopes of an element show: the same chemical properties same electronic configuration and the same number of valence electrons.

For example, for the three isotopes of oxygen

Characteristics	8O16	8O17	8O18
No. of protons	8	8	8
No. of neutrons	8	9	10
No. of electrons	8	8	8
Electronic configuration	2,6	2,6	2,6
No. of valence electrons	6	6	6

The isotope of an element have different masses. So the properties which depend upon atomic mass should be different for different isotopes. Many physical properties, e.g., melting point, boiling point, density, etc., depends upon atomic mass. So, different isotopes of an element show different physical properties. For example, the physical properties of the two isotopes of hydrogen are different.

Property	Deuterium (1H2)	Protium (1H1)
Mass Number	2	1
Melting point	18.65 K	13.95 K
Boiling point	23.59 K	20.38 K
Density (g/L)	0.167	0.083

Fractional atomic masses of certain elements

How to calculate the atomic mass of an element from the mass numbers of its isotopes?

The atomic mass of an element is the weighted arithmetic mean of the atomic mass of its isotopes present in the sample of the element.

Let us consider a sample of an element X containing its two isotopes X₁ and X₂.

This method is illustrated by taking the example of chlorine.

The two isotopes of chlorine ₁₇Cl³⁵ and ₁₇Cl³⁷ occur in 3:1. Then,

The calculated atomic mass of chlorine is thus a weighted average of the mass number of all the isotope present in its sample.

What are the applications of isotopes or radioisotopes?

Isotopes or radioisotopes are useful in many ways. Some main uses of radioactive isotopes, called radioisotopes, are:

• For estimating the age of old archaeological samples: Radiocarbon dating or in general radioisotopic dating method is used for estimating the age of old archaeological samples. For example, age of the earth, moon, rocks and mineral-deposits can be determined by using the principle of radioisotopic dating.
• For the treatment of diseases: Radioisotopes are widely used for the treatment of diseases like cancer.

1. The isotope, cobalt-60, is used for destroying malignant cells in patients suffering from cancer.
2. The isotope, iodine-131, is used for studying disorders of the thyroid gland.
3. The isotope, sodium-24, is used for examining the circulation of blood.

• For estimating the age of glaciers: The age of glaciers, snowfields, and even wines can be determined by radioisotopic dating. In these cases, the radioactivity level of tritium is measured.

What is radiocarbon dating?

This technique was developed by William Libby. He was awarded the Nobel prize for this work.

The age of carbon containing object means the number of years ago when the plant should have died.

Estimating the age of carbon-containing object by measuring the concentration or activity of ₆C¹⁴ in it is called radiocarbon dating or carbon dating.

Radioactive carbon-14 gets converted to radioactive carbon dioxide. This radioactive ¹⁴CO₂ is taken up by the plants during photosynthesis. ₆C¹⁴ is radioactive and decays by the beta-emission.

₆C¹⁴ is being continuously formed in the higher atmosphere and consumed due to beta-emission decay. As a result, an equilibrium concentration of ₆C¹⁴ is maintained in all the living plants.

However, when a plant dies, it can no longer fix up radioactive ¹⁴CO₂. As a result concentration of ₆C¹⁴ in it starts decreasing. The half life of ₆C¹⁴ is 5760 years. Thus, in 5760 years, the concentration of ₆C¹⁴ is lowered to half of its initial concentration and after another 5760 years, its concentration gets lowered to 25% of the initial concentration. Thus in 11520 years, the ₆C¹⁴ concentration is reduced to one fourth of its initial concentration. Thus, by measuring the concentration of 6C14,in a dead carbon-containing object and knowing the concentration of ₆C¹⁴ in a living plant, we can estimate the age of the object.

What are isobars?

The word isobar has been derived from the Greek word meaning equally heavy.

The atoms of different elements which have the same mass number but different atomic number are called isobars. As the mass number is equal to sum of number of protons and neutrons inside the nucleus of an atom, therefore, isobars may also be defined as follows:

The atoms of different elements which have different number of protons but equal sum of the number of protons and neutrons are called isobars.

₁₈Ar⁴⁰, ₁₉K⁴⁰ and ₂₀Ca⁴⁰ are some typical isobars. Each of these have the same mass number but different atomic number.

What are isotones?

Isotones are atoms which have same neutron number but different proton number.

₆C¹⁴, ₇N¹⁵, ₈O¹⁶ are isotones as they contain same neutrons but different protons.

Read More: Rutherford Alpha Scattering Experiment

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