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Hydrogen and its Isotopes Notes for Class 11 Chemistry


Hydrogen and its Isotopes Notes for Class 11 Chemistry

Hydrogen is a chemical element represented by a symbol H and has an atomic number as 1. The standard atomic weight of hydrogen is 1.008 hence, it is the lightest element in the periodic table. The monatomic form (H) is the most abundant chemical substance in the universe.

Hydrogen and its Isotopes Notes for Class 11 Chemistry

Gaseous hydrogen occurs in nature in fairly small amounts. The earth’s atmosphere contains about one part of hydrogen in 15000 to 20000 of air (in molecules number), but this proportion increases with altitude. Hydrogen is also the simplest of all elements. Its atoms consist (usually) of one proton and one electron. Hydrogen was first discovered in 1766 by English chemist and physicist Henry Cavendish (1731-1810). He was also the first person to prove that water is a compound of hydrogen and oxygen.

Hydrogen occurs throughout the universe in two forms. First, it occurs in stars. The stars use hydrogen as a fuel in order to produce energy. The process in which stars use hydrogen is known as fusion. It is the process in which two or more small atoms are pushed together to make one large atom. Hydrogen also occurs in the “empty” spaces between stars. Sometimes back, scientists thought that this space was really empty and that it contained no atoms of any kind. But, truly, this interstellar space (space between stars) contains a small number of atoms and most of which are hydrogen atoms. Hydrogen occurs on the Earth primarily in the form of water. Every molecule of water (H2O) consists of two hydrogen atoms and one oxygen atom. Hydrogen is also present in many rocks and minerals. Hydrogen is not abundant in the atmosphere because it has a very low density. The Earth’s gravity is not able to hold on to hydrogen atoms very well, so they drift away into outer space very easily. Most of the hydrogen that was once available in the atmosphere has now escaped into outer space.

Diatomic Hydrogen (H2) is a colourless, odourless, non- metallic, tasteless, highly flammable.

Hydrogen and its Isotopes Notes for Class 11 Chemistry  Hydrogen and its Isotopes Notes for Class 11 Chemistry

Elemental hydrogen resembles alkali metals as well as with the halogens because it has only one electron in its K shell which can accommodate a maximum of two electrons. It has an equal capacity to lose or gain one electron. When it loose one electron, it forms H+ ions and in such a case, it resembles alkali metals. On the other side, when it gains on electron, it forms Hand in such case, it resembles halogens.

Read another topic for Chemistry class 11 VALENCE BOND

The Resemblance with Alkali Metals

i. Electronic Configuration: Both Hydrogen and alkali metals (Lithium Li, Sodium Na etc.) have similar electronic configurations having one electron in their outermost shells.

1H = 1s3Li = 1s22s111Na = 1s22s22p63s1

ii. Electropositive Character: Both have a strong electropositive character they have a tendency to lose the valence electron.

iii. Valency: Both are monovalent (having 1 valency)

iv.Formation of cations: They lose the valence electrons and form cations; like H+, Li+, Na+

v. Electrolysis of their halides: The halides of hydrogen and the alkali metals are strong electrolytes hence, they are electrolysed, releasing hydrogen and alkali metals at the cathode.

vi. Affinity for electronegative elements: Hydrogen and alkali metals have a great affinity for non-metals. They combine with oxygen, sulphur, halogens to form oxides, sulphides and halides, respectively. Like H2O, Na2O; H2S, Na2S; HCl, NaCl.

Resemblance with Halogens

i. Electronic configuration: Halogens and hydrogen have a similar electronic configuration as they have one electron less than the inert gas type of electronic configuration in their valency shells.

1H = 1s19F =1s22s22p517Cl= 1s22s22p63s23p5

ii. Non-metallic character: Both are non- metals and are poor conductors of heat and electricity. Hydrogen is a gas just like the first three halogens.

iii. Atomicity: It is defined as the total number of atoms present in one molecule of an element, compound or a substance. Both hydrogen and halogen molecules are diatomic, as H2, F2, Cl2 Allalkali metals are monoatomic.

iv. Formation of negative ions: They both have the potential to gain one electron to form the negative ions, i.e., H, Cl, Br

v. Reaction with metals: Hydrogen combines with alkali and alkaline earth metals to form hydrides and halides, respectively.

2Na + H2 → 2NaH                                   Ca + H2→ CaH2

2Na + Cl2 → 2NaCl                                  Ca + Cl2→ CaCl2

Upon electrolysis, of these salts, that is NaH and NaCl, H2 and Cl2 are liberated at the anode.

vi. Formation of similar type of covalent compounds: Both combine with carbon, silicon, nitrogen, phosphorus, arsenic etc., to form similar types of covalent compounds.

CH4 SiH4 NH3 PH AsH3

CCl4  SiCl4 NCl3 PCl3 AsCl3

In some cases, hydrogen differs from alkali metals as well as from that of halogens. For example, the oxides of hydrogen (H2O) is neutral, while that of halogens (Cl2O7) and alkali metals (Na2O) are acidic and basic respectively.

Resemblance with carbon

i. Electronegativity: Both hydrogen and carbon have similar electronegativity values, H = 2.1 and C = 2.5

ii. Electronic Configuration: Both have a half-filled outer most shells. Hydrogen has one electron in the outermost shell, and it can have the maximum of two Similarly, carbon has four electrons in the outermost shell, and it can have a maximum of eight electrons.

iii. Formation of interstitial compounds: Both carbon and hydrogen form interstitial compounds (compounds that are formed when an atom with a small radius sits in an interstitial also known as “hole” in a metal lattice. Carbon and hydrogen are small in size, so they can easily fit in the holes.


Hydrogen is an element that occurs as a diatomic molecule. Diatomic molecule means that they are formed from two atoms of the same element. These elements are so reactive that they need to bound another atom. Thus, hydrogen is very reactive, so it occurs as a diatomic molecule. The reactivity of hydrogen gives rise to many of its unique properties.




Isotopes are atoms having the same number of protons but having a different number of neutrons. The atomic number of an element is equal to the number of protons and the atomic mass is the sum of protons and neutrons, therefore isotopes are elements with the same atomic number but different mass numbers.

There are three isotopes of hydrogen; 1H, 2H, and 3H. The first two isotopes; protium and deuterium are stable, while 3H (Tritium) has a half-life of 12.32 year. Hydrogen is the only element whose isotopes are known by different names and are in common use today.

Hydrogen and its Isotopes Notes for Class 11 Chemistry


1H (Protium) is the most common hydrogen isotope with an abundance of more than 99.98%. The nucleus of this isotope consists of only one proton. Protium is considered to be a stable isotope. Many theories proposed in the 1970s predict that proton decay can occur with a half-life between 1031 and 1036 years.



2H (Deuterium)is another stable isotope of hydrogen, it contains one proton and one neutron in its nucleus. The nucleus of deuterium is called a deuteron. Deuterium is not radioactive and does not have significant toxicity hazard. Water that is enriched in molecules that include deuterium instead of protium is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents used for 1H-NMR spectroscopy. Heavy water is used as a coolant for nuclear reactors. Deuterium is also a possible fuel for commercial nuclear fusion.


3H (Tritium) contains one proton and two neutrons in its nucleus. It is radioactive, which decays into helium-3 through βdecay. It has a half-life of 12.32 years. Trace amounts of tritium occur naturally due to the interaction of cosmic rays with atmospheric gases. This isotope has also been released during nuclear weapons tests. It is used in thermonuclear fusion weapons. Commonly, tritium is produced by bombarding a natural isotope of lithium, lithium-6, with neutrons in a nuclear reactor. Tritium was once used routinely in chemical and biological labelling experiments as a radiolabel, but it has become less common in recent times.


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October 9, 2019

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