How was the problem Related to the position of hydrogen solved in the modern periodic table

Text Solution

Solution : Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. <br> Hydrogen has electronic configuration `1s^1` on one hand, its electron configuration is similar to the outer electronic configuration II `(ns^1)` of alkali metals, which belong to the first group of the periodic table. <br> On the other hand, like halogens, it is short by, one electron to the corresponding noble gas configuration. <br> Hydrogen, therefore has resemblance to alkali metals, which lose one electron to form unipositive ions. Like alkali metals hydrogen forms oxides, halides and sulphides. <br> However, unlike alkali metals, it has a very high ionization enthalpy and does not possess metallic characteristics under normal conditions. `Delta_i H` of Li is 520 kJ `"mol"^(-1)`,F is 1680 kJ `"mol"^(-1)` and that of H is 1312 kJ `"mol"^(-1)` <br> Like halogens, hydrogen forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. However, in terms of reactivity, it is very low as compared to halogens. <br> Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals and Halogens, it differs from them as well. Loss of the electron from Hydrogen atom results in nucleus `(H^+)` of ~ `1.5xx10^(-3)` pm size. <br> This is extremely small as compared to normal atomic and ionic sizes of 50 to 200 pm. <br> As a consequence, `H^+` does not exist freely and is always associated with other atoms or molecules. Thus, it is unique in behaviour and is, therefore, best placed separately in the periodic table

We have studies that there are three types of elements in the modern periodic table. There are metals, nonmetals and metalloids or semimetals. We have studied how the characteristics vary. Most of the elements are predominantly metals and the metals are situated on the left-hand side of the periodic table. The metallic character decreases as we move towards the right. The elements on the extreme right are all non-metals. In between the metals and non-metals, there are Metalloids. The only exception to this rule is Hydrogen. Hydrogen has a very special position in the periodic table due to its various unique properties. Let’s study the Position of Hydrogen in the Periodic Table in detail.

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• Hydrogen element has been placed at the top of group 1, above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals.
• Since hydrogen atoms are very small in size, many properties of hydrogen are different from those of alkali metals. Therefore, while discussing the alkali metals of group 1, hydrogen is never included.
• Hydrogen resembles alkali metals as well as halogens in the following properties, so it is best placed separately in the periodic table.

Electronic Configuration:

Both hydrogen and alkali metals have 1 valence electron.

Like alkali metals, it needs to gain one electron in its valence shell to achieve a stable configuration as that of the next noble gas.

Learn more about the Blocks of the Periodic Table, here.

Good Reducing Agent:

Like all the other alkali metals, Hydrogen is a strong reducing agent. A reducing agent is an element or compound that loses or donates an electron.

Formation of Halides:

Like alkali metals, hydrogen combines with electronegative Halogen elements from group 17 to form halides.

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Hydrogen shows the following differences with the metals.

Hydrogen is essentially not a metal-like all alkali metals, but a nonmetal.

Loss of Electron:

Even though Hydrogen has only one electron in its outer shell, it cannot easily lose or donate this electron to become an electropositive ion. All other alkali metals can do this with ease. Hydrogen can comparatively gain an electron easily like non-metals.

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State at Room Temperature:

At room temperature, all the alkali metals exist in solid-state, while hydrogen alone is a gas.

Size of Atom:

The H+ ion of hydrogen is much smaller than ions of alkali metals.

Ionization Potential:

The ionization potential of hydrogen is more than 300 Kcal per mole, The maximum ionization potential for metals is 147 Kcal per mole.

Isotopes of Hydrogen

Hydrogen has three isotopes, namely Protium, Deuterium, Tritium.

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Preparation of Hydrogen

• High purity hydrogen is obtained by the electrolysis of water containing traces of acid and alkali or the electrolysis of an aqueous solution of sodium hydroxide or potassium hydroxide using a nickel anode and iron cathode.
• Hydrogen can be prepared in the laboratory by the reaction of metals, such as zinc, iron, tin with dilute acid.

Properties of Hydrogen

Physical Properties:

• Hydrogen is a colorless, odorless, tasteless, lightest and highly flammable gas.
• It can be liquefied under low temperature and high pressure, hydrogen is a good reducing agent.

Chemical Properties:

• Hydrogen reacts with oxygen to give water. Hydrogen itself acts as a reducing agent.
• Hydrogen reacts with halogens to give halides.
• Deuterium reacts with oxygen in order to give heavy water.

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Uses of Hydrogen

• Majorly hydrogen is used in synthetic applications, one such process is the Haber process which is used to synthetic ammonia on large scales.
• Ammonia is used for the manufacture of chemicals such as nitric acid, fertilizers, and explosives.
• Liquid Hydrogen is used in rocket fuel.
• Hydrogen is also used in fuel cells for generating electrical energy. The reversible uptake of hydrogen in metals is also attractive for rechargeable metal hydride batteries.
• Points to Remember
• Only 63 elements were discovered at the time of Mendeleev when he was composing the periodic table.
• Inert gases were not found at Mendeleev’s time.
• Hydrogen is the only element that exists without neutrons.
• Hydrogen is a very light gas and is not held by the earth’s gravity. Thus no free hydrogen is present on earth.
• The solar energy of the sun is due to the conversion of hydrogen into helium.
• Hydrogen is not used in balloons as it is inflammable.
• The majority of elements in the periodic table are metals.
• There are 7 periods and 18 groups in the modern periodic table.
• Group 18 elements are called noble gases.

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Trends in Periodic Table FAQs

Q.1 What is the position of Hydrogen in the Periodic Table?

Ans.1 In the modern periodic table, Hydrogen is placed at the start as the very first element in group 1.

Q.2 Why is the position of Hydrogen in the periodic table controversial?

Ans.2 Hydrogen has an electron configuration similar to those of alkali metals. However, it is a non-metallic gas and shows its properties. Hence, the position of Hydrogen in the periodic table is controversial.

Q.3 What is one metallic property of hydrogen?

Ans.3 Like all the other alkali metals, Hydrogen is a strong reducing agent. A reducing agent is an element or compound that loses or donates an electron.

Q.4 What is one non-metallic property of hydrogen?

Ans.4 Even though Hydrogen has only one electron in its outer shell, it cannot easily lose or donate this electron to become an electropositive ion. All other alkali metals can do this with ease. Hydrogen can comparatively gain an electron easily like non-metals.

Q.5 How many isotopes of Hydrogen are there?

Ans.5 Hydrogen has three isotopes namely Protium, Deuterium, Tritium.

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