Key Properties of Hydrogen

Q: Why is the position of hydrogen in the periodic table anomalous?

Hydrogen has one valence electron like an alkali metal, yet it also needs one electron like a halogen. It shows +1 in HCl and -1 in NaH, its ionisation enthalpy matches the halogens, and its physical properties match neither group. No single position fits every fact.

Q: How does hydrogen resemble the alkali metals?

It has a single valence electron, it loses that electron to give H⁺, it shows an oxidation state of +1, it acts as a reducing agent, and it forms compounds of matching formulae with electronegative elements: HCl beside NaCl, H₂O beside Na₂O, H₂S beside Na₂S.

Q: How is hydrogen different from the halogens?

Hydrogen forms hydride ions with only a few very electropositive metals, while halogens form halides with almost all metals. The H₂ molecule has no lone pairs, its oxide (water) is neutral instead of acidic, and hydrogen halides are low-boiling covalent compounds unlike ionic metal halides.

Q: Why does the hydrogen ion not exist freely in water?

H⁺ is a bare proton about 1.5× 10⁻³ pm across, against 50 to 220 pm for ordinary ions. Its charge density is enormous, so it immediately bonds to the lone pair of a water molecule and exists as the hydronium ion H₃O⁺, itself hydrated as H₉O₄⁺.

Q: What are the three isotopes of hydrogen?

Protium has no neutron and makes up 99.985% of natural hydrogen. Deuterium, written D, has one neutron and accounts for 0.0156%. Tritium, written T, has two neutrons and is only about one atom in 10¹⁸. Only tritium is radioactive, emitting β particles with a half-life of 12.33 years.

Q: What is the isotope effect in hydrogen?

Isotopes have identical electronic configurations, so they give the same products, but different masses change how fast bonds break. Protium reacts with chlorine about 13.4 times faster than deuterium, and ordinary water is electrolysed faster than heavy water. That rate gap is used to concentrate D₂O.

Q: Is the Hydrogen chapter part of the NEET syllabus?

No. NMC removed the Hydrogen chapter when it rationalised the NEET syllabus, and the chapter has also left the NCERT Class 11 book. The ideas are still worth knowing, because hydrogen bonding, oxidation numbers and the hydride ion reappear in chapters that NEET does test.

Q: Is hydrogen asked in JEE Main or JEE Advanced?

Not in JEE Main, where NTA has removed the Hydrogen chapter. It is in JEE Advanced: the official syllabus lists the position of hydrogen in the periodic table, its isotopes, preparation and properties, hydrides, water, heavy water, hydrogen peroxide and hydrogen as a fuel, so everything on this page can be asked there.

ChemistryHydrogenFor JEE aspirants

Hydrogen is the first and the lightest element, and every one of the key properties of hydrogen follows from its single $1 s^{1}$ electron. It can lose that electron to give $H^{+}$ like an alkali metal, take one more to give $H^{-}$ like a halogen, or share it, so its position in the periodic table is called anomalous. This page tests these properties of hydrogen against both groups and covers its three isotopes: protium, deuterium and tritium. The Hydrogen chapter is in the JEE Advanced syllabus; JEE Main and NEET have dropped it.

On this page1Three ways2Like Group 1?3Like Group 17?4Rogue element5Isotopes6Isotope effect7Flowchart8Examples

Key Formulas - Quick Reference

★ Must learn Electronic configuration: $1 s^{1}$ . Hydrogen is one electron short of helium and one electron away from a bare proton.
★ Must learn Ionisation enthalpy of hydrogen $= 1312 kJ mol^{- 1}$ (lithium $520$ , fluorine $1681$ , chlorine $1256$ ).
★ Must learn Losing the electron: $H$ gives $H^{+}$ + $e^{-}$ , so the oxidation state is $+ 1$ in $HCl$ , $H_{2} O$ , $N H_{3}$ .
★ Must learn Gaining an electron: $H$ + $e^{-}$ gives the hydride ion $H^{-}$ , so the oxidation state is $- 1$ in $NaH$ , $Ca H_{2}$ , $LiH$ .
Bond dissociation enthalpy of $H - H$ $= 435.9 kJ mol^{- 1}$ , the strongest single bond between two identical atoms, which is why $H_{2}$ is unreactive at room temperature.
Radius of the bare proton is about $1.5 \times 1 0^{- 3} pm$ against $50$ to $220 pm$ for ordinary ions, so $H^{+}$ in water is always $H_{3} O^{+}$ (truly $H_{9} O_{4}^{+}$ ).
★ Must learn Natural abundance: protium $99.985%$ , deuterium $0.0156%$ , tritium about one atom in $1 0^{18}$ .
Tritium decay: $_{1}^{3} H \to_{2}^{3} He + β^{-}$ , half-life $12.33$ years.
Isotope effect: $H_{2}$ reacts with $C l_{2}$ about $13.4$ times faster than $D_{2}$ does, and ordinary water is electrolysed faster than heavy water.

1. Hydrogen: the First Element

Hydrogen has atomic number $1$ : one proton in the nucleus and one electron around it. It is the lightest element known and the most abundant element in the universe. A free hydrogen atom survives only at very high temperature; under ordinary conditions two atoms pair up and hydrogen exists as the diatomic molecule $H_{2}$ , called dihydrogen.

Why hydrogen is special: it is the only element whose positive ion, $H^{+}$ , is a bare nucleus with no electron at all. Every other cation still keeps an electron cloud around it.

With one electron and one vacancy, hydrogen has three different ways of reaching a stable arrangement, and each one makes it look like a different family:

Lose the electron to give $H^{+}$ : this is what an alkali metal does.
Gain one electron to give the hydride ion $H^{-}$ and the helium configuration: this is what a halogen does.
Share the electron in a covalent bond, as in $H_{2}$ , $C H_{4}$ and $H_{2} O$ : this is what a group 14 element does with its half-filled shell.

Figure 1: With one electron and one vacancy, hydrogen can lose, gain or share. Each choice makes it look like a different family, which is the whole puzzle of its position.

Key idea

One electron and one vacancy: hydrogen can lose it (

+ 1

), gain one (

- 1

) or share it in a covalent bond. Everything else on this page follows from that choice.

2. Position of Hydrogen in the Periodic Table

Neither Mendeleev's table nor the modern periodic table could give hydrogen a settled address. Its electronic configuration argues for Group 1, its chemistry argues just as strongly for Group 17, and every point in its favour on one side has a point against it on the other.

Figure 2: Five resemblances on each side, and a mismatch under each: hydrogen fits neither Group 1 nor Group 17 completely.

2.1 Resemblance with alkali metals

Electronic configuration. Hydrogen has one electron in its valence shell, exactly like every alkali metal.

Element Configuration

H $1 s^{1}$
Li $[He] 2 s^{1}$
Na $[Ne] 3 s^{1}$
K $[Ar] 4 s^{1}$
Rb $[Kr] 5 s^{1}$
Electropositive character. Like an alkali metal, hydrogen can lose its only electron to form a positive ion: $H$ gives $H^{+}$ + $e^{-}$ .
Oxidation state. Hydrogen shows $+ 1$ in its compounds, just as alkali metals do: $HCl$ , $NaCl$ , $KCl$ .
Reducing agent. Alkali metals reduce because they give away an electron easily. Dihydrogen is a good reducing agent too:
$F e_{3} O_{4} + 4 H_{2} Δ 3Fe + 4 H_{2} O$
$CuO + H_{2} Δ Cu + H_{2} O$
Similar compounds with electronegative elements. Hydrogen and the alkali metals form compounds with matching formulae.

Type Hydrogen compound Sodium compound

Halide $HCl$ $NaCl$
Oxide $H_{2} O$ $N a_{2} O$
Sulphide $H_{2} S$ $N a_{2} S$

Element	Configuration
H	$1 s^{1}$
Li	$[He] 2 s^{1}$
Na	$[Ne] 3 s^{1}$
K	$[Ar] 4 s^{1}$
Rb	$[Kr] 5 s^{1}$

Type	Hydrogen compound	Sodium compound
Halide	$HCl$	$NaCl$
Oxide	$H_{2} O$	$N a_{2} O$
Sulphide	$H_{2} S$	$N a_{2} S$

2.2 Differences from alkali metals

The resemblance stops as soon as numbers are put on it. Hydrogen holds its electron far more tightly than any alkali metal.

Figure 3: Hydrogen needs about

2.5

times more energy to lose its electron than lithium does (

1312

against

520 kJ mo l^{- 1}

), which is why it cannot be a true Group 1 member.

Ionisation enthalpy is far too high. Hydrogen needs $1312 kJ mol^{- 1}$ against $520$ for lithium and $419$ for potassium. An element that guards its electron this well cannot be a true Group 1 member.
The $H^{+}$ ion cannot exist on its own. A bare proton has a radius of about $1.5 \times 1 0^{- 3} pm$ , while ordinary ions measure $50$ to $220 pm$ . Its charge density is so large that it immediately attaches to a water molecule, as the figure below shows.
The halides are not alike. Pure $HCl$ is a covalent gas at room temperature, while $NaCl$ is an ionic solid that melts at $1074 K$ . The formulae match; the bonding does not.
Physical state and character. Hydrogen is a light, colourless non-metallic gas. The alkali metals are soft, shiny, solid metals that conduct electricity.

Figure 4: A bare

H^{+}

is so small that its charge density is enormous, so in water it is always found as

H_{3} O^{+}

In water the proton is therefore present as the hydronium ion $H_{3} O^{+}$ , which is itself hydrated and better written $H_{9} O_{4}^{+}$ . Alkali metal ions need no such rescue: they simply sit in a shell of six water molecules, as in $[Na (H_{2} O)_{6}]^{+}$ .

2.3 Resemblance with halogens

Electronic configuration. Like a halogen, hydrogen is one electron short of the nearest noble gas configuration: $H 1 s^{1}$ , $F 1 s^{2} 2 s^{2} 2 p^{5}$ , $Cl 1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{5}$ .
Atomicity. Hydrogen exists as $H_{2}$ , exactly as the halogens exist as $F_{2}$ , $C l_{2}$ and $B r_{2}$ . The atomicity is two in both cases.
Electrochemical behaviour. When molten $LiH$ or $Ca H_{2}$ is electrolysed, hydrogen is set free at the anode, which means it travelled as a negative ion. Halogens behave the same way.
Oxidation state $- 1$ . In metal hydrides hydrogen takes the $- 1$ state, just as a halogen does in a metal halide: $NaH$ and $NaCl$ , $Ca H_{2}$ and $CaC l_{2}$ .
Compounds with alkali metals. Both families give salts of the same formula type: $NaH$ , $LiH$ , $Ca H_{2}$ against $NaCl$ , $LiCl$ , $CaC l_{2}$ .
Compounds with non-metals. Both give simple covalent compounds with carbon, silicon and germanium: $C H_{4}$ , $Si H_{4}$ , $Ge H_{4}$ against $CC l_{4}$ , $SiC l_{4}$ , $GeC l_{4}$ .
Ionisation energy. The value for hydrogen, $1312 kJ mol^{- 1}$ , sits in the halogen band, close to fluorine at $1681$ , chlorine at $1256$ and bromine at $1142$ , and nowhere near the alkali metals, as the ionisation enthalpy chart above shows.

2.4 Differences from halogens

Hydride formation is rare. Halogens form halides with almost every metal. Hydrogen forms hydride ions only with strongly electropositive metals such as sodium and calcium, because adding an electron to hydrogen releases little energy: only $73 kJ mol^{- 1}$ , against $328$ to $349$ for the halogens.
No unshared electron pairs. The $H_{2}$ molecule has one shared pair and nothing else, while $C l_{2}$ carries six unshared pairs. This changes the whole chemistry: halogen molecules can donate lone pairs, $H_{2}$ cannot.
Nature of the oxide. Oxides of the halogens, such as $C l_{2} O_{7}$ and $I_{2} O_{5}$ , are acidic. The oxide of hydrogen is water, which is neutral.
Nature of the compounds with metals. Hydrogen halides ( $HF$ , $HCl$ , $HBr$ , $HI$ ) are low-boiling covalent compounds, whereas alkali metal halides ( $LiF$ , $NaCl$ , $KBr$ , $CsI$ ) are high-melting ionic solids.

Figure 5: Adding an electron releases only

73 kJ mo l^{- 1}

for hydrogen against

328

349

for the halogens. This is why the hydride ion forms only with the most electropositive metals.

The proton,

H^{+}

electron lost: a bare nucleus
radius about

1.5 \times 1 0^{- 3} pm

never free:

H_{3} O^{+}

in water
oxidation state

+ 1

, as in

HCl

The hydride ion,

H^{-}

electron gained:

1 s^{2}

, like He
radius about

208 pm

strong base and reducing agent
oxidation state

- 1

, as in

NaH

2.5 The verdict: a rogue element

Hydrogen resembles the alkali metals in five ways and the halogens in seven, and differs from each of them in four. No single group can hold it, so its place in the periodic table is described as anomalous and hydrogen is often called a rogue element.

Exam Trick

Decide what hydrogen is by looking at its partner. With a non-metal it plays the metal and takes $+ 1$ , as in $HCl$ and $H_{2} O$ . With a metal it plays the halogen and takes $- 1$ , as in $NaH$ and $Ca H_{2}$ . One question, one rule, every time.

Key idea

On paper (

1 s^{1}

) hydrogen looks like Group 1; in its numbers (ionisation enthalpy, diatomic molecule, the

- 1

state) it looks like Group 17. It fits neither, so it is kept on its own.

Quick Recall: tap to check

Why can

H^{+}

never exist free in water?

It is a bare proton about

1.5 \times 1 0^{- 3} pm

across. Its charge density is so high that it bonds to a water molecule at once, giving

H_{3} O^{+}

In which compound is hydrogen

- 1

HCl

NaH

NaH

. Sodium is less electronegative than hydrogen, so hydrogen takes the electron and becomes the hydride ion.

Which number puts hydrogen next to the halogens?

Its ionisation enthalpy,

1312 kJ mol^{- 1}

: close to chlorine (

1256

), far above lithium (

520

3. Isotopes of Hydrogen

Mass spectrograph work showed that hydrogen occurs as three isotopes: protium, deuterium and tritium. All three have one proton and one electron, so their chemistry is the same. They differ only in the number of neutrons, which is why hydrogen is the one element whose isotopes are given separate names and symbols.

Figure 6: All three isotopes carry one proton and one electron, so their chemistry is identical; only the number of neutrons, and therefore the mass, changes.

Property	Protium	Deuterium	Tritium
Symbol	$_{1}^{1} H$	$_{1}^{2} H$ or D	$_{1}^{3} H$ or T
Protons	$1$	$1$	$1$
Neutrons	$0$	$1$	$2$
Mass number	$1$	$2$	$3$
Natural abundance	$99.985%$	$0.0156%$	1 atom in $1 0^{18}$
Radioactive	No	No	Yes, $β^{-}$ emitter
Half-life	Stable	Stable	$12.33$ years

The isotopes also differ in every physical constant that depends on mass. The heavier the molecule, the higher its melting point, boiling point and bond dissociation enthalpy:

Figure 7: The heavier the isotope, the higher its melting point, boiling point and bond enthalpy (

435.9

443.4

446.9 kJ mo l^{- 1}

). Mass changes the physical constants, not the chemistry.

3.1 The three isotopes

Protium, $_{1}^{1} H$ . One proton, no neutron, one electron in the $1 s$ orbital. Ordinary hydrogen is $99.985%$ protium, so nearly every hydrogen atom met in the laboratory is this one.
Deuterium, $_{1}^{2} H$ or D. Also called heavy hydrogen: one proton and one neutron. It makes up $0.0156%$ of natural hydrogen, mostly as HD rather than $D_{2}$ . Its oxide $D_{2} O$ is heavy water.
Tritium, $_{1}^{3} H$ or T. One proton and two neutrons. It is extremely rare, roughly one atom in $1 0^{18}$ hydrogen atoms, and it is radioactive, emitting low energy $β$ particles with a half-life of $12.33$ years.

The decay of tritium gives an isotope of helium:

_{1}^{3} H \to_{2}^{3} He +_{- 1}^{0} e

A neutron inside the nucleus turns into a proton and an electron, so the atomic number rises by one while the mass number stays at $3$ . Half of any sample is gone every $12.33$ years:

Figure 8: Tritium is a

β^{-}

emitter with a half-life of

12.33

years: half is left after

12.33

years, a quarter after

24.66

and an eighth after

36.99

, whatever the starting amount.

Exam Trick

Neutrons = mass number minus one: protium $0$ , deuterium $1$ , tritium $2$ . Only the one with two neutrons is radioactive: T for Ticking, half-life $12.33$ years. Abundance falls just as fast: $99.985%$ , then $0.0156%$ , then 1 atom in $1 0^{18}$ .

3.2 The isotope effect

Because the electronic configuration is identical, the three isotopes give the same products in the same reactions. The mass, however, is different by a factor of two or three, and mass controls how fast a bond can be broken. So the isotopes react at different rates.

Protium reacts with chlorine about $13.4$ times faster than deuterium does.
Ordinary water is electrolysed faster than heavy water, which is exactly how $D_{2} O$ is concentrated from ordinary water.

Isotope effect: the difference in physical properties and reaction rates that arises purely from the difference in mass number between isotopes of the same element.

JEE Advanced

The reason is the zero-point energy of the bond. A heavier atom vibrates more slowly, so a $D - X$ bond sits lower in energy than the matching $H - X$ bond and needs more energy to break. When that bond is broken in the rate-determining step, the reaction with deuterium is measurably slower. For a typical X-H stretch at $2900 cm^{- 1}$ the zero-point energy is about $17.3 kJ mol^{- 1}$ against $12.3$ for X-D, so the X-D bond needs about $5 kJ mol^{- 1}$ more, and the step can be up to about eight times slower at room temperature. This is the kinetic isotope effect, and chemists use it to find out which bond breaks first in a mechanism.

Figure 9: For a

2900 c m^{- 1}

X-H stretch the zero-point energy is

17.3 kJ mo l^{- 1}

; for X-D it is

12.3

. The X-D bond needs about

5.1 kJ mo l^{- 1}

more to break, so at

298 K

it can react up to about eight times more slowly.

3.3 Uses of deuterium and tritium

Tritium in fusion. It is used in thermonuclear devices and in research on fusion as an energy source.
Storing tritium. The gas is usually stored as uranium tritide, $U T_{3}$ . Heating this solid to $673 K$ releases $T_{2}$ again, which is far safer than keeping a radioactive gas under pressure.
Tracers. Both deuterium and tritium label a particular atom in a molecule, so chemists can follow where that atom ends up during a reaction.
Heavy water. $D_{2} O$ slows down neutrons in a nuclear reactor. Its preparation and properties are covered in the concept on water.

Key idea

The three isotopes differ only in neutrons: the same chemistry and the same products, but different masses, so different physical constants and different reaction rates.

Quick Recall: tap to check

How many neutrons does tritium have?

Two: mass number

3

, one proton.

Why do the isotopes give the same products?

They have the same electronic configuration,

1 s^{1}

; only the mass differs.

How much tritium is left after two half-lives?

One quarter, after

24.66

years.

4. Oxidation State Flowchart and Mind Map

Most questions about hydrogen in a compound come down to one decision: is its partner more or less electronegative than hydrogen? The flowchart makes that decision; the mind map after it puts the whole page on one screen.

Figure 10: Flowchart: one comparison of electronegativity decides whether hydrogen plays the metal (

+ 1

) or the halogen (

- 1

Figure 11: Mind map: the whole page on one screen. Every branch goes back to one fact, the single

1 s

electron.

5. Solved Examples

Solved Example 1

The decay product of tritium is
(A)

_{1}^{1} H

(B)

_{1}^{2} H

(C)

_{2}^{3} He

(D)

_{2}^{4} He

Solution:

Answer: (C). Tritium is a $β$ emitter. A neutron in its nucleus turns into a proton and an electron, so the atomic number rises by one while the mass number stays at $3$ .

_{1}^{3} H \to_{2}^{3} He +_{- 1}^{0} e

Solved Example 2

Which property of hydrogen does not match the alkali metals?
(A) one electron in the valence shell
(B) an oxidation state of

+ 1

Solution:

Answer: (C). The ionisation enthalpy of hydrogen is $1312 kJ mol^{- 1}$ , about two and a half times that of lithium at $520 kJ mol^{- 1}$ and three times that of potassium at $419 kJ mol^{- 1}$ . The other three properties are genuine resemblances.

Solved Example 3

Natural hydrogen contains

0.0156%

deuterium by number of atoms. How many deuterium atoms are present in one mole of naturally occurring hydrogen atoms?

Solution:

One mole of atoms contains $6.022 \times 1 0^{23}$ atoms, and the fraction that is deuterium is $0.0156/100 = 1.56 \times 1 0^{- 4}$ .

N_{D} = 6.022 \times 1 0^{23} \times 1.56 \times 1 0^{- 4} = 9.39 \times 1 0^{19}

Answer: about $9.4 \times 1 0^{19}$ deuterium atoms, which is still a huge number even though the percentage looks tiny.

Solved Example 4

Assign the oxidation number of hydrogen in

NaH

H_{2} O

H_{2}

Ca H_{2}

and

HCl

Solution:

Hydrogen takes $- 1$ when the other element is less electronegative than it, and $+ 1$ when the other element is more electronegative.

$NaH$ : $- 1$ (sodium is a metal)
$H_{2} O$ : $+ 1$ (oxygen is more electronegative)
$H_{2}$ : $0$ (both atoms are identical)
$Ca H_{2}$ : $- 1$ (calcium is a metal)
$HCl$ : $+ 1$ (chlorine is more electronegative)

Answer: $- 1, + 1, 0, - 1, + 1$ in that order.

Solved Example 5

Why does the

H^{+}

ion never exist freely in aqueous solution?

Solution:

$H^{+}$ is a bare proton: removing the only electron leaves nothing but the nucleus. Its radius is about $1.5 \times 1 0^{- 3} pm$ , while normal ions measure $50$ to $220 pm$ , so the charge is packed into a volume roughly $1 0^{14}$ times smaller.

Such an intense positive field attracts the lone pair of a water molecule at once:

H^{+} + H_{2} O H_{3} O^{+}

Answer: the proton is captured by water and exists as the hydronium ion $H_{3} O^{+}$ , itself hydrated further as $H_{9} O_{4}^{+}$ . Written as $H^{+}$ only for convenience.

Solved Example 6

Ordinary water is decomposed by electrolysis faster than heavy water. Name and explain the effect responsible.

Solution:

This is the isotope effect. Protium and deuterium have the same electronic configuration, so the products of electrolysis are the same, but deuterium is twice as heavy. The $O - D$ bond is therefore harder to break than the $O - H$ bond and $D_{2} O$ reacts more slowly.

Answer: the isotope effect. It is put to practical use: the residue left after prolonged electrolysis is rich in $D_{2} O$ , which is the standard way heavy water is prepared.

Solved Example 7

A sealed sample contains

8.0 mg

of tritium. How much tritium is left after

37

years? (

t_{1/2} = 12.33

years)

Solution:

The number of half-lives is $37/12.33 = 3.0$ . Each half-life halves the amount:

m = 8.0 \times (\frac{1}{2})^{3} = 1.0 mg

Answer: about $1.0 mg$ ; the other $7.0 mg$ has become helium-3.

Solved Example 8

Which of the following has the largest radius?
(A)

H

(B)

H^{+}

(C)

H^{-}

(D)

He

Solution:

Answer: (C). The hydride ion has one proton holding two electrons. The repulsion between the electrons and the weak pull of a single proton spread the cloud out to about $208 pm$ . The H atom (covalent radius $37 pm$ ) and He are far smaller, and $H^{+}$ , a bare proton, is smallest of all.

Practice Questions

Which of the following statements about protium, deuterium and tritium is not true?
(A) They are isotopes of each other
(B) They have similar electronic configurations
(C) They occur in nature in the ratio $1 : 2 : 3$
(D) Their atomic masses are in the ratio $1 : 2 : 3$ Answer: (C). The natural abundances are $99.985%$ , $0.0156%$ and only 1 atom in $1 0^{18}$ , nothing like $1 : 2 : 3$ . The mass numbers, however, really are $1 : 2 : 3$ .
Why is hydrogen shown separately, above the main body of the table, in many modern periodic tables?Answer: Because its properties match Group 1 and Group 17 partly but neither fully, so placing it inside either group would be misleading.
In which respect does hydrogen resemble the elements of group 14 rather than group 1 or group 17?Answer: Its valence shell is exactly half filled, so like carbon it prefers to share electrons and form covalent bonds.
Tritium occurs to the extent of about one atom in $1 0^{18}$ hydrogen atoms. Estimate the number of tritium atoms in one mole of hydrogen atoms.Answer: About $6 \times 1 0^{5}$ atoms, from $6.022 \times 1 0^{23} \times 1 0^{- 18}$ .
A metal M gives $MH$ with hydrogen and $MCl$ with chlorine. What does this tell you about the oxidation state of hydrogen in $MH$ ?Answer: It is $- 1$ : hydrogen has taken the place a halogen would take, so it behaves as the hydride ion.
Arrange $H$ , $H^{+}$ and $H^{-}$ in increasing order of size.Answer: $H^{+} < H < H^{-}$ . Removing the electron leaves a bare proton; adding one expands the cloud to about $208 pm$ .
Which has the higher bond dissociation enthalpy, $H_{2}$ or $D_{2}$ ?Answer: $D_{2}$ , $443.4$ against $435.9 kJ mol^{- 1}$ , because its zero-point energy is lower.

Common Mistakes to Avoid

Watch out

Writing hydrogen as single atoms at room temperature. Free H atoms exist only at very high temperature; the ordinary form is dihydrogen, $H_{2}$ .
Treating $H^{+}$ as a normal ion floating in solution. It is a bare proton and is always attached to water as $H_{3} O^{+}$ .
Saying hydrogen is placed in Group 1 because it is a metal. It is a non-metallic gas; only its configuration, $1 s^{1}$ , is metal-like.
Mixing up $H^{+}$ and $H^{-}$ . The proton has lost the electron; the hydride ion has gained one and is a strong base and reducing agent.
Claiming isotopes differ in the number of electrons. They differ only in neutrons, which is why their chemistry is identical and only their rates differ.
Calling deuterium and tritium different elements. They are the same element; D and T are just convenient symbols for $_{1}^{2} H$ and $_{1}^{3} H$ .
Describing tritium as an $α$ emitter. It emits low energy $β$ particles, with a half-life of $12.33$ years.
Assuming the oxide of hydrogen is acidic because halogen oxides are. Water is neutral to litmus, and this is one of the clearest differences from Group 17.

Frequently Asked Questions

Why is the position of hydrogen in the periodic table anomalous?

Hydrogen has one valence electron like an alkali metal, yet it also needs one electron like a halogen. It shows $+ 1$ in $HCl$ and $- 1$ in $NaH$ , its ionisation enthalpy matches the halogens, and its physical properties match neither group. No single position fits every fact.

How does hydrogen resemble the alkali metals?

It has a single valence electron, it loses that electron to give $H^{+}$ , it shows an oxidation state of $+ 1$ , it acts as a reducing agent, and it forms compounds of matching formulae with electronegative elements: $HCl$ beside $NaCl$ , $H_{2} O$ beside $N a_{2} O$ , $H_{2} S$ beside $N a_{2} S$ .

How is hydrogen different from the halogens?

Hydrogen forms hydride ions with only a few very electropositive metals, while halogens form halides with almost all metals. The $H_{2}$ molecule has no lone pairs, its oxide (water) is neutral instead of acidic, and hydrogen halides are low-boiling covalent compounds unlike ionic metal halides.

Why does the hydrogen ion not exist freely in water?

$H^{+}$ is a bare proton about $1.5 \times 1 0^{- 3} pm$ across, against $50$ to $220 pm$ for ordinary ions. Its charge density is enormous, so it immediately bonds to the lone pair of a water molecule and exists as the hydronium ion $H_{3} O^{+}$ , itself hydrated as $H_{9} O_{4}^{+}$ .

What are the three isotopes of hydrogen?

Protium has no neutron and makes up $99.985%$ of natural hydrogen. Deuterium, written D, has one neutron and accounts for $0.0156%$ . Tritium, written T, has two neutrons and is only about one atom in $1 0^{18}$ . Only tritium is radioactive, emitting $β$ particles with a half-life of $12.33$ years.

What is the isotope effect in hydrogen?

Isotopes have identical electronic configurations, so they give the same products, but different masses change how fast bonds break. Protium reacts with chlorine about $13.4$ times faster than deuterium, and ordinary water is electrolysed faster than heavy water. That rate gap is used to concentrate $D_{2} O$ .

Is the Hydrogen chapter part of the NEET syllabus?

No. NMC removed the Hydrogen chapter when it rationalised the NEET syllabus, and the chapter has also left the NCERT Class 11 book. The ideas are still worth knowing, because hydrogen bonding, oxidation numbers and the hydride ion reappear in chapters that NEET does test.

Is hydrogen asked in JEE Main or JEE Advanced?

Not in JEE Main, where NTA has removed the Hydrogen chapter. It is in JEE Advanced: the official syllabus lists the position of hydrogen in the periodic table, its isotopes, preparation and properties, hydrides, water, heavy water, hydrogen peroxide and hydrogen as a fuel, so everything on this page can be asked there.

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