What is hydrogen?
Hydrogen is one of the chemical elements that exist in nature. An element has one
type of atom, and it cannot be broken down into other substances. Hydrogen exists
naturally as a molecule, consisting of two hydrogen atoms. The chemical formula of
hydrogen is H₂.
With a standard atomic weight of 1.008, hydrogen is the lightest element in the
periodic table. Its monatomic form (H) is the most abundant chemical substance in
the Universe, constituting roughly 75% of all mass. Non-remnant stars are mainly
composed of hydrogen in the plasma state. The most common isotope of hydrogen,
termed protium (name rarely used, symbol 1H), has one proton and no neutrons.
Since hydrogen has one proton, it has the atomic number 1 and it is in group IA.
The Sun also consists of hydrogen for the most part (75%). When two hydrogen
atoms combine their nuclei, they turn into helium. This is what heats up the Sun.
After 4-5 billion years, the Sun will run out of hydrogen and life on Earth will cease to
exist.
Hydrogen atoms were the first to be formed after the Big Bang. Out of all atoms in
the universe, about 89% are hydrogen. On Earth, the amount of hydrogen is much
lower. Most of hydrogen atoms on Earth are in water molecules. In human body,
hydrogen is the third most abundant element. About 10% of all body mass is
hydrogen.
Practice!
Which statements are true?
● In a human that weighs 50kg, there is 5kg of hydrogen atoms.
● 89% of all atoms on Earth are hydrogen.
● Hydrogen atom has a simple structure: it consists of only one proton.
● Hydrogen’s atomic nr is 1 and it is the first element in the periodic table.
● Most of the hydrogen atoms on Earth are in water molecules.
Hydrogen as pure substance
Hydrogen atom is chemically unstable on its own and usually only exists as a
molecule or different compounds. Hydrogen as pure substance exists as a diatomic
molecule (H​2​) that’s molecular mass is approximately 2. Compared with air that’s
average molecular mass is 29, it is so light that gravity cannot hold it in the
atmosphere. Hydrogen will quickly rise higher and gather in the upper layers of the
Earth’s atmosphere.
Practice!
How many times smaller is the molecular mass of hydrogen compared with the
molecular mass of air?
Properties of hydrogen
Hydrogen as a pure substance is colorless, transparent and odorless. Hydrogen as a
pure substance is not very
stable. It can ignite easily and it
burns with a soft blue flame.
High amount of heat energy is
released when hydrogen is
burning, that’s why this reaction
is used in welding and cutting
metals.
Due to the danger of explosion, before igniting hydrogen it is important to check its
purity. Often in experiment demonstrations, a mixture of oxygen and hydrogen is
ignited and there is an explosion. As a result of this reaction, water is formed.
A mixture that contains ⅔ of hydrogen and ⅓ of oxygen is called oxyhydrogen or
knallgas (​ “bang-gas”). Hydrogen is dangerous even starting from low concentration
(5%) so it is very important to be cautious when using it.
Even though hydrogen is in IA group with alkali metals, it doesn’t have metallic
properties. Alkali metals are metals as pure substances and have metallic properties
but hydrogen is a nonmetal and gaseous at its standard state.
Practice!
Mixture of hydrogen and oxygen is explosive when there is at least _____% of
hydrogen in the mixture.
Hydrogen is a ____________________ and ___________________ gas that burns
with a _______________ flame.
What is hydrogen as a pure substance used for?
Additional reading - hydrogen cars
Most cars burn petrol or diesel in their engines. These chemical reactions make
carbon dioxide and water. Carbon dioxide is a greenhouse gas, which means too
much of it can promote global warming. Global warming causes climate change,
resulting in droughts, floods and extreme weather.
Hydrogen cars help to reduce greenhouse gas emissions. They either burn hydrogen
in an engine or they react both hydrogen and oxygen together in a fuel cell. Both
processes produce electricity which powers an electric motor. Hydrogen cars
produce one harmless exhaust gas – water vapour.
Key ideas to remember
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Hydrogen is an element; it exists naturally as a molecule.
Hydrogen atom consists of one proton and one electron in its elemental form.
Each hydrogen molecule is made up of two hydrogen atoms.
*In petrol and diesel cars, the burning of fuel produces carbon dioxide and
water. Extra carbon dioxide in the air promotes global warming.
● *In hydrogen cars hydrogen reacts with oxygen in a fuel cell, making
electricity to run the car. The only waste product of this process is water
vapour.