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We've established that atoms are the fundamental, tiny building blocks of all matter. But how do these atoms actually exist in the world around us? Interestingly, atoms of most elements are not able to exist independently. Instead, they form larger, more stable entities called molecules and ions. These molecules or ions then aggregate in vast numbers to form the matter that we can see, feel, or touch.

What is a Molecule?

A molecule is generally defined as a group of two or more atoms that are chemically bonded together, held tightly by attractive forces. A molecule is the smallest particle of an element or a compound that is capable of independent existence under ordinary conditions and exhibits all the properties of that substance.

Molecules can be formed in two main ways:

1. Molecules of Elements:

These molecules are constituted by the same type of atoms.

• Some elements, primarily noble gases like argon (Ar) and helium (He), exist as single, isolated atoms. They are called monoatomic.
• However, many non-metals form molecules consisting of two or more atoms. For example, a molecule of oxygen consists of two oxygen atoms and is therefore known as a diatomic molecule, represented as O₂. If three oxygen atoms unite, they form ozone (O₃).
• The number of atoms constituting a molecule is known as its atomicity. For example, oxygen and hydrogen are diatomic; phosphorus is tetra-atomic (P₄); and sulphur is poly-atomic (S₈).
• Metals and some other elements, such as carbon, do not have a simple molecular structure but instead consist of a very large and indefinite number of atoms bonded together in a vast network.

2. Molecules of Compounds:

Atoms of different elements join together in definite proportions to form molecules of compounds. These proportions are fixed by mass, as seen in the Law of Constant Proportions.

• Water (H₂O): Composed of hydrogen and oxygen, with a mass ratio of 1:8. This means for every 1 unit of hydrogen mass, there are 8 units of oxygen mass. By number of atoms, water has 2 hydrogen atoms for every 1 oxygen atom.
• Ammonia (NH₃): Composed of nitrogen and hydrogen, with a mass ratio of 14:3. By number of atoms, ammonia has 1 nitrogen atom for every 3 hydrogen atoms.
• Carbon dioxide (CO₂): Composed of carbon and oxygen, with a mass ratio of 3:8. By number of atoms, carbon dioxide has 1 carbon atom for every 2 oxygen atoms.

What is an Ion?

When compounds are composed of metals and non-metals, their constituent particles are often charged species known as ions. Ions are formed when atoms (or groups of atoms) gain or lose electrons, resulting in a net electrical charge.

• A negatively charged ion is called an anion.
• A positively charged ion is called a cation.

For example, sodium chloride (NaCl), common table salt, consists of positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl⁻).

Sometimes, a group of atoms carries a net charge. These are known as polyatomic ions. Examples include ammonium (NH₄⁺), hydroxide (OH⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), sulphite (SO₃²⁻), sulphate (SO₄²⁻), and phosphate (PO₄³⁻).

Both molecules and ions are crucial concepts in chemistry, as they represent how atoms achieve stability and interact to form the vast diversity of matter we observe.

Question for You: If a molecule of phosphorus is tetra-atomic, how many atoms of phosphorus are present in one molecule?