What Is an Atom? / What Are Atoms Made Of? / Dalton’s Atomic Theory / What Is a Molecule

An atom is defined as the basic building block of all matter. Everything around us—air, water, food, metals, living organisms—is ultimately made up of atoms.

In scientific terms, an atom is the smallest unit of an element that retains the chemical properties of that element. While atoms are extremely small and invisible to the naked eye, they play a central role in determining the structure and behavior of matter.

Atoms do not usually exist independently in nature. Instead, they combine with other atoms to form molecules or ions, which then assemble in vast numbers to create the substances we can see, touch, and use.

Structure of an Atom

Every atom is composed of three fundamental subatomic particles:

• Protons – Positively charged particles located in the nucleus

• Neutrons – Neutral particles located in the nucleus

• Electrons – Negatively charged particles that move around the nucleus

Key Characteristics:

• The nucleus is the dense central core containing protons and neutrons.

• Electrons occupy shells (energy levels) surrounding the nucleus.

• Most of the atom’s mass is concentrated in the nucleus.

• An atom is electrically neutral because it has equal numbers of protons and electrons.

Atomic Number and Atomic Mass

• Atomic Number (Z): The number of protons in the nucleus. It defines the identity of an element.

• Atomic Mass (A): The combined mass of protons and neutrons, expressed in atomic mass units (u).

1 atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom.

Examples of Atomic Masses

Element	Atomic Mass (u)
Hydrogen	1
Carbon	12
Nitrogen	14
Oxygen	16
Sodium	23
Chlorine	35.5

Size of an Atom

Atoms are extraordinarily small. To put this into perspective:

• Millions of atoms placed side by side would form a layer as thin as a sheet of paper.

• Atomic size is measured in nanometers (nm).

• The exact size of an isolated atom cannot be measured directly because electrons do not have fixed positions.

Scientists estimate atomic size using the distance between neighboring atoms in substances.

What Are Atoms Made Of?

• All atoms contain protons and electrons.

• Most atoms also contain neutrons (hydrogen is an exception).

• Protons and neutrons together are called nucleons.

• Atoms are mostly empty space, with a dense nucleus at the center.

Dalton’s Atomic Theory (Salient Features)

John Dalton proposed one of the earliest scientific theories of atoms. Its key ideas include:

1. Matter is made up of extremely small particles called atoms.

2. Atoms cannot be created or destroyed in chemical reactions.

3. All atoms of a given element are identical in mass and properties.

4. Atoms of different elements differ in mass and properties.

5. Atoms combine in simple whole-number ratios to form compounds.

6. Chemical reactions involve the rearrangement of atoms.

Although later discoveries refined this theory, it laid the foundation of modern chemistry.

What Is a Molecule? (Molecule Definition)

A molecule is defined as the smallest unit of a substance that retains the chemical properties of that substance.

In simple terms, a molecule is formed when two or more atoms chemically bond together by sharing or transferring electrons. These bonds are responsible for holding atoms together in a stable arrangement.

Key Differences Between Atoms and Molecules

Atoms	Molecules
Smallest unit of an element	Smallest unit of a compound
May or may not exist independently	Always consists of two or more atoms
Defined by atomic number	Defined by chemical bonding

Examples of Molecules

• H₂O – Water

• O₂ – Oxygen

• O₃ – Ozone

• CO₂ – Carbon dioxide

• C₆H₁₂O₆ – Glucose

• N₂ – Nitrogen

Note: Ionic compounds such as NaCl (table salt) are not molecules; they are ionic lattices.

Forces Between Atoms and Molecules

Atoms bond together due to electrostatic forces arising from electron interactions.

Example: Sodium Chloride (NaCl)

• Sodium loses one electron → becomes Na⁺

• Chlorine gains one electron → becomes Cl⁻

• The oppositely charged ions attract each other through Coulombic forces, forming a strong ionic bond.

These forces are fundamental to the formation of solids, liquids, and gases.

Frequently Asked Questions (FAQs)

Q1. How do atoms form molecules?

Atoms form molecules by sharing or transferring electrons through chemical bonds, primarily involving electrons in the outermost shell.

Q2. Can a molecule consist of a single atom?

No. A molecule must contain two or more atoms chemically bonded together.

Q3. Is oxygen a molecule?

Yes. Oxygen exists naturally as O₂, a diatomic molecule.

Q4. Is salt a molecule?

No. Sodium chloride (NaCl) is an ionic compound, not a molecule.

Q5. What is the basic structure of an atom?

An atom consists of a central nucleus (protons and neutrons) surrounded by electrons in energy levels.

Final Note

Atoms and molecules form the foundation of chemistry and all material science. Understanding these concepts is essential for fields ranging from medicine and engineering to environmental science and technology.

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