Atoms and Molecules From invisible particles to measurable realities.

What is an Atom?

Atom

The fundamental unit of an element. It is the smallest particle that retains all the element’s chemical properties.

What is a Molecule?

Molecule

A molecule is two or more atoms joined by chemical bonds, acting as one independent unit.

Atom vs. Molecule

Atom

Fundamental unit of an element.
Composed of protons, neutrons, and electrons.
Indivisible by chemical reactions.
Exists alone or joins others to form molecules.

Molecule

Two or more atoms chemically bonded.
Smallest unit of a compound or elemental gas.
Has fixed composition and atom ratio.
Acts as an independent particle in reactions.

Key Similarities

Both are electrically neutral overall.
Obey the laws of chemical combination.
Together make up all matter around us.

Atomicity

Atomicity

Atomicity is the number of atoms present in one molecule of an element or compound.

Key Classifications:

  • Monoatomic: single atom molecules – He, Ne
  • Diatomic: two atoms per molecule – O₂, N₂
  • Polyatomic: three or more atoms – O₃ (tri-atomic), P₄ (tetra-atomic)

Example:

He is monoatomic, O₂ is diatomic, P₄ is polyatomic.

Law of Conservation of Mass

Mass is Conserved

Mass can neither be created nor destroyed during a chemical reaction; reactant mass equals product mass.

Law of Constant Proportions

Proust’s Law

Any chemical compound has its elements in a fixed mass ratio, no matter the sample’s origin or amount.

Also called Proust’s Law (1799).

Source: NCERT, Grade 10 Science

Conservation of Mass: Experimental Illustration

Sealed flask before and after reaction

Sealed flask weighed before and after mixing the solutions.

How the Closed-Flask Test Works

A sealed flask holds separate NaCl and AgNO₃ solutions; its total mass is recorded before mixing.

After shaking, AgCl precipitate forms; the flask’s mass remains unchanged, proving mass is conserved.

Key Points:

  • Closed system prevents matter escape.
  • Initial mass equals final mass.
  • Confirms the law of conservation of mass.

Match the Element Symbols

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Drag each chemical symbol to its correct element name to master common periodic table symbols.

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Draggable Items

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Drop Zones

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Tip:

Symbols are one or two letters; the first letter is always uppercase.

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Writing Chemical Formulae

Use valency rules to write accurate chemical formulae.

1

Write element symbols

Place the metal symbol first and the non-metal second.

2

Note & cross valencies

Write each valency above its symbol, then criss-cross the numbers to the opposite element.

3

Simplify subscripts

Divide crossed numbers by their common factor to get the simplest whole-number ratio.

4

Write final formula

Write symbols with simplified subscripts; omit subscript 1 to complete the chemical formula.

Pro Tip:

Ensure total positive and negative charges balance—this confirms the formula obeys valency rules.

Calculating Molecular Mass

\[ M_{\text{H}_2\text{O}} = (2 \times 1\,\text{u}) + (1 \times 16\,\text{u}) = 18\,\text{u} \]

Variable Definitions

M Molecular mass of the compound (u)
n Number of atoms of each element
A Atomic mass of the element (u)
u Unified atomic mass unit

Applications

Stoichiometry

Compute reactant and product amounts in chemical equations.

Mass–Mole Conversions

Translate grams of a substance into moles using its molecular mass.

Purity Check

Compare measured mass with theoretical value to assess sample purity.

The Mole Concept

Mole

One mole contains exactly 6.022 × 10²³ entities, called the Avogadro number. It connects microscopic particles to measurable mass in chemistry.

Unit symbol: mol — essential for converting grams to particles and back.

Mole-Mass-Number Conversions

Use Avogadro’s number and molar mass to switch between mass, moles, and particles.

1

Identify Given Mass

Example: Water sample has 36 g. Note formula H₂O and molar mass 18 g mol⁻¹.

2

Convert Mass → Moles

Moles = mass ÷ molar mass = \(36\,\text{g} \div 18\,\text{g mol}^{-1} = 2\ \text{mol}\).

3

Convert Moles → Particles

Molecules = moles × \(6.022\times10^{23}\) = \(2 \times 6.022\times10^{23} = 1.2044\times10^{24}\).

Pro Tip:

Keep units visible at every step to avoid conversion errors.

Key Takeaways

Atoms are the smallest units of matter and join to form molecules.

Laws of chemical combination fix the ratio in which atoms unite.

The mole links atomic scale to lab scale: \(6.022 \times 10^{23}\) particles per mole.

minimalistic atom network graphic

Thank You!

We hope you found this lesson informative and engaging.