Atoms and molecules

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Lesson notes

What are Atoms and Molecules?

An **atom** is the smallest particle of an element that still has the properties of that element.
A **molecule** is a group of two or more atoms held together by **chemical bonds**.
Molecules can be **homonuclear** (same element, e.g., O₂) or **heteronuclear** (different elements, e.g., H₂O).
Noble gases like helium (He) exist as single atoms and are sometimes considered molecules in gas theory.

Atomic Structure

Atoms consist of a **nucleus** (protons and neutrons) surrounded by **electrons** in shells.
**Protons** have a positive charge, **neutrons** have no charge, and **electrons** have a negative charge.
The number of protons determines the **element** (e.g., 6 protons = carbon).
Electrons are arranged in **shells**; the first shell holds up to 2 electrons, the second up to 8.

Chemical Bonds

**Chemical bonds** are attractive forces that hold atoms together in a molecule.
**Covalent bonds** form when atoms share electrons (e.g., H₂O).
**Ionic bonds** form when atoms transfer electrons (e.g., NaCl).
Molecules are neutral; **polyatomic ions** are charged groups of atoms.

Representing Molecules

**Chemical formulas** show the types and numbers of atoms in a molecule (e.g., H₂O means 2 H and 1 O).
**Ball-and-stick models** show atoms as spheres and bonds as sticks.
**Space-filling models** show the relative sizes and arrangement of atoms.
The subscript number in a formula indicates how many atoms of that element are present.

States of Matter and the Particle Model

In **solids**, particles are tightly packed in a fixed arrangement and vibrate in place.
In **liquids**, particles are close but can slide past each other.
In **gases**, particles are far apart and move freely at high speeds.
The **particle model** explains properties like shape, volume, and compressibility.

History of the Molecule Concept

Ancient Greeks like Democritus proposed that matter is made of atoms and void.
Robert Boyle (1661) suggested matter is composed of clusters of particles called **corpuscles**.
Amedeo Avogadro (1811) coined the term **molecule** and distinguished atoms from molecules.
Jean Perrin (1926) proved the existence of molecules by calculating Avogadro's number.

Bohr model of an oxygen atom: 8 protons and 8 neutrons in the nucleus, with 2 electrons in the first shell and 6 in the second.

Particle arrangement in solid, liquid, and gas states.

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Practice questions

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1.What is a molecule?
Easy
- AA group of two or more atoms held together by chemical bonds
- BA single atom of any element
- CA mixture of different substances
- DA type of chemical bond
2.Which of the following is an example of a homonuclear molecule?
Easy
- AO2
- BH2O
- CCO2
- DNaCl
3.Water (H2O) is an example of a:
Easy
- Aheteronuclear molecule
- Bhomonuclear molecule
- Cnoble gas
- Dionic compound
4.Which of the following is NOT a molecule?
Medium
- AA sodium atom (Na)
- BAn oxygen molecule (O2)
- CA water molecule (H2O)
- DA carbon dioxide molecule (CO2)
5.The word 'molecule' comes from the Latin word 'moles', which means:
Medium
- Aa small unit of mass
- Ba chemical bond
- Ca type of atom
- Da particle of energy
6.Which scientist is credited with coining the word 'molecule'?
Medium
- AAmedeo Avogadro
- BRobert Boyle
- CLinus Pauling
- DJean Perrin
7.According to the kinetic theory of gases, which of the following is considered a molecule?
Hard
- AA single atom of helium
- BA pair of oxygen atoms bonded together
- CBoth A and B
- DNeither A nor B
8.Which of the following is typically NOT considered a molecule?
Hard
- AA sodium chloride (NaCl) unit in a crystal
- BA water molecule (H2O)
- CAn oxygen molecule (O2)
- DA carbon dioxide molecule (CO2)

Revision notes

## Atoms and Molecules ### Key Ideas - **Atoms** are the basic building blocks of matter. They combine to form **molecules**. - A **molecule** is a group of two or more atoms held together by **chemical bonds**. - Molecules can be **homonuclear** (same element, e.g., O₂) or **heteronuclear** (different elements, e.g., H₂O). - Chemical formulas (like H₂O) show the types and numbers of atoms in a molecule. - Models (ball-and-stick, space-filling) help visualize molecular structure. ### Important Definitions - **Molecule**: "a group of two or more atoms that are held together by attractive forces known as chemical bonds" - **Homonuclear molecule**: consists of atoms of one element (e.g., O₂, N₂) - **Heteronuclear molecule**: consists of atoms of more than one element (e.g., H₂O, CO₂) - **Chemical bond**: the force that holds atoms together in a molecule ### Examples - Water (H₂O): 2 hydrogen atoms + 1 oxygen atom - Oxygen gas (O₂): 2 oxygen atoms - Carbon dioxide (CO₂): 1 carbon atom + 2 oxygen atoms ### History & Scientists - **Robert Boyle** (1661): proposed matter is made of clusters of particles ("corpuscles") - **Amedeo Avogadro** (1811): coined "molecule" and stated gases are made of molecules, not just atoms - **Jean Perrin** (1926): proved existence of molecules by calculating Avogadro's constant - **Linus Pauling**: applied quantum mechanics to chemical bonding ### Key Points for Exam - Molecules are the smallest units of a compound that retain its properties. - Not all substances are made of discrete molecules; some (like salts) form extended networks. - Noble gases (He, Ne, Ar) exist as single atoms, but are sometimes called "molecules" in gas theory. - Chemical formulas show the number of each atom: subscript indicates count (e.g., H₂ means 2 hydrogen atoms).

Atom: The smallest unit of an element that retains the chemical properties of that element.
Molecule: A group of two or more atoms held together by chemical bonds.
Chemical bond: An attractive force that holds atoms together in a molecule.
Homonuclear molecule: A molecule consisting of atoms of the same chemical element (e.g., O₂).
Heteronuclear molecule: A molecule consisting of atoms of two or more different elements (e.g., H₂O).
Chemical formula: A notation using element symbols and subscripts to show the types and numbers of atoms in a molecule (e.g., H₂O).

Flashcards

What is a molecule?

A molecule is a group of two or more atoms held together by chemical bonds.

What are the two types of molecules based on element composition?

Homonuclear molecules (same element, e.g., O₂) and heteronuclear molecules (different elements, e.g., H₂O).

Give an example of a homonuclear molecule.

Oxygen molecule (O₂), consisting of two oxygen atoms.

Give an example of a heteronuclear molecule.

Water (H₂O), consisting of two hydrogen atoms and one oxygen atom.

What holds atoms together in a molecule?

Chemical bonds, which are attractive forces between atoms.

Are noble gases considered molecules in the kinetic theory of gases?

Yes, in kinetic theory, any gaseous particle (including single atoms like helium) is called a molecule.

What is a chemical formula?

A chemical formula uses element symbols and subscripts to show the number of atoms of each element in a molecule, e.g., H₂O.

What does the subscript in a chemical formula indicate?

The subscript indicates the number of atoms of that element in one molecule. For example, in H₂O, the '2' means two hydrogen atoms.

What is a ball-and-stick model?

A ball-and-stick model is a 3D representation of a molecule where balls represent atoms and sticks represent chemical bonds.

What is the difference between an atom and a molecule?

An atom is the smallest unit of an element; a molecule is two or more atoms bonded together.

Can a molecule consist of only one atom?

Typically no, but in the kinetic theory of gases, single atoms of noble gases are considered molecules.

What are the four 'elements' imagined by Empedocles?

Fire, earth, air, and water.

Who first hypothesized that matter is composed of clusters of particles?

Robert Boyle in 1661, in his work 'The Sceptical Chymist'.

Who coined the word 'molecule'?

Amedeo Avogadro in 1811.

What did Avogadro propose about gas particles?

He proposed that the smallest particles of gases are not necessarily single atoms but are made up of atoms united to form molecules.

Who proved the existence of molecules conclusively?

Jean Perrin in 1926, by calculating Avogadro's constant using three methods.

What is the origin of the word 'molecule'?

From French 'molécule' (1678), from Neo-Latin 'molecula', diminutive of Latin 'moles' meaning 'mass'.

What is a polyatomic ion?

A polyatomic ion is a charged molecule (ion) composed of two or more atoms covalently bonded, e.g., NH₄⁺.

Are ionic compounds like NaCl considered molecules?

Typically no; they form extended crystal lattices, not discrete molecules.

What is the difference between a molecule and a compound?

A molecule is two or more atoms bonded; a compound is a substance with two or more different elements. All compounds are molecules, but not all molecules are compounds (e.g., O₂ is a molecule but not a compound).

What does the term 'homonuclear' mean?

Homonuclear means consisting of atoms of the same chemical element.

What does the term 'heteronuclear' mean?

Heteronuclear means consisting of atoms of different chemical elements.

What is the modern definition of a molecule?

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds.

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