Atoms and molecules

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Atoms and Sub-atomic Particles

Atoms are the basic building blocks of all matter.
Atoms are made of three sub-atomic particles: protons, neutrons, and electrons.
Protons are positively charged and are found in the nucleus.
Neutrons have no charge (neutral) and are also in the nucleus.
Electrons are negatively charged, much smaller, and move around the nucleus in an electron cloud.
The number of protons determines the element; in a neutral atom, protons equal electrons.

A model of an atom, showing the nucleus (protons and neutrons) at the centre and electrons orbiting around it.

Elements and the Periodic Table

An element is a pure substance made of only one kind of atom.
There are more than 100 known elements, each with a unique name, symbol, and atomic number.
Elements are arranged in the Periodic Table in order of increasing atomic number.
Atoms of different elements have different numbers of protons and different masses.
Metals are found on the left and non-metals on the right of the Periodic Table; semi-metals are in between.

A collection of identical atoms representing an element

Molecules

A molecule forms when two or more atoms link together.
Molecules can be made of the same type of atoms (e.g., O2) or different types (e.g., H2O).
A molecule always has the same types and proportions of atoms; for example, water is always H2O.
Some elements exist as diatomic molecules (two atoms), such as H2, N2, O2, F2, Cl2, Br2, and I2.
Molecules of compounds contain at least two different kinds of atoms.

Some elements exist as diatomic molecules.

Compounds and Chemical Formulas

A compound is a pure substance made of two or more different elements chemically combined.
Compounds have a fixed ratio of atoms; for example, water has 2 hydrogen atoms to 1 oxygen atom.
Each compound has a unique chemical formula that shows the types and numbers of atoms (e.g., CO2).
Subscripts in a formula indicate the number of atoms of each element; '1' is not written.
Compounds form during chemical reactions when atoms rearrange and new bonds form.

A compound consists of two or more kinds of atoms combined in a fixed ratio.

Naming Compounds

The IUPAC system provides unique names for compounds.
For compounds of a metal and non-metal, the metal comes first, and the non-metal ends in -ide (e.g., sodium chloride).
For compounds of only non-metals, the element further left on the Periodic Table comes first, and the second ends in -ide.
Prefixes like mono-, di-, tri-, tetra- indicate the number of atoms when needed (e.g., carbon dioxide).
The prefix mono- is usually omitted from the first element (e.g., CO is carbon monoxide, not monocarbon monoxide).

Mixtures vs. Compounds

A mixture contains two or more substances that are not chemically combined.
Mixtures can be separated by physical means, such as filtering or evaporation.
Compounds are chemically combined and can only be separated by chemical reactions.
In a mixture, the substances retain their individual properties; in a compound, the properties are different from the constituent elements.

A mixture of hydrogen and oxygen molecules — the two elements are not chemically combined.

Models of the Atom

Scientific models help us understand atoms, which are too small to see.
The model shows protons and neutrons in the nucleus and electrons in a cloud around it.
The nucleus is very dense and makes up almost all of the atom's mass.
Atoms are mostly empty space; the nucleus is about 1 trillionth of the atom's size.
The model is not to scale but effectively shows the parts and their locations.

The scientific model of the atom has changed over time, from a solid sphere to the modern electron-cloud model.

Properties of Molecules

The properties of a molecule are different from the properties of the atoms that form it.
For example, sodium (explosive) and chlorine (poisonous) combine to form harmless table salt (NaCl).
A pure substance has the same composition throughout; all water molecules are identical.
Different combinations of atoms give molecules unique properties (e.g., water vs. hydrogen peroxide).

Sodium and chlorine combine to form sodium chloride — safe, unreactive table salt.

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1.What are the three sub-atomic particles that make up an atom?
Easy
- Aprotons, neutrons, electrons
- Bprotons, electrons, molecules
- Cneutrons, electrons, atoms
- Dprotons, neutrons, nuclei
2.All elements consist of molecules.
Easy
True or false?
3.Which of the following is a diatomic molecule?
Easy
- AO2
- BHe
- CH2O
- DCO2
4.What does the formula H2O tell us about a water molecule?
Easy
- AIt has two hydrogen atoms and one oxygen atom.
- BIt has one hydrogen atom and two oxygen atoms.
- CIt has two hydrogen atoms and two oxygen atoms.
- DIt has one hydrogen atom and one oxygen atom.
5.Which of the following is a compound?
Medium
- AWater (H2O)
- BOxygen (O2)
- CHydrogen (H2)
- DGold (Au)
6.The properties of a molecule are the same as the properties of the atoms that form it.
Medium
True or false?
7.Match each sub-atomic particle with its charge.
Easy
- Proton
- Neutron
- Electron
- Positive
- Neutral
- Negative
8.Which of the following are elements that exist as diatomic molecules? (Select all that apply)
Easy
- AHydrogen (H2)
- BOxygen (O2)
- CHelium (He)
- DNitrogen (N2)
- ECarbon (C)

Plus de questions

9.Arrange the following in order of increasing size (smallest first).
Medium
- Electron
- Atom
- Molecule
- Nucleus
10.The diagram shows a sodium chloride crystal lattice. Which of the following correctly describes the arrangement of atoms in it?
Medium
- ASodium and chlorine atoms are in a fixed 1:1 ratio.
- BSodium and chlorine atoms are in a random ratio.
- CSodium and chlorine atoms are in a 2:1 ratio.
- DSodium and chlorine atoms are in a 1:2 ratio.
11.Match each chemical formula with the correct name.
Medium
- CO
- CO2
- SO2
- SO3
- Carbon monoxide
- Carbon dioxide
- Sulfur dioxide
- Sulfur trioxide
12.Which of the following statements about molecules are true? (Select all that apply)
Medium
- AA molecule can be made of atoms of the same element.
- BA molecule can be made of atoms of different elements.
- CThe properties of a molecule are the same as the properties of its atoms.
- DA molecule always has the same kind and proportion of atoms.
13.In a chemical reaction, atoms are rearranged to form new molecules.
Easy
True or false?
14.The formula for hydrogen peroxide is H2O2. How does this differ from water (H2O)?
Medium
- AIt has one more oxygen atom per molecule.
- BIt has one more hydrogen atom per molecule.
- CIt has the same ratio of hydrogen to oxygen.
- DIt is a mixture of hydrogen and oxygen.
15.Arrange the following steps in the correct order to name a compound containing only non-metals.
Medium
- Identify the elements in the formula.
- Determine which element is further left on the Periodic Table.
- Write the first element's name.
- Add the second element's name with the -ide suffix and appropriate prefix.
16.Which of the following are examples of compounds? (Select all that apply)
Medium
- AWater (H2O)
- BOxygen (O2)
- CCarbon dioxide (CO2)
- DSodium chloride (NaCl)
- EHelium (He)
17.The number of protons in an atom determines which element it is.
Medium
True or false?
18.The diagram shows five particle models, numbered 1 to 5 from top to bottom. Match each one to what it represents.
Medium
- Model 1
- Model 2
- Model 3
- Model 4
- Model 5
- Single atom
- Molecule (2 atoms)
- Molecule (4 atoms)
- Single atom
- Molecule (5 atoms)
19.Which of the following statements about the model of the atom are accurate? (Select all that apply)
Medium
- AThe nucleus is very small compared to the whole atom.
- BElectrons move in fixed orbits around the nucleus.
- CThe nucleus contains most of the atom's mass.
- DThe atom is mostly empty space.
20.Which of the following is the correct name for the compound with the formula N2O4?
Medium
- ADinitrogen tetroxide
- BNitrogen oxide
- CNitrogen dioxide
- DDinitrogen oxide
21.Match each sub-atomic particle with its relative mass (proton = 1).
Medium
- Proton
- Neutron
- Electron
- 1
- 1
- 1/2000
22.Which of the following are examples of pure substances? (Select all that apply)
Medium
- AWater
- BSalt
- CAir
- DIron
- ESugar
23.Atoms of different elements have different masses.
Easy
True or false?
24.Arrange the following in order of increasing number of oxygen atoms per molecule.
Medium
- CO2
- H2O
- N2O4
- SO3
25.The prefix 'mono-' is used to indicate one atom in a compound name, and it is always used for the first element.
Medium
True or false?
26.The table below shows the number of atoms of each element in one molecule of four substances. Which substance is a compound? | Substance | Atoms present | |---|---| | A | 2 hydrogen atoms | | B | 1 carbon atom, 2 oxygen atoms | | C | 2 oxygen atoms | | D | 1 helium atom |
Medium
- ASubstance A
- BSubstance B
- CSubstance C
- DSubstance D
27.Which of the following is the correct formula for a molecule containing three hydrogen atoms and one nitrogen atom?
Medium
- AH3N
- BNH3
- C3HN
- DH3N3
28.A model of a molecule has one central carbon atom bonded to four hydrogen atoms. What is the formula of this molecule?
Medium
- ACH4
- BC4H
- CC1H4
- DH4C
29.The chemical formula of a compound tells you the ratio of atoms of each element in the compound.
Easy
True or false?
30.Match each chemical formula with the correct number of atoms in one molecule.
Medium
- CO2
- NH3
- H2SO4
- 3 atoms
- 4 atoms
- 7 atoms
31.A student builds a model of a molecule using play dough. The model has a central atom with two other atoms attached to it, and the atoms are arranged in a straight line. Which of the following molecules could this model represent?
Medium
- ACO2
- BH2O
- CNH3
- DCH4
32.Which of the following statements about the chemical reaction CO + H2O → CO2 + H2 are correct? (Select all that apply)
Medium
- AThe reactants are CO and H2O.
- BThe products are CO2 and H2.
- CThe number of atoms of each element is the same on both sides of the equation.
- DThe reaction shows that atoms are created during a chemical reaction.
- EThe reaction shows that atoms are rearranged to form new molecules.
33.Fill in the blanks to complete the sentence.
Easy
The three sub-atomic particles are ____, ____, and ____.
34.What is the term for a molecule consisting of two atoms?
Easy
35.How many atoms are in one molecule of carbon dioxide (CO2)?
Medium
36.Complete the sentence about the composition of a water molecule.
Easy
A water molecule is made of two ____ atoms and one ____ atom.
37.How many total atoms are in one molecule of glucose (C6H12O6)?
Medium
38.Complete the sentence about naming compounds.
Medium
In the IUPAC system, the name of the second element in a compound often ends in the suffix ____.
39.What is the name of the force that holds atoms together in a molecule?
Easy
40.How many atoms are in one molecule of ammonium sulfate ((NH4)2SO4)?
Medium
41.What is the term for the smallest unit of an element that retains its properties?
Easy
42.Explain why the model of the atom is not completely accurate.
Medium
43.Explain why the properties of a compound are different from the properties of the elements that form it. Use sodium chloride as an example.
Hard
44.Describe how a mixture of hydrogen and oxygen molecules is different from a compound like water.
Hard
45.The table below shows the number of atoms of each element in one molecule of four substances. | Substance | Atoms present | |---|---| | A | 2 hydrogen atoms | | B | 1 carbon atom, 2 oxygen atoms | | C | 2 oxygen atoms | | D | 1 helium atom | Which substance is a compound? Explain your answer.
Hard
46.Propane (a fuel gas used in barbecue tanks) has the chemical formula C3H8. How many different types of atoms make up one molecule of propane?
Medium
- A8
- B2
- C3
- D11
47.A student draws two molecule diagrams. Diagram 1 shows one carbon atom bonded to four hydrogen atoms (methane, CH4). Diagram 2 shows two carbon atoms bonded together, with hydrogen atoms attached, making a molecule with six hydrogen atoms in total (ethane, C2H6). What do these two diagrams represent?
Medium
- ATwo different elements
- BTwo different compounds
- CTwo different forms of the same mixture
- DTwo different forms of the same compound

Notes de révision

## 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).

Cartes mémo

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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