Elements, Atoms & Compounds

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Elements, Atoms & Compounds

  • November 7, 2023
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Year 8 Chemistry – Elements, Atoms & Compounds
Year 8 Chemistry ⚛️ 🧪 🧬 🌡️

Elements, Atoms, Compounds & Chemical Formula

Understand what matter is made of, how atoms join together, and how to read and write chemical formulas confidently.

Elements Atoms & Molecules Compounds Chemical Formula

Core Ideas Class Notes

Use these notes while teaching or revising. They cover all the key ideas students need before practising questions.

E What is an Element?
An element is a pure substance made from only one type of atom.
  • Examples: Oxygen (O), Hydrogen (H), Carbon (C), Iron (Fe).
  • Each element has its own symbol on the periodic table.
Pure substance One type of atom Shown by symbol
⚛️ Atoms & Molecules
Atom: the smallest particle of an element that still has that element’s properties.
Molecule: two or more atoms chemically joined together.
  • O₂ – two oxygen atoms (molecule of an element)
  • H₂O – two hydrogen + one oxygen (molecule of a compound)
Atoms = single particles Molecules = 2+ atoms
C What is a Compound?
A compound is a substance made when two or more different elements join together chemically.
  • Water (H₂O) – hydrogen + oxygen
  • Carbon dioxide (CO₂) – carbon + oxygen
  • Compounds have new properties, different from the elements they came from.
Fixed ratio Chemical bond New properties
F Chemical Formula Basics
A chemical formula tells you:
  • Which elements are present.
  • How many atoms of each element.
Examples:
  • H₂O – 2 H atoms, 1 O atom
  • CO₂ – 1 C atom, 2 O atoms
  • NaCl – 1 Na atom, 1 Cl atom
Subscript = number Symbol = element

True / False 10 Questions

Decide if each statement is true or false. Then reveal the answer.

Q1 True / False
An element contains only one type of atom.
Show Answer
True.
Q2 True / False
A compound can be separated by physical methods.
Show Answer
False. It needs a chemical change.
Q3 True / False
Water (H₂O) is a compound.
Show Answer
True.
Q4 True / False
Carbon dioxide contains three carbon atoms.
Show Answer
False. It has one carbon and two oxygen.
Q5 True / False
Atoms and molecules mean the same thing.
Show Answer
False.
Q6 True / False
Oxygen is an element.
Show Answer
True.
Q7 True / False
NaCl contains sodium and chlorine atoms.
Show Answer
True.
Q8 True / False
A molecule must always be a compound.
Show Answer
False. H₂ and O₂ are molecules of elements.
Q9 True / False
Compounds always form by chemical reactions.
Show Answer
True.
Q10 True / False
Molecules contain only one atom.
Show Answer
False. They contain two or more atoms.

Multiple-Choice Questions 10 Questions

Choose the best option for each question, then check your answer.

Q11 MCQ
Which best describes an element?
  • A) Two atoms joined
  • B) One type of atom only
  • C) A mixture
Show Answer
B) One type of atom only.
Q12 MCQ
H₂O has how many hydrogen atoms?
  • A) 1
  • B) 2
  • C) 3
Show Answer
B) 2 hydrogen atoms.
Q13 MCQ
Which is a compound?
  • A) O₂
  • B) Fe
  • C) CO₂
Show Answer
C) CO₂.
Q14 MCQ
Which symbol represents iron?
  • A) Fe
  • B) F
  • C) I
Show Answer
A) Fe.
Q15 MCQ
Which statement is correct?
  • A) Compounds have same properties as elements
  • B) Compounds have new properties
Show Answer
B) They have new properties.
Q16 MCQ
CO₂ contains:
  • A) 1 carbon, 2 oxygen
  • B) 2 carbon, 1 oxygen
  • C) Only oxygen
Show Answer
A) 1 C and 2 O.
Q17 MCQ
The tiny number in a chemical formula is called:
  • A) Superscript
  • B) Coefficient
  • C) Subscript
Show Answer
C) Subscript.
Q18 MCQ
Which is a molecule but NOT a compound?
  • A) CO₂
  • B) H₂
  • C) H₂O
Show Answer
B) H₂ (only hydrogen atoms).
Q19 MCQ
Which is NOT an element?
  • A) Sodium
  • B) Hydrogen
  • C) Water
Show Answer
C) Water is a compound.
Q20 MCQ
Which formula represents a compound made of sodium and chlorine?
  • A) HCl
  • B) NaCl
  • C) CO₂
Show Answer
B) NaCl.

Fill in the Blanks 10 Questions

Type or say the missing word, then click to reveal the correct answer.

Q21 Fill
An element is made of only one type of ______.
Show Answer
Atom.
Q22 Fill
CO₂ contains ______ oxygen atoms.
Show Answer
Two.
Q23 Fill
A compound is formed when atoms ______ join.
Show Answer
Chemically.
Q24 Fill
Water has the chemical formula ______.
Show Answer
H₂O.
Q25 Fill
The smallest part of an element is an ______.
Show Answer
Atom.
Q26 Fill
A molecule contains two or more ______ joined together.
Show Answer
Atoms.
Q27 Fill
Sodium chloride has the formula ______.
Show Answer
NaCl.
Q28 Fill
The number written small in a formula is called a ______.
Show Answer
Subscript.
Q29 Fill
Hydrogen gas exists as a molecule ______.
Show Answer
H₂.
Q30 Fill
The formula for carbon dioxide is ______.
Show Answer
CO₂.

Short-Answer Questions 10 Questions

Great for quick quizzes or oral questioning during live class.

Q31 Short
What is an element?
Show Answer
A pure substance made of only one type of atom.
Q32 Short
Give an example of a compound.
Show Answer
Any valid example, e.g. H₂O, CO₂, NaCl.
Q33 Short
What is a molecule?
Show Answer
Two or more atoms chemically joined.
Q34 Short
How many oxygen atoms are in CO₂?
Show Answer
2 oxygen atoms.
Q35 Short
What is the difference between atoms and molecules?
Show Answer
Atoms are single particles; molecules are made from 2 or more atoms joined.
Q36 Short
Name the two elements in NaCl.
Show Answer
Sodium and chlorine.
Q37 Short
Give one reason compounds differ from mixtures.
Show Answer
Compounds are joined by chemical bonds and need chemical reactions to separate; mixtures do not.
Q38 Short
Write the formula for oxygen gas.
Show Answer
O₂.
Q39 Short
Is O₂ an element or a compound?
Show Answer
It is an element (only oxygen atoms).
Q40 Short
What does the formula H₂O tell us?
Show Answer
It has 2 hydrogen atoms and 1 oxygen atom.

Long-Answer / Exam Practice 10 Questions

Use these for written practice, homework, or extended-response tasks.

Q41 Long
Explain the difference between an element and a compound with examples.
Show Answer
An element contains only one type of atom, such as oxygen (O) or iron (Fe). A compound contains two or more different elements chemically joined, such as water (H₂O) or carbon dioxide (CO₂).
Q42 Long
How is a molecule different from a compound?
Show Answer
A molecule is made of two or more atoms chemically joined together. A compound is a molecule that contains different kinds of elements. Therefore all compounds are molecules, but not all molecules are compounds (for example, H₂ is a molecule but not a compound).
Q43 Long
What information does a chemical formula provide?
Show Answer
A chemical formula tells us which elements are in a substance and how many atoms of each element are present. For example, CO₂ shows that the compound has 1 carbon atom and 2 oxygen atoms.
Q44 Long
Why do elements in a compound have different properties from the original elements?
Show Answer
When elements chemically react, their atoms join in fixed ratios to form new substances with new arrangements of atoms. This changes their properties. For example, sodium is a soft, reactive metal and chlorine is a poisonous gas, but together they form sodium chloride (table salt), which is safe to eat.
Q45 Long
Describe the formation of water from hydrogen and oxygen.
Show Answer
Hydrogen gas (H₂) reacts with oxygen gas (O₂) in a chemical reaction. Each water molecule (H₂O) is formed when two hydrogen atoms combine with one oxygen atom. This is a chemical change and produces a compound with different properties from either hydrogen or oxygen.
Q46 Long
Explain why a molecule of oxygen is written as O₂ instead of just O.
Show Answer
In nature, oxygen atoms usually pair up and form a molecule containing two atoms joined together. This is called a diatomic molecule. The formula O₂ shows that each molecule has 2 oxygen atoms bonded together.
Q47 Long
Compare CO₂ and O₂ in terms of elements and compounds.
Show Answer
O₂ is a molecule that contains only oxygen atoms, so it is an element. CO₂ contains carbon and oxygen atoms, so it is a compound. Both are molecules, but only CO₂ is a compound because it has more than one type of element.
Q48 Long
Why is an atom considered the basic unit of matter?
Show Answer
An atom is the smallest particle of an element that still behaves like that element. If you break an atom into smaller particles, such as protons and electrons, it no longer keeps the same chemical properties. So atoms are the basic building blocks of all matter.
Q49 Long
Explain how to determine the number of atoms in a formula such as C₆H₁₂O₆.
Show Answer
Look at each element symbol and its subscript. In C₆H₁₂O₆ there are: 6 carbon atoms (C₆), 12 hydrogen atoms (H₁₂), and 6 oxygen atoms (O₆). Add them if you want the total number of atoms in one molecule.
Q50 Long
Describe the structure of a compound using NaCl as an example.
Show Answer
Sodium chloride (NaCl) is made of sodium ions and chloride ions arranged in a regular lattice. In the formula NaCl there is a 1:1 ratio of sodium to chlorine. The particles are held together by strong ionic bonds, and the atoms cannot be separated by physical means, only by chemical reactions.
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