Grade VII science worksheet on matter and chemical formulae, including questions on particles, chemical symbols, and definitions.

Let's solve each part of the worksheet step by step and explain the answers clearly.

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1. Fill in the blanks

a. The smallest particle of a compound is ________

✔ Answer: Molecule

🔹 Explanation: A molecule is the smallest unit of a compound that retains its chemical properties. It consists of two or more atoms chemically bonded together.

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b. An ________ is formed when an atom loses or gains electrons.

✔ Answer: Ion

🔹 Explanation: When an atom loses electrons, it becomes a positively charged ion (cation). When it gains electrons, it becomes a negatively charged ion (anion). This process forms an ion.

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c. A molecule of CO₂ consists of ____ carbon and ____ oxygen atoms.

✔ Answer: 1 carbon and 2 oxygen atoms

🔹 Explanation: The chemical formula CO₂ tells us:
- C = one carbon atom
- O₂ = two oxygen atoms

So, one molecule of carbon dioxide has 1 carbon and 2 oxygen atoms.

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d. The formulae for magnesium chloride is ________

✔ Answer: MgCl₂

🔹 Explanation: Magnesium (Mg) has a +2 charge (Mg²⁺), and chlorine (Cl) has a -1 charge (Cl⁻). To balance the charges, we need two Cl⁻ ions for every Mg²⁺ ion → MgCl₂.

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2. Choose the correct answer

A. What is the chemical symbol for Silver?

✔ Answer: c. Ag

🔹 Explanation:
- Si = Silicon
- Au = Gold
- Ag = Silver (from Latin *Argentum*)
- S = Sulfur

So, Ag is correct.

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B. Sodium chloride molecule is an example of:

✔ Answer: b. A compound

🔹 Explanation:
- Sodium chloride (NaCl) is made of two different elements (Na and Cl) chemically combined in a fixed ratio.
- A compound is a substance formed when two or more elements combine chemically.
- It is not an element (only one type of atom), nor a mixture (which is physically combined).

So, compound is correct.

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C. The correct formulae for Calcium oxide is:

✔ Answer: b. CaO

🔹 Explanation:
- Calcium (Ca) forms a +2 ion (Ca²⁺)
- Oxygen (O) forms a -2 ion (O²⁻)
- Charges balance directly: Ca²⁺ + O²⁻ → CaO

Other options:
- Ca₂O₂ → incorrect (not balanced)
- CaO₂ → calcium peroxide (different compound)
- Ca(OH)₂ → calcium hydroxide

So, CaO is correct.

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3. Define

a. Molecule

✔ Definition: A molecule is the smallest particle of a substance that retains the chemical properties of that substance. It consists of two or more atoms chemically bonded together.

🔹 Example: H₂O (water molecule)

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b. Monoatomic

✔ Definition: Monoatomic means consisting of only one atom.

🔹 Example: Noble gases like helium (He), neon (Ne) exist as single atoms — they are monoatomic.

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c. Atomicity

✔ Definition: Atomicity is the number of atoms present in one molecule of an element.

🔹 Examples:
- O₂ → atomicity = 2 (diatomic)
- O₃ → atomicity = 3 (triatomic)
- He → atomicity = 1 (monoatomic)

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4. Balance the following equations

⚠️ Note: The balancing equations section is not visible in your image upload. Please provide the equations you'd like me to balance, or I can give examples if needed.

But since it's missing, here’s how you’d approach it:

To balance a chemical equation:
1. Write the unbalanced equation.
2. Count atoms on both sides.
3. Use coefficients (numbers in front of formulas) to balance atoms.
4. Never change subscripts.

Example:
Unbalanced: Fe + O₂ → Fe₂O₃
Balanced: 4Fe + 3O₂ → 2Fe₂O₃

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✔ Final Answers Summary:

#### 1. Fill in the blanks:
a. Molecule
b. Ion
c. 1 carbon and 2 oxygen atoms
d. MgCl₂

#### 2. Multiple Choice:
A. c. Ag
B. b. A compound
C. b. CaO

#### 3. Definitions:
a. Molecule – Smallest unit of a compound with chemical properties; made of bonded atoms.
b. Monoatomic – Made of a single atom (e.g., noble gases).
c. Atomicity – Number of atoms in one molecule of an element.

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If you can share the equations from question 4, I’ll balance them for you too!