Elements, Molecules & Compounds — Key Concepts

ELEMENTS – Key Notes

• An element is a substance that consists of only one type of atom.
  This means every particle in the element is made of atoms with the same number of protons.
• Elements cannot be broken down into simpler substances by ordinary chemical means.
• There are over 118 known elements, each represented by a chemical symbol (e.g., H for Hydrogen, O for Oxygen, Na for Sodium).

EXAMPLE: HYDROGEN

• Hydrogen is the simplest and lightest element.
• A hydrogen atom contains:
  • 1 proton (in the nucleus)
  • 1 electron (orbiting around the nucleus)
• Most hydrogen atoms have no neutrons, though some isotopes do.

ADDITIONAL POINTS ABOUT ELEMENTS

• The atomic number of an element equals the number of protons in its atoms.
  For hydrogen, the atomic number is 1.
• Elements are organized in the Periodic Table based on their atomic number and properties.
• Examples of elements include:
  • Oxygen (O)
  • Carbon (C)
  • Iron (Fe)
  • Gold (Au)
• Two or more atoms of the same element can join to form molecules, e.g.:
  • O₂ (oxygen molecule)
  • N₂ (nitrogen molecule)

 

COMPOUNDS – Key Notes

• A compound is a substance made up of two or more different elements that are chemically combined in fixed proportions.
• Compounds have different properties from the elements that form them.
• Examples of compounds:
  • Water (H₂O) – hydrogen + oxygen
  • Carbon monoxide (CO) – carbon + oxygen
  • Ethanol (C₂H₆O) – carbon + hydrogen + oxygen
  • Sulphuric acid (H₂SO₄) – hydrogen + sulphur + oxygen

IMPORTANT CHECKPOINT

Is the statement true or false?

“All molecules are not compounds, but all compounds are molecules.”

✅ TRUE

Explanation:

• A molecule can contain atoms of the same element → e.g. O₂, H₂, Cl₂
  These are molecules but not compounds.
• But every compound is made up of atoms of different elements bonded together → which means every compound is a molecule.

Why O₂, H₂, Cl₂ Are NOT Compounds

• O₂ (oxygen gas) → only oxygen atoms
• H₂ (hydrogen gas) → only hydrogen atoms
• Cl₂ (chlorine gas) → only chlorine atoms

Since each consists of only one type of element, they are molecules, not compounds.

Why CO, Ethane, and Ammonia ARE Compounds

• Carbon monoxide (CO) → carbon + oxygen
• Ethane (C₂H₆) → carbon + hydrogen
• Ammonia (NH₃) → nitrogen + hydrogen

These all contain different elements, so they are compounds.

QUESTION: Are the following molecules or compounds?

Let’s classify each one:

1️⃣ Water (H₂O)

• Contains H and O
  ➡️ Compound (and also a molecule)

2️⃣ Nitric Acid (HNO₃)

• Contains H, N, and O
  ➡️ Compound

3️⃣ Fluorine (F₂)

• Contains only fluorine atoms
  ➡️ Molecule, not a compound

4️⃣ Iodine (I₂)

• Contains only iodine atoms
  ➡️ Molecule, not a compound

✔ Summary Table

Substance	Type
H₂O	Compound
HNO₃	Compound
F₂	Molecule (not compound)
I₂	Molecule (not compound)

 

 

✅ Are All Compounds Molecules?

Yes.
✔ All compounds are molecules
✘ But not all molecules are compounds

This is because:

• Molecules = two or more atoms chemically bonded (same element OR different elements).
• Compounds = two or more different elements chemically bonded.

So every compound fits the definition of a molecule, but some molecules (like O₂ or N₂) contain only one type of element — so they are not compounds.

✅ Summary (Clear and Correct)

• Element:
  A substance that consists of only one type of atom.
  Example: O, N, H, Fe
• Molecule:
  Two or more atoms chemically joined.
  These atoms can be:
  • the same element (e.g., O₂, N₂), or
  • different elements (e.g., H₂O, CO₂)
• Compound:
  Two or more atoms from different elements chemically joined.
  Examples: H₂O, CO₂, NH₃

🧪 Classify the Following: Molecule, Compound, or Both

Remember:

• Molecule only (NOT compound): same element
• Compound (and also molecule): different elements
• Both = it is a molecule AND a compound

Let’s classify:

1️⃣ Oxygen (O₂)

• Two oxygen atoms (same element)
  ➡️ Molecule only (NOT a compound)

2️⃣ CO₂ (Carbon dioxide)

• Carbon + oxygen (different elements)
  ➡️ Both (molecule + compound)

3️⃣ N₂ (Nitrogen gas)

• Two nitrogen atoms (same element)
  ➡️ Molecule only

4️⃣ H₂O (Water)

• Hydrogen + oxygen (different elements)
  ➡️ Both (molecule + compound)

5️⃣ O₃ (Ozone)

• Three oxygen atoms (same element)
  ➡️ Molecule only (NOT a compound)

6️⃣ Sugar – C₆H₁₂O₆ (Glucose)

• Carbon, hydrogen, oxygen (different elements)
  ➡️ Both (molecule + compound)
  Note: It is a complex organic compound.

✔ Final Answers Table

Substance	Molecule?	Compound?	Classification
O₂	Yes	No	Molecule
CO₂	Yes	Yes	Both (molecule + compound)
N₂	Yes	No	Molecule
H₂O	Yes	Yes	Both
O₃	Yes	No	Molecule
C₆H₁₂O₆	Yes	Yes	Both