Elements, Symbols & Compounds

🎯 Learning Objective

Recall the Latin-based element symbols and the formulas of common compounds, and tell elements, compounds and mixtures apart.

💡 Concept

• An element is a pure substance made of one type of atom (e.g. iron, oxygen, gold); about 118 elements are known
• A compound is two or more elements chemically combined in a FIXED ratio; its properties differ from its elements (e.g. water, common salt)
• A mixture is a physical combination with no fixed ratio, separable by physical means (e.g. air, salt water)
• Latin-based symbols: Sodium = Na (Natrium), Potassium = K (Kalium), Iron = Fe (Ferrum)
• More Latin symbols: Gold = Au (Aurum), Silver = Ag (Argentum), Lead = Pb (Plumbum)
• Also: Copper = Cu (Cuprum), Tin = Sn (Stannum), Mercury = Hg (Hydrargyrum), Tungsten = W (Wolfram)
• Common compound formulas: Water = H₂O, Carbon dioxide = CO₂, Common salt = NaCl
• Calcium carbonate (limestone/marble) = CaCO₃, Ammonia = NH₃, Methane = CH₄
• Glucose = C₆H₁₂O₆, Sulphuric acid = H₂SO₄, Hydrochloric acid = HCl
• In a symbol the first letter is capital and the second is small (convention by Berzelius)

🧮 Key Formulas

Water H₂O · Carbon dioxide CO₂ · Common salt NaCl

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Limestone CaCO₃ · Ammonia NH₃ · Methane CH₄

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Na (Sodium) · K (Potassium) · Fe (Iron) · Au (Gold) · Ag (Silver) · Pb (Lead)

✏️ Easy Example

Q. What is the chemical symbol of Gold? (a) Go (b) Gd (c) Au (d) Ag

1. Gold's symbol comes from its Latin name 'Aurum'
2. So the symbol is Au (Ag is silver)

Answer: (c) Au

🇮🇳 Real-Life Example

Gold jewellery is stamped 'Au' and iron gates rust because of 'Fe' — these Latin symbols aren't textbook trivia, they're literally printed on the metals around you. Ag on silverware is another everyday example.

📝 Exam-Level Example

Q. The chemical formula of common salt is: (a) NaCl (b) Na₂CO₃ (c) NaHCO₃ (d) NaOH

1. Common salt is sodium chloride
2. Sodium (Na) + Chlorine (Cl) = NaCl

Answer: (a) NaCl

📝 Exam-Level Example

Q. Which of these is a compound, not a mixture? (a) Air (b) Sea water (c) Carbon dioxide (d) Brass

1. Air, sea water and brass are mixtures (no fixed ratio)
2. Carbon dioxide has a fixed ratio C:O = 1:2 → a compound

Answer: (c) Carbon dioxide

🪄 Memory Trick

For coins/metals recall the Latin: Sodium→Natrium(Na), Iron→Ferrum(Fe), Gold→Aurum(Au), Silver→Argentum(Ag), Lead→Plumbum(Pb) — 'plumber' works with lead pipes!

⚠️ Common Mistakes

• ❌ Writing gold as 'Go' or silver as 'Si' — they are Au and Ag from Latin
• ❌ Confusing Co (cobalt) with CO (carbon monoxide) — capitalisation matters
• ❌ Calling air or brass a compound — they are mixtures

🏆 Exam Tips

• ✅ Memorise the 8–10 Latin-symbol elements as a block; they appear almost every exam
• ✅ Compound = fixed ratio + chemical bond; mixture = variable ratio + physical mix

📌 Summary

• Element (one atom type), compound (fixed ratio, bonded), mixture (variable, physical)
• Latin symbols: Na, K, Fe, Au, Ag, Pb, Cu, Hg
• H₂O, CO₂, NaCl, CaCO₃, NH₃, CH₄ — learn these formulas cold
• Symbol rule: first letter capital, second small