ATOMS AND MOLECULES

Ancient philosophers ne matter ke smallest particles ke baare mein socha tha. Maharishi Kanad ne 'Parmanu' aur Democritus ne 'Atoms' ka idea diya tha. Eighteenth century mein, Lavoisier aur Joseph L. Proust ne chemical reactions ko observe karke do important laws diye:

1. Law of Conservation of Mass

• Statement: Mass can neither be created nor destroyed in a chemical reaction.
• Matlab: Kisi bhi chemical reaction mein, products ka total mass reactants ke total mass ke barabar hota hai.
• Example: Agar 5.3 g sodium carbonate (reactant) 6 g acetic acid (reactant) se react karta hai, toh total reactant mass = $5.3 + 6 = 11.3$ g. Products mein 2.2 g carbon dioxide, 0.9 g water, aur 8.2 g sodium acetate bante hain. Total product mass = $2.2 + 0.9 + 8.2 = 11.3$ g. Dekha, dono sides ka mass equal hai.
• Experimental Verification (Activity 3.1):
• Ek conical flask mein solution Y lo (e.g., Barium Chloride). Ignition tube mein solution X (e.g., Sodium Sulphate) lo.
• Ignition tube ko flask mein aise suspend karo ki solutions mix na ho.
• Flask ko cork se band karke uska mass weigh karo.
• Flask ko tilt karke solutions ko mix karo. Chemical reaction hogi (e.g., precipitate banega).
• Dobara flask ka mass weigh karo.
• Observation: Mass mein koi change nahi aata. Total mass before reaction = Total mass after reaction.

2. Law of Constant Proportions (Law of Definite Proportions)

• Statement: In a chemical substance, the elements are always present in definite proportions by mass, irrespective of its source or method of preparation.
• Matlab: Koi bhi compound, jaise water ($H_2O$), hamesha same elements (Hydrogen aur Oxygen) se bana hota hai aur un elements ka mass ratio hamesha fixed hota hai.
• Example:
• Water ($H_2O$): Hydrogen aur Oxygen ka mass ratio hamesha $1:8$ hota hai. Agar 9 g water decompose hota hai, toh 1 g Hydrogen aur 8 g Oxygen hi milenge.
• Ammonia ($NH_3$): Nitrogen aur Hydrogen ka mass ratio hamesha $14:3$ hota hai.
• Significance: Yeh law batata hai ki compounds ki composition fixed hoti hai. Har compound ka apna unique mass ratio hota hai constituent elements ka.

Dalton's Atomic Theory

John Dalton ne in laws ko explain karne ke liye apni atomic theory di. Yeh theory matter ke nature ke baare mein ek revolutionary idea thi.

• Postulates:

1. Matter is made of atoms: All matter, chahe woh element ho, compound ho ya mixture, sab bahut chote particles se bane hote hain jinhe atoms kehte hain.
2. Atoms are indivisible: Atoms indivisible particles hote hain, jinhe chemical reaction mein na toh create kiya ja sakta hai aur na hi destroy.

• _Explanation for Law of Conservation of Mass: Atoms sirf rearrange hote hain, destroy ya create nahi hote, isliye total mass constant rehta hai._

1. Identical atoms: Ek given element ke saare atoms mass aur chemical properties mein identical hote hain.
2. Different atoms: Different elements ke atoms ka mass aur chemical properties different hoti hain.
3. Combine in whole numbers: Atoms small whole numbers ke ratio mein combine hoke compounds banate hain.

• _Explanation for Law of Constant Proportions: Jab atoms fixed ratio mein combine hote hain, toh unka mass ratio bhi fixed ho jaata hai._

1. Constant ratio in compounds: Kisi bhi given compound mein relative number aur kinds of atoms constant rehte hain.

• Limitations (Modern Context mein):
• Atoms indivisible nahi hote, unke andar electrons, protons, neutrons hote hain.
• Ek hi element ke atoms ka mass different ho sakta hai (Isotopes).
• Different elements ke atoms ka mass same ho sakta hai (Isobars).
• Atoms hamesha simple whole number ratios mein combine nahi hote (e.g., complex organic compounds).

Atoms: The Building Blocks

• Definition: Atom matter ka smallest building block hai. Jaise ek building bricks se banti hai, waise hi saara matter atoms se bana hai.
• Size: Atoms bahut chote hote hain. Itne chote ki hum unhe naked eye se nahi dekh sakte. Millions of atoms ko stack karne par bhi ek paper sheet jitni thickness nahi banti.
• Atomic Radius: Atoms ka size nanometres (nm) mein measure kiya jaata hai.
• $1 \text{ nm} = 10^{-9} \text{ m}$ (1 meter = $10^9$ nanometers)
• Hydrogen atom ka radius approx $10^{-10} \text{ m}$ hota hai.
• Importance: Poori duniya atoms se bani hai. Hum unhe dekh nahi sakte, but they are constantly affecting everything around us.

Modern Day Symbols of Atoms

• Early Symbols: Dalton ne elements ke liye specific symbols use kiye the (e.g., circle with a dot for Hydrogen).
• Berzelius's Suggestion: Berzelius ne suggest kiya ki elements ke symbols unke naam ke ek ya do letters se banaye jaayen.
• IUPAC (International Union of Pure and Applied Chemistry): Yeh international scientific organization hai jo elements ke names, symbols, aur units ko approve karti hai.
• Rules for Symbols:

1. First letter: Hamesha capital (uppercase) hota hai. Example: Hydrogen (H), Oxygen (O).
2. Second letter (if any): Hamesha small (lowercase) hota hai. Example: Aluminium (Al), Cobalt (Co).

• Common Mistake: AL ya CO nahi, Al aur Co.

1. Latin/German/Greek names: Kuch elements ke symbols unke Latin, German, ya Greek names se liye gaye hain.

• Iron (Fe) from Ferrum
• Sodium (Na) from Natrium
• Potassium (K) from Kalium
• Significance: Har element ka ek unique name aur chemical symbol hota hai, jo uski identity define karta hai.

Atomic Mass

• Dalton's Concept: Dalton ne kaha ki har element ka ek characteristic atomic mass hota hai.
• Relative Atomic Mass: Individual atom ka mass measure karna difficult tha, isliye relative atomic mass ka concept aaya.
• Initially, scientists ne $1/16^{th}$ mass of Oxygen atom ko unit mana.
• Universally Accepted Standard (1961): Carbon-12 isotope ko standard reference mana gaya.
• Atomic Mass Unit (u): One atomic mass unit (amu, ab 'u' se denote karte hain) is exactly one-twelfth ($1/12^{th}$) the mass of one atom of carbon-12.
• Definition: Relative atomic mass of an element is the average mass of the atom, as compared to $1/12^{th}$ the mass of one carbon-12 atom.
• Importance: Atomic mass se hum elements ke atoms ka relative weight compare kar sakte hain aur chemical calculations mein use kar sakte hain.

How Do Atoms Exist?

• Most elements ke atoms independently exist nahi kar sakte.
• Atoms combine hoke molecules aur ions banate hain.
• Yeh molecules aur ions large numbers mein aggregate hoke woh matter banate hain jo hum dekh sakte hain, feel kar sakte hain, ya touch kar sakte hain.

Molecules

• Definition: A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces.
• Independent Existence: Molecule element ya compound ka smallest particle hai jo independently exist kar sakta hai aur us substance ki saari properties show karta hai.
• Formation: Atoms of the same element ya different elements combine hoke molecules banate hain.

Molecules of Elements

• Same type of atoms: Elements ke molecules same type ke atoms se bane hote hain.
• Atomicity: The number of atoms constituting a molecule is called its atomicity.
• Monoatomic: Ek atom (e.g., Argon (Ar), Helium (He) - Noble gases).
• Diatomic: Do atoms (e.g., Oxygen ($O_2$), Hydrogen ($H_2$), Nitrogen ($N_2$), Chlorine ($Cl_2$)).
• Triatomic: Teen atoms (e.g., Ozone ($O_3$)).
• Tetra-atomic: Chaar atoms (e.g., Phosphorus ($P_4$)).
• Poly-atomic: Chaar se zyada atoms (e.g., Sulphur ($S_8$)).
• Metals: Metals aur kuch non-metals (jaise Carbon) ka simple molecular structure nahi hota. Unmein bahut saare atoms ek saath bonded hote hain.

Molecules of Compounds

• Different elements: Different elements ke atoms definite proportions mein combine hoke compounds ke molecules banate hain.
• Examples:
• Water ($H_2O$): Hydrogen aur Oxygen $1:8$ mass ratio mein.
• Ammonia ($NH_3$): Nitrogen aur Hydrogen $14:3$ mass ratio mein.
• Carbon dioxide ($CO_2$): Carbon aur Oxygen $3:8$ mass ratio mein.
• Ratio by Number of Atoms (Activity 3.2):
• Elements ke mass ratio aur unke atomic mass ko use karke, hum atoms ka simplest whole number ratio nikal sakte hain.
• For Water ($H_2O$):
• Hydrogen: Mass ratio = 1, Atomic mass = 1u. Ratio by mass/atomic mass = $1/1 = 1$.
• Oxygen: Mass ratio = 8, Atomic mass = 16u. Ratio by mass/atomic mass = $8/16 = 1/2$.
• Simplest ratio (divide by smallest, i.e., $1/2$): Hydrogen ($1 / (1/2) = 2$), Oxygen ($(1/2) / (1/2) = 1$).
• So, H:O = $2:1$. Formula $H_2O$.

What is an Ion?

• Definition: Ions are charged species. Yeh single charged atom ho sakte hain ya group of atoms jin par net charge hota hai.
• Formation: Compounds jo metals aur non-metals se bane hote hain (ionic compounds) mein charged species hote hain.
• Types of Ions:
• Cation: Positively charged ion (e.g., $Na^+$, $Mg^{2+}$, $Al^{3+}$).
• Anion: Negatively charged ion (e.g., $Cl^-$, $O^{2-}$, $N^{3-}$).
• Polyatomic Ions: A group of atoms carrying a charge is called a polyatomic ion.
• Examples: Ammonium ($NH_4^+$), Hydroxide ($OH^-$), Nitrate ($NO_3^-$), Carbonate ($CO_3^{2-}$), Sulphate ($SO_4^{2-}$), Phosphate ($PO_4^{3-}$).
• Valency: Ions par jo charge hota hai, woh unki valency hoti hai. Valency combine karne ki capacity ko batati hai.

Chemical formula kisi compound ke composition ka symbolic representation hota hai. Isse humein pata chalta hai ki compound mein kaun se elements hain aur kitne atoms hain har element ke.

Rules for Writing Chemical Formulae

1. Valencies/Charges Balance: Ions par jo valencies ya charges hote hain, unhe balance hona chahiye. Overall compound neutral hona chahiye.
2. Metal First: Jab compound metal aur non-metal se bana ho, toh metal ka naam ya symbol pehle likha jaata hai. Non-metal right side mein aata hai.

• Examples: Calcium oxide (CaO), Sodium chloride (NaCl), Iron sulphide (FeS).

1. Polyatomic Ions (Brackets): Agar compound mein polyatomic ion ho aur uske ek se zyada units hon, toh polyatomic ion ke formula ko bracket mein rakhte hain aur number of units ko bracket ke bahar subscript mein likhte hain.

• Example: Magnesium hydroxide $Mg(OH)_2$. Yahan do hydroxide ions hain.
• Agar polyatomic ion ka sirf ek unit ho, toh bracket ki zaroorat nahi hoti. Example: Sodium hydroxide $NaOH$.

Formulae of Simple Compounds (Criss-Cross Method)

Chemical formulae likhne ke liye criss-cross method bahut useful hai:

• Elements ke symbols likho.
• Har element ki valency uske symbol ke neeche likho.
• Valencies ko criss-cross karo (ek element ki valency doosre element ke subscript mein jaati hai).
• Agar valencies same hon, toh unhe cancel kar do (e.g., $Ca^{2+}O^{2-}$ becomes $CaO$).
• Polyatomic ions ke liye brackets use karna mat bhoolna.

Examples:

1. Hydrogen Chloride:

• Symbols: H, Cl
• Valencies: 1, 1
• Criss-cross: $H_1Cl_1 \rightarrow HCl$

1. Hydrogen Sulphide:

• Symbols: H, S
• Valencies: 1, 2
• Criss-cross: $H_2S_1 \rightarrow H_2S$

1. Carbon Tetrachloride:

• Symbols: C, Cl
• Valencies: 4, 1
• Criss-cross: $C_1Cl_4 \rightarrow CCl_4$

1. Magnesium Chloride:

• Symbols: Mg, Cl
• Charges: $Mg^{2+}$, $Cl^-$
• Criss-cross: $Mg_1Cl_2 \rightarrow MgCl_2$

1. Aluminium Oxide:

• Symbols: Al, O
• Charges: $Al^{3+}$, $O^{2-}$
• Criss-cross: $Al_2O_3$

1. Calcium Oxide:

• Symbols: Ca, O
• Charges: $Ca^{2+}$, $O^{2-}$
• Criss-cross: $Ca_2O_2$. Simplify to $CaO$ (since valencies are same).

1. Sodium Nitrate:

• Symbols: Na, $NO_3$
• Charges: $Na^+$, $NO_3^-$
• Criss-cross: $Na_1(NO_3)_1 \rightarrow NaNO_3$

1. Calcium Hydroxide:

• Symbols: Ca, OH
• Charges: $Ca^{2+}$, $OH^-$
• Criss-cross: $Ca_1(OH)_2 \rightarrow Ca(OH)_2$
• _Note:_ Brackets are essential here because there are two hydroxide groups.

1. Sodium Carbonate:

• Symbols: Na, $CO_3$
• Charges: $Na^+$, $CO_3^{2-}$
• Criss-cross: $Na_2(CO_3)_1 \rightarrow Na_2CO_3$

1. Ammonium Sulphate:

• Symbols: $NH_4$, $SO_4$
• Charges: $NH_4^+$, $SO_4^{2-}$
• Criss-cross: $(NH_4)_2SO_4$

Molecular Mass

• Definition: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
• Units: Isse atomic mass units (u) mein express kiya jaata hai.
• Calculation Steps:

1. Compound ka chemical formula pata karo.
2. Har element ke atomic mass ko uske number of atoms se multiply karo.
3. Sabhi elements ke products ko add kar do.

• Example 1: Water ($H_2O$)
• Atomic mass of H = 1 u
• Atomic mass of O = 16 u
• Molecular mass of $H_2O = (2 \times \text{Atomic mass of H}) + (1 \times \text{Atomic mass of O})$
• $= (2 \times 1 \text{ u}) + (1 \times 16 \text{ u}) = 2 \text{ u} + 16 \text{ u} = 18 \text{ u}$

• Example 2: Nitric Acid ($HNO_3$)
• Atomic mass of H = 1 u
• Atomic mass of N = 14 u
• Atomic mass of O = 16 u
• Molecular mass of $HNO_3 = (1 \times \text{H}) + (1 \times \text{N}) + (3 \times \text{O})$
• $= (1 \times 1 \text{ u}) + (1 \times 14 \text{ u}) + (3 \times 16 \text{ u})$
• $= 1 \text{ u} + 14 \text{ u} + 48 \text{ u} = 63 \text{ u}$

Formula Unit Mass

• Definition: The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound.
• Difference from Molecular Mass: Calculation method same hai. Fark sirf term mein hai. Formula unit mass उन substances ke liye use hota hai jinke constituent particles ions hote hain (ionic compounds).
• Units: Isse bhi atomic mass units (u) mein express kiya jaata hai.

• Example: Calcium Chloride ($CaCl_2$)
• Atomic mass of Ca = 40 u
• Atomic mass of Cl = 35.5 u
• Formula unit mass of $CaCl_2 = (1 \times \text{Atomic mass of Ca}) + (2 \times \text{Atomic mass of Cl})$
• $= (1 \times 40 \text{ u}) + (2 \times 35.5 \text{ u}) = 40 \text{ u} + 71 \text{ u} = 111 \text{ u}$

• Example: Sodium Chloride (NaCl)
• Atomic mass of Na = 23 u
• Atomic mass of Cl = 35.5 u
• Formula unit mass of $NaCl = (1 \times 23 \text{ u}) + (1 \times 35.5 \text{ u}) = 23 \text{ u} + 35.5 \text{ u} = 58.5 \text{ u}$