Atoms, Molecules and Chemical Formulae
NCERT Class 9 • Science • Chapter "Atoms and Molecules"
Atoms, Molecules and Chemical Formulae
A molecule is the smallest particle of an element or compound that can exist independently and shows all the properties of that substance. The rules for how atoms combine into molecules were fixed by two laws established long before atoms could be seen.
The Laws and Basic Quantities
- The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.
- The law of constant proportions states that a given compound always contains the same elements in the same proportion by mass.
- Atomic mass is expressed in atomic mass units, taken as one twelfth of the mass of a carbon twelve atom.
- Molecular mass is the sum of the atomic masses of all atoms in a molecule.
- Atomicity is the number of atoms in a molecule: helium is monoatomic, oxygen diatomic, ozone triatomic and phosphorus tetratomic.
- An ion is a charged particle; a positive ion is a cation and a negative ion an anion.
- A polyatomic ion is a group of atoms carrying a charge, such as sulphate, nitrate, carbonate and ammonium.
The Mole and Chemical Formulae
- A mole is the amount of substance containing as many particles as there are atoms in twelve grams of carbon twelve.
- That number is called Avogadro's number and is about 6.022 × 10<sup>23</sup>.
- The mass of one mole in grams is the molar mass, numerically equal to the atomic or molecular mass.
- To write a formula, the valency of each ion is crossed over to become the subscript of the other.
- In a formula the metal or positive ion is written first, followed by the non metal or negative ion.
- A polyatomic ion taken more than once is enclosed in brackets, as in calcium hydroxide and aluminium sulphate.
| Quantity | Meaning |
|---|---|
| Atomic mass unit | One twelfth the mass of a carbon twelve atom |
| Molecular mass | Sum of atomic masses in a molecule |
| Mole | Amount containing Avogadro's number of particles |
| Molar mass | Mass of one mole in grams |
Mole — the amount of a substance containing as many particles as there are atoms in twelve grams of carbon twelve.
Exam me kaise aata hai
- Avogadro's number is about — 6.022 × 10<sup>23</sup>
- Mass is neither created nor destroyed is which law — law of conservation of mass
- A positively charged ion is called a — cation
- The number of atoms in a molecule is its — atomicity
UPSC/State PSC ke liye
- The mole exists because chemists need to count particles that are far too small to count directly, so mass is used as a proxy for number.
- The law of constant proportions is what separates a compound from a mixture, since a mixture can be made in any proportion.
Yahan confuse hote hain
✗ An atom and a molecule are the same | A molecule is made of atoms and can exist independently | ✓
✗ A cation is negatively charged | A cation is positive; an anion is negative | ✓
✗ Molar mass and molecular mass have the same unit | Molecular mass is in atomic mass units, molar mass in grams per mole | ✓
Ek nazar me
- Conservation of mass and constant proportions govern chemical combination.
- Atomic and molecular mass are measured in atomic mass units.
- A mole contains Avogadro's number of particles.
- Formulae are written by crossing over valencies with the positive ion first.