Comprehensive study guide covering Structure Of Atom Molecules for UPSSSC Lower Mains. UPSSSC लोअर मेन्स के लिए Structure Of Atom Molecules को कवर करने वाली मार्गदर्शिका।
mindmap
root((Atom))
Subatomic
Proton
Neutron
Electron
Models
Dalton
Thomson
Rutherford
Bohr
Quantum
Orbitals
Configuration
Molecules
Valency
Bonding
FormulaeThe atom is the smallest unit of matter that retains the properties of an element. It consists of a dense central nucleus surrounded by electrons. परमाणु पदार्थ की सबसे छोटी इकाई है जो एक तत्व के गुणों को बरकरार रखती है। इसमें इलेक्ट्रॉनों से घिरा एक सघन केंद्रीय नाभिक होता है।
The three primary subatomic particles define the identity and reactivity of an element. Their properties are summarized below.
| Particle | Symbol | Relative Mass | Charge |
|---|---|---|---|
| Proton | p+ | 1.007 u | +1 |
| Neutron | n0 | 1.008 u | 0 |
| Electron | e- | 0.0005 u | -1 |
John Dalton (1808) proposed that matter consists of indivisible atoms. He suggested that atoms of a given element are identical in mass and properties.
Postulates include: Matter is made of indestructible atoms; Compounds are formed by fixed ratios of atoms. पदार्थ अविनाशी परमाणुओं से बना है; यौगिक परमाणुओं के निश्चित अनुपात से बनते हैं।
Thomson discovered the electron using a cathode ray tube. His model visualized the atom as a positively charged sphere with electrons embedded like seeds in a watermelon.
Rutherford bombarded a thin gold foil with alpha particles. Most passed through, but some deflected, leading to the discovery of the 'Nucleus'.
He concluded that the atom is mostly empty space with a tiny, positively charged, dense center called the nucleus.
Niels Bohr (1913) proposed that electrons revolve in discrete orbits (shells) with fixed energy levels (K, L, M, N).
Electrons do not radiate energy while in stable orbits. Energy is emitted or absorbed only when an electron jumps between orbits (Quantum jump).
Modern physics uses the Schrodinger equation to define 'Orbitals'—regions of space where the probability of finding an electron is highest.
| Quantum Number | Represents |
|---|---|
| Principal (n) | Shell number |
| Azimuthal (l) | Subshell shape |
| Magnetic (ml) | Orientation |
| Spin (ms) | Electron spin |
The distribution of electrons follows the Aufbau Principle (filling lower energy first), Pauli Exclusion Principle (no two electrons have same 4 quantum numbers), and Hund’s Rule (filling orbitals singly before pairing).
Valency is the combining capacity of an element. Atoms bond to achieve a stable octet configuration (Noble gas state).
Elements with the same atomic number but different mass numbers are Isotopes (e.g., Carbon-12, Carbon-14).
Isobars have the same mass number but different atomic numbers (e.g., Argon-40 and Calcium-40).
Molecular mass is the sum of atomic masses of all atoms in a molecule. Example: H2O = (2 * 1) + 16 = 18 u.
| Molecule | Composition | Molecular Mass (u) |
|---|---|---|
| CO2 | 1C, 2O | 44 |
| NH3 | 1N, 3H | 17 |
| CH4 | 1C, 4H | 16 |
1 mole is the amount of substance containing 6.022 x 10^23 particles (Avogadro’s Number).
The mass of 1 mole of a substance in grams is equal to its atomic/molecular mass in atomic mass units (amu).
Atomic radius decreases across a period due to increased effective nuclear charge. It increases down a group due to the addition of new shells.
Mnemonic for Subatomic particles: PEN (Proton, Electron, Neutron). Remember: Proton = Positive, Electron = Extra-negative, Neutron = Neutral.
Use the 'Aufbau' mnemonic: 1s, 2s, 2p, 3s, 3p, 4s, 3d... Remember the sequence of energy levels for UPSSSC exams.