Atomic Structure
NCERT Class 9 • Science • Chapter "Structure of the Atom" | NCERT Class 9 • Science • Chapter "Atoms and Molecules"
Atomic Structure
An atom is the smallest particle of an element that takes part in a chemical reaction. It was once thought to be indivisible, but a series of experiments showed that it contains still smaller subatomic particles arranged in a definite structure.
The Models of the Atom
- Dalton proposed that matter is made of indivisible atoms, that atoms of an element are identical, and that compounds form when atoms combine in fixed whole number ratios.
- Thomson discovered the electron and pictured the atom as a sphere of positive charge with electrons embedded in it.
- Rutherford fired alpha particles at a thin gold foil; most passed straight through, a few deflected, and a very few bounced back.
- He concluded that the atom is mostly empty space with a tiny, dense, positively charged nucleus at the centre.
- Bohr explained why the atom is stable by proposing that electrons move only in fixed shells of definite energy without radiating energy.
- Electron shells are named K, L, M, N and hold a maximum of 2, 8, 18 and 32 electrons respectively.
- The outermost shell can never hold more than 8 electrons.
Subatomic Particles and Isotopes
- The proton is positively charged and lies in the nucleus; the neutron has no charge and also lies in the nucleus.
- The electron is negatively charged, moves around the nucleus, and is far lighter than the other two.
- Atomic number is the number of protons and is denoted by Z; it decides which element the atom is.
- Mass number is the total of protons and neutrons and is denoted by A.
- Isotopes are atoms of the same element with the same atomic number but different mass numbers, as in hydrogen's protium, deuterium and tritium.
- Isobars are atoms of different elements with the same mass number.
- Valency is the combining capacity of an atom and depends on the electrons in its outermost shell.
| Particle | Charge | Location |
|---|---|---|
| Proton | Positive | Nucleus |
| Neutron | No charge | Nucleus |
| Electron | Negative | Shells around nucleus |
Isotopes — atoms of the same element having the same atomic number but different mass numbers.
Hindi me samjhein
परमाणु क्रमांक प्रोटॉनों की संख्या है और यही तय करता है कि तत्व कौन सा है। द्रव्यमान संख्या प्रोटॉन और न्यूट्रॉन का योग है। समस्थानिकों में परमाणु क्रमांक समान पर द्रव्यमान संख्या भिन्न रहती है।
Exam me kaise aata hai
- Who discovered the electron — Thomson
- Atomic number is the number of — protons
- Same atomic number but different mass number means — isotopes
- Maximum electrons in the outermost shell — 8
UPSC/State PSC ke liye
- Rutherford's key evidence was that a very few alpha particles bounced straight back, which can only happen if the positive charge is concentrated in a tiny nucleus.
- Chemical properties depend on electrons, so isotopes of an element behave the same chemically but differ in mass and in nuclear behaviour.
Yahan confuse hote hain
✗ Isotopes have different atomic numbers | Isotopes have the same atomic number, different mass numbers | ✓
✗ The nucleus contains electrons | The nucleus contains protons and neutrons | ✓
✗ Mass number is the number of protons | Mass number is protons plus neutrons | ✓
Ek nazar me
- Dalton, Thomson, Rutherford and Bohr built the picture of the atom step by step.
- The atom is mostly empty space with a dense positive nucleus.
- Protons and neutrons are in the nucleus; electrons move in shells.
- Atomic number is protons; mass number is protons plus neutrons.