Modern & Nuclear Physics

The smallest unit of an element that still keeps the element's properties.

A tiny heavy central nucleus with light electrons moving around it in shells.

Protons (positive charge) and neutrons (no charge), together called nucleons.

The number of protons equals the number of electrons.

In the nucleus — electrons are extremely light.

Discovered the electron and gave the plum-pudding model (electrons stuck in a positive sphere).

Did the gold-foil experiment and gave the nuclear model (mass and positive charge in a tiny nucleus).

Said electrons revolve only in fixed energy levels (shells) K, L, M, N = n 1, 2, 3, 4.

Discovered the neutron, completing the three-particle picture.

Gave the Theory of Relativity and explained the photoelectric effect.

The number of protons; it decides the identity of the element.

Protons + neutrons (total nucleons).

Neutrons = A − Z.

The number of atoms in one molecule: He is monatomic, O₂ diatomic, O₃ triatomic.

Same atomic number Z, different mass number A — same element, different neutrons.

Protium, deuterium and tritium, with 0, 1 and 2 neutrons.

Carbon: C-12, C-13, C-14 (radio-carbon dating); chlorine: Cl-35 and Cl-37.

Different elements with the same mass number A, e.g. argon-40 and calcium-40.

Atoms having the same number of neutrons but different Z and A.

Isotope same protons, isobar same mass, isotone same neutrons.

The unified atomic mass unit (amu or u) — a special unit for very light atoms.

1/12 of the mass of one carbon-12 atom; a C-12 atom is exactly 12 u.

1961 (earlier hydrogen was used).

The same number with unit changed from u to gram.

One mole of atoms equals about 6.022 × 10²³ atoms.

The nuclear force (strong force) binds nucleons together despite proton repulsion.

Short-range, acting only across the tiny distance inside the nucleus.

It is much stronger than the electric (Coulomb) repulsion between protons.

Yes — it acts equally between proton-proton, neutron-neutron and proton-neutron pairs.

The strongest of the four fundamental forces of nature.

The nucleus mass is slightly less than the total mass of its nucleons; this loss is the mass defect.

The released energy that holds the nucleus together.

A more stable nucleus; iron is among the most stable.

Both increase binding energy per nucleon.

E = mc² — the lost mass is converted into energy.

The spontaneous emission of radiation by unstable heavy nuclei, discovered by Henri Becquerel.

Positively charged helium nuclei (2 protons + 2 neutrons) — the positive particle in PYQs.

Fast-moving negatively charged electrons.

Neutral, high-energy electromagnetic waves with the highest penetrating power.

Alpha bends one way, beta the opposite way, and gamma goes undeflected.

SI unit is the becquerel; the older unit is the curie.

The splitting of a heavy nucleus (like uranium-235) into lighter nuclei, releasing huge energy.

Fission produces extra neutrons that split more nuclei.

The atom bomb works on uncontrolled fission; a nuclear reactor uses controlled fission.

Slows down neutrons — e.g. heavy water or graphite. Heavy water acts as both moderator and coolant.

Cadmium or boron rods absorb extra neutrons to control the reaction.

Uranium-235 and plutonium-239.

The joining of light nuclei (like hydrogen) to form a heavier nucleus, releasing enormous energy.

Very high temperature and pressure.

From fusion of hydrogen into helium.

The hydrogen bomb — uncontrolled thermonuclear fusion.

The Lawrence Livermore National Laboratory (USA) claimed a fusion ignition breakthrough.

Fusion releases more energy per unit mass and produces less radioactive waste.

Converts light energy into electrical energy / current.

Packets of energy called photons.

Convert solar energy directly into electric energy.

Automatic doors, burglar alarms and light meters.

Einstein — the emission of electrons from a metal when light falls on it.

Wilhelm Roentgen in 1895.

No — they are high-energy EM waves and are chargeless.

Soft tissue — used to photograph bones and detect fractures.

Medical imaging, airport security and crystal study.

Light Amplification by Stimulated Emission of Radiation.

Intense, highly coherent and monochromatic (single-colour), and highly directional — travels long distances without spreading.

Stimulated emission of light.

Surgery, CD/DVD players, barcode scanners, cutting and communication.

To give relief from body aches and muscle pain (heat therapy).

Radiotherapy to treat cancer (a gamma source).

The treatment of blood cancer (leukaemia).

Treating and studying the thyroid gland.

Radio-carbon dating of old fossils and objects.

To preserve food and to sterilise medical equipment.

High-energy charged particles (mostly protons) reaching Earth from outer space.

Yes — being charged, they are deflected, including by Earth's field.

A rapidly rotating neutron star that emits regular pulses of radiation.

An extremely dense remnant left after a massive star explodes.

A region with gravity so strong that even light cannot escape.

Light, gamma rays and X-rays — about 3 × 10⁸ m/s.

Gamma rays / light.

Alpha particles; gamma rays are fastest.

Positive emitted particle = alpha particle; negatively charged = electron (beta); zero charge = neutron.

gamma > beta > alpha.