Summary & Quick Quiz ആറ്റത്തിന്റെ ഘടന
The whole chapter on one page: the key boxes of the textbook, every fact and formula you need, the scientists and what they found, the mistakes to avoid, flashcards, a scored quiz and your study checklist.
1. The chapter in one picture
Read it clockwise from the top left: discharge-tube experiments revealed the electron and the proton; the atom models improved from Thomson to Rutherford to Bohr; the nucleus holds the protons and neutrons (Z and A); the electrons fill the shells by 2n²; atoms of one element can differ in neutrons (isotopes); and radioactive isotopes are useful in medicine, industry and dating.
2. The key boxes of the textbook
A molecule is the smallest particle of a substance that has all the properties of the substance and can exist independently. – page 4
The atom has a central part, the nucleus, which is very small compared with the atom. All the positive charge and almost all the mass are in the nucleus. Electrons revolve very fast round the nucleus in circular orbits (solar system model). – page 6
- Electrons revolve in definite orbits.
- Each orbit has a definite energy (energy levels).
- In an orbit the electron’s energy does not change (stationary energy levels).
- The energy of the orbits increases with distance from the nucleus.
- Higher → lower orbit: energy given out; lower → higher: energy absorbed.
- Orbits 1, 2, 3, 4, 5 … = shells K, L, M, N, O.
– page 7
The unified atomic mass unit (u) is the unit used to express the masses of atoms. Proton ≈ 1 u, neutron ≈ 1 u, electron ≈ 0. – page 7
A = protons + neutrons
neutrons = A − Z
– page 8
Different atoms of the same element with the same atomic number and different mass numbers: 11H, 21H, 31H. – page 10
Atoms with the same mass number and different atomic numbers: 4018Ar, 4019K, 4020Ca. – page 10
Geissler (1854) made discharge tubes and suction pumps; Plücker saw a glow opposite the cathode move in a magnetic field; Hittorf (1869) and Goldstein (1876) showed that rays from the cathode cause it. – page 4
Thomson: e/m = 1.76 × 1011 C/kg. Millikan’s oil drop: charge 1.6 × 10−19 C. So mass = 9.1 × 10−31 kg. – page 4
Uranium, thorium … give out radiations by themselves (Becquerel, 1896): α – positive, with mass; β – negative; γ – no charge, no mass. – page 5
The electron cannot be divided further – it is a fundamental particle. Protons and neutrons are each made of 3 quarks, so they are not. – page 7
Atoms with the same number of neutrons: 157N and 146C both have 8 neutrons. – page 10
3. Facts & formulas
📐 Facts & formulas card
maximum electrons in shell n = 2n² · the outermost shell holds at most 8 · fill the lower shell first
symbol: AZX (A on top, Z below), e.g. 3517Cl → 17 p, 17 e, 18 n
| Particle (Table 1.2) | Where | Charge | Mass | Mass used in practice |
|---|---|---|---|---|
| proton പ്രോട്ടോൺ | nucleus | +1 | 1.00727 u | 1 u |
| neutron ന്യൂട്രോൺ | nucleus | 0 | 1.00866 u | 1 u |
| electron ഇലക്ട്രോൺ | shells round the nucleus | −1 | 0.000548 u (1/1837 of a proton) | 0 |
| Shell | K | L | M | N |
|---|---|---|---|---|
| n | 1 | 2 | 3 | 4 |
| maximum 2n² (Table 1.3) | 2 × 1² = 2 | 2 × 2² = 8 | 2 × 3² = 18 | 2 × 4² = 32 |
Electron configurations, Z = 1–20 ഇലക്ട്രോൺ വിന്യാസം
1
2
2, 1
2, 2
2, 3
2, 4
2, 5
2, 6
2, 7
2, 8
2, 8, 1
2, 8, 2
2, 8, 3
2, 8, 4
2, 8, 5
2, 8, 6
2, 8, 7
2, 8, 8
2, 8, 8, 1
2, 8, 8, 2
Z = 1–18 from Table 1.4; potassium and calcium from Fig 1.12 – for elements up to 20 the outermost shell takes at most 8. Higher elements: higher classes.
| Idea | Remember |
|---|---|
| Discharge tube | air is an insulator; at very low pressure and high voltage current flows (electric discharge); the pale-green glow on zinc sulphide behind a perforated anode is caused by cathode rays |
| Cathode rays | travel in straight lines (shadow) · have mass (paddle wheel turns) · negative (bend to the + plate) · bent by a magnet · the same for every gas and metal → electrons (Thomson) |
| Canal (anode) rays | Goldstein 1886, perforated cathode · positive · depend on the gas · hydrogen gives the lightest particle → proton (named by Rutherford) |
| Electron numbers | e/m = 1.76 × 1011 C/kg · charge 1.6 × 10−19 C (negative) · mass 9.1 × 10−31 kg |
| Gold foil (1911) | most α straight → mostly empty space · some bend a little → repelled by a positive part · about 1 in 20 000 bounce back → tiny positive nucleus |
| Rutherford’s limitation | a revolving charged electron should give out energy and spiral into the nucleus – could not explain the stability of the atom |
| Neutron | Chadwick 1932 · no charge · mass about that of a hydrogen atom |
| Isotopes of hydrogen | protium (1p, 0n), deuterium (1p, 1n), tritium (1p, 2n); heavy water D2O is used in nuclear power stations |
| Isotopes of carbon | 12C most abundant and stable · 13C ≈ 1.1 %, studies of life processes · 14C radioactive, age of fossils |
| Uses (Table 1.6) | iodine-131 – thyroid · uranium-235 – nuclear fuel · cobalt-60 – cancer treatment · sodium-24 – pipeline leaks · iron-59 – anaemia |
| Properties of isotopes | same chemical properties; small differences in physical properties |
4. Scientists and discoveries
| Year | Scientist | Discovery or model |
|---|---|---|
| 1854 | Heinrich Geissler | discharge tubes (Geissler tubes) and better suction pumps |
| – | Julius Plücker | a glow opposite the cathode that shifts in a magnetic field |
| 1869 | Johann Wilhelm Hittorf | continued the study: rays from the cathode cause the glow |
| 1875 | William Crookes (1832–1919) | discharge tube with high voltage and gas at low pressure; cathode rays |
| 1876, 1886 | Eugen Goldstein (1850–1930) | studied the rays from the cathode (1876); canal rays with a perforated cathode (1886) |
| 1896 | Henri Becquerel | radioactivity |
| Nobel 1906 | J. J. Thomson (1856–1940) | the electron, its e/m ratio; the plum pudding model |
| – | Robert Millikan | oil-drop experiment: the charge of the electron |
| 1911 | Ernest Rutherford (1871–1937), with Geiger and Marsden | gold foil experiment, the nucleus, the solar system model; named the proton |
| 1913 | Niels Bohr (1885–1962) | definite orbits and energy levels (K, L, M, N) |
| 1932 | James Chadwick (1891–1974) | the neutron |
- Mixing up Z and A, or writing them on the wrong side. In 2311Na the mass number 23 is on top and the atomic number 11 below, both on the left.
- Neutrons = A + Z. Wrong – neutrons = A − Z.
- Putting electrons in the nucleus. The nucleus has only protons and neutrons; electrons are in the shells outside it.
- Saying cathode rays are positive, canal rays negative, or that canal rays come from the cathode. Cathode rays are negative and start at the cathode; canal rays are positive and come from the anode side, through the holes of the cathode.
- “Isotopes are different elements.” Isotopes are atoms of the same element; isobars are atoms of different elements.
- Putting 8 electrons in K, filling M before L is full, or more than 8 in the outermost shell. K holds 2, L holds 8 – and potassium is 2, 8, 8, 1, not 2, 8, 9.
- Writing the mass of the electron as 1 u. It is 0.000548 u – taken as 0 in practice.
- Saying electrons in Bohr’s model lose energy while revolving. That was the problem with Rutherford’s model; in Bohr’s model the energy stays constant in an orbit.
- Thinking the atom is a solid ball. The gold foil experiment showed it is mostly empty space.
- Mixing up the scientists. Thomson – electron, plum pudding; Goldstein – canal rays; Rutherford – nucleus, proton, solar system model; Chadwick – neutron; Bohr – energy levels.
5. Flashcards
Click a card to flip it.
6. Scored quiz
Answer all sixteen, then press Check my answers. Each wrong answer shows the page to revise. For number answers type only the number.
7. Your study checklist
Ticks come from “Mark this page as completed” at the bottom of each page.
Next: the model test paper ▶ – attempt it on paper with a timer first, then check with the solved paper.