Atom and Dalton's atomic theory
If you keep breaking a chalk stick into smaller and smaller pieces, you wonder: is there a smallest piece that cannot be broken further? Thinkers in India and Greece asked this question more than 2,000 years ago.
An atom (अणू) is the smallest particle of an element that keeps its chemical identity.
In the figure : In the figure of Dalton's picture (1803) the atoms of one element (X) are identical, indivisible and indestructible solid balls, and they differ from the atoms of another element (Y).
Rule 1: Early ideas — Indian philosopher Kanad (6th century B.C.) called the indivisible particles Paramanu. Greek philosopher Democritus (5th century B.C.) named them atom ('atomos', cannot be cut).
- Easy : Kanad's word is Paramanu.
- Medium : Democritus's 'atomos' means 'cannot be cut'.
- Exam-type : Kanad lived in the 6th century B.C. and Democritus in the 5th century B.C., so Kanad was about one century earlier.
Rule 2: Dalton's atomic theory (1803) — Matter is made of atoms. Atoms are indivisible and indestructible. All atoms of an element are alike. Mass is uniformly distributed, like a hard solid sphere.
- Easy : Dalton imagined atoms as tiny hard balls.
- Medium : All hydrogen atoms are alike, but a hydrogen atom differs from an oxygen atom.
- Exam-type : Today we know that atoms can be divided into protons, neutrons and electrons, so 'indivisible' is not exactly right.
Atom: smallest particle. Dalton 1803: solid ball. Kanad: Paramanu; Democritus: atom.
Believing atoms cannot be divided at all. They contain smaller particles.
Memory clue: "Dalton = Dense Dot": a dense solid ball.
Spot the difference
| Person | Year/Time | Idea |
|---|---|---|
| Kanad | 6th century B.C. | Paramanu |
| Democritus | 5th century B.C. | Atom, cannot be cut |
| Dalton | 1803 | Solid ball atom |
Practice questions
- 1What is an atom?
- 2What did Kanad call the smallest particle?
- 3What does 'atomos' mean?
- 4In which year did Dalton give his theory?
- 5What did Dalton think about atoms of one element?
- 6How many years after Dalton was Thomson's model of 1904?
- 7Which part of Dalton's theory is not correct now?
- 8How is Dalton's picture of the atom different from a modern picture?
Show answers (try them yourself first)
- 1The smallest particle of an element that keeps its chemical identity.
- 2Paramanu
- 3Cannot be cut.
- 41803
- 5They are all alike.
- 61904 − 1803 = 101 years
- 7That atoms are indivisible, since they have protons, neutrons and electrons.
- 8It shows a solid ball with no parts, but now we know it has a nucleus and electrons.
Thomson and Rutherford models
Imagine throwing small balls at a covered box and learning the shape inside only by how the balls bounce. Scientists studied the atom this way: they fired particles at it and watched where they went.
The nucleus (अणुकेंद्रक) is the tiny, dense, positively charged centre of an atom that holds almost all of its mass.
In the figure : In the figure Thomson (1904) shows a sphere of positive charge with electrons embedded in it, like a plum pudding. Rutherford (1911) shows a tiny positive nucleus at the centre with electrons revolving around it and empty space in between.
Rule 1: Thomson's plum pudding model — Thomson (1904): positive charge is spread through the atom with negative electrons embedded in it, like plums in a pudding. The atom as a whole is neutral. The mass of an electron is about 1800 times less than a hydrogen atom.
- Easy : Watermelon seeds in the red pulp show the idea of electrons in positive charge.
- Medium : The atom is neutral because positive and negative charges are equal.
- Exam-type : An electron is about 1800 times lighter than a hydrogen atom, so 1800 electrons weigh about as much as one hydrogen atom.
In the figure : In the figure of the gold foil experiment P went straight through, Q was deflected a little, R was deflected through a large angle and S bounced back (about 1 in 20000). A tiny, heavy, positive nucleus explains this.
Rule 2: Rutherford's gold foil experiment — In 1911 Rutherford bombarded a thin gold foil with alpha particles. Most passed straight through, a few were deflected, and about one in 20000 bounced back. He proposed the nuclear model: a tiny, dense, positive nucleus holds almost all the mass, the electrons revolve around it, and there is a lot of empty space in between.
- Easy : Most alpha particles went straight, which means most of the atom is empty space.
- Medium : About 1 in 20000 bouncing back means that out of 100000 particles about 5 bounce back (100000 ÷ 20000 = 5).
- Exam-type : A tiny heavy nucleus can turn back a fast particle, as a heavy stone wall turns back a rubber ball while the wind passes through an open window.
Thomson 1904: plum pudding. Rutherford 1911: nucleus; most of the atom is empty.
Thinking the nucleus is large. It is tiny but holds almost all the mass.
Memory clue: "Plum in Pudding, Point at the centre": Thomson = pudding, Rutherford = point (nucleus).
Spot the difference
| Point | Thomson (1904) | Rutherford (1911) |
|---|---|---|
| Positive charge | Spread over the atom | In a tiny nucleus |
| Electrons | Embedded | Revolve around |
| Atom | Filled | Mostly empty |
Practice questions
- 1What was Thomson's model called?
- 2What did Rutherford bombard on gold foil?
- 3What did most alpha particles do?
- 4Where is almost all the mass of the atom?
- 5Is the atom electrically charged as a whole?
- 6What did the few particles that bounced back show?
- 7How many of 400000 alpha particles bounce back if the rate is 1 in 20000?
- 8Which observation of the experiment shows that most of the atom is empty?
Show answers (try them yourself first)
- 1Plum pudding model
- 2Alpha particles
- 3Passed straight through.
- 4In the nucleus
- 5No, it is neutral.
- 6A tiny, dense, positive nucleus exists.
- 7400000 ÷ 20000 = 20
- 8Most particles passed straight through the foil.
Bohr's model and subatomic particles
Early models were like rough sketches, and each new experiment added a better drawing. Bohr put electrons on fixed 'lanes', like runners on a track, each lane with its own energy.
Atomic number (अणुक्रमांक, Z) is the number of protons. Mass number (वस्तुमान अंक, A) is the number of protons plus neutrons.
In the figure : In the figure the four models in order are Dalton 1803 (solid ball), Thomson 1904 (plum pudding), Rutherford 1911 (nucleus) and Bohr 1913 (fixed orbits).
Rule 1: Bohr's model — Bohr (1913) removed the instability of Rutherford's model. Electrons revolve in stable, fixed orbits with constant energy. Energy is absorbed when an electron jumps outward and emitted when it jumps inward.
- Easy : In Bohr's model each orbit has a fixed energy.
- Medium : Jumping outward from K to L absorbs energy.
- Exam-type : Order of the models: 1803, 1904, 1911, 1913. Gaps: 101, 7 and 2 years.
In the figure : In the figure of Bohr's orbits K, L and M, an electron jumping outwards (K to L in P) absorbs energy, and one jumping inwards (M to L in Q) emits energy.
Rule 2: Orbit jumps — Going to a higher orbit needs energy (absorbed). Coming down to a lower orbit gives out energy (emitted).
- Easy : K to L: energy absorbed.
- Medium : M to L: energy emitted.
- Exam-type : Like climbing stairs needs effort and coming down gives some energy back.
In the figure : In the figure a nucleus has protons (red) and neutrons (grey). Carbon has Z = 6 protons and A = 12, so 12 − 6 = 6 neutrons.
Rule 3: Subatomic particles — Proton: charge +1e, mass about 1 u. Neutron: neutral, mass about 1 u. Electron: charge −1e, negligible mass. Neutrons = A − Z.
- Easy : Sodium: Z = 11, A = 23, so neutrons = 23 − 11 = 12.
- Medium : Oxygen: Z = 8, A = 16, so neutrons = 8 and electrons (neutral atom) = 8.
- Exam-type : Aluminium: Z = 13, A = 27 gives 27 − 13 = 14 neutrons and 13 electrons.
Bohr's model and subatomic particles
In the app you can move this figure with your finger.
Open in the appZ = protons = electrons (neutral atom). A = protons + neutrons. Bohr: fixed orbits.
Counting electrons in the mass number. Electron mass is negligible.
Memory clue: "Neutrons = A minus Z" (N = A − Z).
Spot the difference
| Particle | Charge | Mass |
|---|---|---|
| Proton | +1e | About 1 u |
| Neutron | 0 | About 1 u |
| Electron | −1e | Negligible |
Practice questions
- 1What is the charge of a proton?
- 2What is atomic number?
- 3What is mass number?
- 4How many neutrons does carbon (Z = 6, A = 12) have?
- 5What happens when an electron jumps outward?
- 6Find the neutrons in sodium (Z = 11, A = 23).
- 7Which problem of Rutherford's model did Bohr remove?
- 8An atom has 17 protons and mass number 35. Find neutrons and electrons (neutral atom).
Show answers (try them yourself first)
- 1+1e
- 2The number of protons in the atom.
- 3Number of protons plus neutrons.
- 412 − 6 = 6
- 5Energy is absorbed.
- 623 − 11 = 12
- 7Its instability. Electrons in fixed orbits are stable.
- 8Neutrons = 35 − 17 = 18; electrons = 17.
Electronic configuration and valency
Seats in a cinema row fill from the front. Electrons also fill the nearest shells first, and each shell has a fixed number of seats. The last row decides how the atom behaves.
Electronic configuration (इलेक्ट्रॉनिक रचना) is the way electrons are arranged in the shells. The outermost shell is the valence shell (संयुजा कवच).
In the figure : In the figure sodium (Z = 11) has 11 electrons that fill the K shell (2) and the L shell (8), and the last one goes into the M shell, so the configuration is 2, 8, 1.
Rule 1: Shell capacity — A shell number n can hold 2n2 electrons: K (n = 1) holds 2, L holds 8, M holds 18 and N holds 32. Electrons fill the inner shells first. Helium has a duplet; neon and argon have an octet.
- Easy : K: 2 × 12 = 2; L: 2 × 22 = 8.
- Medium : The third shell holds 2 × 32 = 18 and the fourth holds 2 × 42 = 32.
- Exam-type : Magnesium (Z = 12): 2, 8, 2. Potassium (Z = 19): 2, 8, 8, 1.
In the figure : In the figure valency from the outermost shell: P has 4 valence electrons so valency 4, R has 2 so valency 2, and Q has 7 (more than 4) so valency = 8 − 7 = 1.
Rule 2: Valency rule — Let x be the number of valence electrons. If x is 4 or less, valency = x. If x is more than 4, valency = 8 − x. Example: chlorine 2, 8, 7: valency = 8 − 7 = 1.
- Easy : Sodium 2, 8, 1: x = 1, valency 1.
- Medium : Oxygen 2, 6: x = 6, valency = 8 − 6 = 2. Nitrogen 2, 5: valency = 8 − 5 = 3.
- Exam-type : Silicon (Z = 14) is 2, 8, 4: x = 4, valency 4. Sulphur (Z = 16) is 2, 8, 6: valency = 8 − 6 = 2.
In the figure : In the figure neon (2, 8) and argon (2, 8, 8) have an octet in the outermost shell, so their valency is zero. Helium (2) has a duplet.
Rule 3: Stable atoms — An outer shell with 8 electrons (octet), or 2 in helium (duplet), is complete, so the atom does not react easily. Its valency is zero.
- Easy : Neon is 2, 8: valency 0.
- Medium : Argon is 2, 8, 8: valency 0.
- Exam-type : Atoms with incomplete outer shells react to complete them.
Capacity 2n2: 2, 8, 18, 32. Valency = x if x is 4 or less, else 8 − x. Octet: valency 0.
Saying the M shell holds only 8 electrons. Its capacity is 18, though the outermost shell of the first 20 elements holds at most 8.
Memory clue: "2, 8, 18, 32 = 2 × (1, 4, 9, 16)".
Spot the difference
| Shell | n | Capacity 2n² |
|---|---|---|
| K | 1 | 2 |
| L | 2 | 8 |
| M | 3 | 18 |
| N | 4 | 32 |
Practice questions
- 1What is the capacity of the L shell?
- 2What is the electronic configuration of sodium?
- 3What is the valence shell?
- 4What is the valency of neon?
- 5Find the valency of chlorine (2, 8, 7).
- 6Write the configuration of magnesium (Z = 12) and its valency.
- 7What is the capacity of the fourth (N) shell using 2n2?
- 8Phosphorus has Z = 15. Write the configuration and valency.
Show answers (try them yourself first)
- 18
- 22, 8, 1
- 3The outermost shell.
- 4Zero
- 58 − 7 = 1
- 62, 8, 2; valency 2.
- 72 × 4 × 4 = 32
- 82, 8, 5; valency = 8 − 5 = 3.
Isotopes and nuclear reactor
The same family can have members of different weights. Similarly the atoms of one element can have different masses. Some of these are used in hospitals and power stations.
Isotopes (समस्थानिके) are atoms of the same element with the same atomic number but different mass numbers.
In the figure : In the figure all isotopes of carbon have 6 protons (red) but 6, 7 and 8 neutrons (C-12, C-13, C-14). The isotopes of hydrogen (hydrogen, deuterium, tritium) have 1 proton and 0, 1 and 2 neutrons.
Rule 1: Isotopes — C-12, C-13 and C-14 have the same Z = 6 but different A. Neutrons: 12 − 6 = 6, 13 − 6 = 7, 14 − 6 = 8. Hydrogen has three isotopes: hydrogen (0 neutrons), deuterium (1) and tritium (2).
- Easy : All isotopes of an element have the same number of protons.
- Medium : Chlorine-35 has 35 − 17 = 18 neutrons and chlorine-37 has 37 − 17 = 20 neutrons.
- Exam-type : Isotopes of the same element have the same chemical properties because the electrons are the same.
In the figure : In the figure the uses of isotopes are: Co-60 in cancer treatment, I-131 in goitre, Na-24 to find leaks in pipes, C-14 to find the age of old objects and U-235 as the fuel of a reactor.
Rule 2: Uses of isotopes — U-235: fission fuel for electricity. Co-60: cancer treatment. I-131: goitre. Na-24: leaks in pipes. C-14: age of objects.
- Easy : C-14 is used to find the age of old objects.
- Medium : I-131 is used in the treatment of goitre.
- Exam-type : Radioactive isotopes are used only under expert control because their radiation can harm living cells.
In the figure : In the figure of a reactor, A controller rods (boron or cadmium) absorb neutrons, B fuel rods (U-235), C moderator (graphite or heavy water) slows the neutrons, D coolant (water) takes the heat out and E is the thick outer wall.
Rule 3: Nuclear reactor — Slow neutrons split U-235 nuclei. Moderator: graphite or heavy water slows neutrons. Controller rods: boron or cadmium absorb neutrons. Coolant: water takes heat out. Apsara at BARC, Mumbai (4 August 1956) was India's first reactor.
- Easy : Controller rods keep the reaction steady.
- Medium : Apsara started in 1956, which is 2026 − 1956 = 70 years ago.
- Exam-type : Heat from the reactor makes steam that turns turbines to generate electricity.
In the figure : In the figure of a chain reaction a slow neutron splits a U-235 nucleus, and the neutrons released (2 or 3 in a fission, 3 in the figure) split more nuclei. The controller rods keep the reaction steady.
Rule 4: Chain reaction — One fission releases 2 or 3 neutrons, which split more nuclei. Without control the number grows very fast; controller rods absorb extra neutrons to keep the reaction steady.
- Easy : With 3 neutrons per fission the growth goes 1, 3, 9, 27 in successive steps.
- Medium : After 5 steps there are 35 = 243 neutrons in the idealised case.
- Exam-type : This fast growth is why a reactor must always have working control rods.
Isotopes: same Z, different A. Reactor: fuel, moderator, controller rods, coolant.
Thinking isotopes are different elements. They are the same element with different mass numbers.
Memory clue: "Iso = Same place (Z), different weight (A)".
Spot the difference
| Isotope | Use |
|---|---|
| U-235 | Reactor fuel |
| Co-60 | Cancer treatment |
| I-131 | Goitre |
| Na-24 | Pipe leaks |
| C-14 | Age of objects |
Practice questions
- 1What are isotopes?
- 2Name the three isotopes of hydrogen.
- 3Which isotope is used in cancer treatment?
- 4Which isotope is used to find the age of old objects?
- 5What is the role of the moderator?
- 6Find the number of neutrons in C-14.
- 7Why are controller rods needed in a reactor?
- 8Chlorine-35 and chlorine-37: are they the same element? Compare protons and neutrons.
Show answers (try them yourself first)
- 1Atoms of the same element with the same atomic number but different mass numbers.
- 2Hydrogen, deuterium and tritium.
- 3Co-60
- 4C-14
- 5It slows down the neutrons.
- 614 − 6 = 8
- 7They absorb extra neutrons and keep the chain reaction steady.
- 8Yes. Both have 17 protons; neutrons are 18 and 20.
Quick revision
- Z = protons; A = protons + neutrons; shell capacity = 2n2.
- Thomson 1904, Rutherford 1911, Bohr 1913.
- Isotopes: same Z, different A.