2.1
Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
Sulphur, gold and bromine look nothing alike. Your book says elements differ because their atoms differ. Democritus first named the atom, and around 1800 John Dalton's experiments gave evidence that atoms exist.
In discharge tube experiments, a very high voltage was passed through a gas at very low pressure. In 1897, J. J. Thomson saw cathode rays bend towards the positive plate, so they are negative. These particles were named electrons.
In 1886, Goldstein used a tube with a perforated cathode and found positive canal rays (anode rays). In 1917, Rutherford showed the hydrogen nucleus, the proton, is inside other nuclei. A proton is 1836 times heavier than an electron.
Your book says the neutron was found in 1933; it has no charge and almost the same mass as a proton. In 1911, Rutherford's gold foil experiment showed a tiny central nucleus with an extra nuclear portion around it.
The nucleus holds the protons and neutrons, bound by the strong nuclear force. It is less than 1/100,000 of the atom's size, yet it has over 99.9% of the mass. Ordinary hydrogen is the only atom with no neutron.
In 1913, Bohr said electrons move round the nucleus in fixed orbits, or shells, each with a fixed energy. The nearest shell (K, n = 1) has the lowest energy; that is the ground state. Electrons cannot sit between shells.
Shells farther out (L, M, N) have more energy. Shell n holds at most 2n2 electrons: K 2, L 8, M 18. Shell n also has n sub-shells: K has s; L has s and p; M has s, p and d; N has s, p, d and f.
Sub-shells s, p, d and f hold at most 2, 6, 10 and 14 electrons. The modern view treats electrons as charged clouds: we can only give the probability of finding one at a certain distance from the nucleus.
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Sulphur, gold and bromine look nothing alike. Your book says elements differ because their atoms differ. Democritus first named the atom, and around 1800 John Dalton's experiments gave evidence that atoms exist.
In discharge tube experiments, a very high voltage was passed through a gas at very low pressure. In 1897, J. J. Thomson saw cathode rays bend towards the positive plate, so they are negative. These particles were named electrons.
In 1886, Goldstein used a tube with a perforated cathode and found positive canal rays (anode rays). In 1917, Rutherford showed the hydrogen nucleus, the proton, is inside other nuclei. A proton is 1836 times heavier than an electron.
Your book says the neutron was found in 1933; it has no charge and almost the same mass as a proton. In 1911, Rutherford's gold foil experiment showed a tiny central nucleus with an extra nuclear portion around it.
The nucleus holds the protons and neutrons, bound by the strong nuclear force. It is less than 1/100,000 of the atom's size, yet it has over 99.9% of the mass. Ordinary hydrogen is the only atom with no neutron.
In 1913, Bohr said electrons move round the nucleus in fixed orbits, or shells, each with a fixed energy. The nearest shell (K, n = 1) has the lowest energy; that is the ground state. Electrons cannot sit between shells.
Shells farther out (L, M, N) have more energy. Shell n holds at most 2n2 electrons: K 2, L 8, M 18. Shell n also has n sub-shells: K has s; L has s and p; M has s, p and d; N has s, p, d and f.
Sub-shells s, p, d and f hold at most 2, 6, 10 and 14 electrons. The modern view treats electrons as charged clouds: we can only give the probability of finding one at a certain distance from the nucleus.
Electrons (negative) are outside; protons (positive) and neutrons (no charge) make the tiny, heavy nucleus. Electrons sit in shells of fixed energy, and shell n holds at most 2n2 electrons.
Q1. How many electrons can the L shell and the M shell hold at most? How does the L shell share its electrons between sub-shells?
L holds 8 (2 in s, 6 in p); M holds 18.
Q2. How did Thomson show that cathode rays are made of negative particles?
Attraction to the positive plate shows negative charge; the measured mass to charge ratio shows they are particles: electrons.
✗ The nucleus has most of the mass, so it must fill most of the atom.
✓ It is less than 1/100,000 of the atom's size; the heavy protons and neutrons are packed into it.
✗ An electron far from the nucleus has less energy than one close to it.
✓ It is the other way round: K has the lowest energy (the ground state), and each shell farther out has more.
✗ We can draw the exact path of every electron, like a planet's orbit.
✓ The modern view gives only a probability of finding the electron at a certain distance from the nucleus.
1. What information was obtained from discharge tube experiments?
(c) Exercise MCQ (ii) (p. 29): the tubes showed electrons and protons. The neutron came later, and the nucleus came from the gold foil experiment.
2. How many electrons can be accommodated at the most in the third shell?
(b) Exercise MCQ (i) (p. 29): n = 3, so 2 × 32 = 18.
3. What keeps the particles present in the nucleus intact?
(a) Exercise MCQ (viii) (p. 30); p. 21: protons and neutrons are held by a strong nuclear force.
4. Goldstein's canal rays (anode rays) showed the presence of which particle?
(d) p. 20: in 1886 Goldstein found positive rays, including hydrogen ions, which are protons.
5. Which sub-shells does the second shell (n = 2) have?
(b) p. 21: shell n has n sub-shells, so the second shell has two: s and p.
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