Structure of the atom practice questions.

In the Geiger–Marsden–Rutherford experiment, a small fraction of the alpha particles were deflected through angles greater than 90°. This observation indicates that:

• the atom's positive charge is spread out perfectly uniformly throughout the whole of its volume
• electrons occupy fixed circular orbits around the atom
• almost all the mass and positive charge of the atom is concentrated in a very small nucleus
• alpha particles are absorbed by gold atoms and re-emitted backwards

Answer: almost all the mass and positive charge of the atom is concentrated in a very small nucleus

Only a tiny, dense, positively charged nucleus can produce the enormous repulsive force needed for large-angle scattering; the Thomson 'plum pudding' model could not explain it.

A sodium nucleus is represented by the notation ²³₁₁Na. The number of neutrons in this nucleus is:

• 11
• 12
• 23
• 34

Answer: 12

The neutron number is N = A − Z = 23 − 11 = 12. Choosing 11 confuses protons with neutrons, and 34 adds A and Z instead of subtracting.

In an atom, an electron makes a transition from an energy level of −1.5 eV to a level of −3.4 eV. The energy of the photon emitted is:

• 1.9 eV
• 4.9 eV
• 3.4 eV
• 1.5 eV

Answer: 1.9 eV

The photon carries away the energy difference between the levels: ΔE = −1.5 − (−3.4) = 1.9 eV. Adding the two magnitudes (4.9 eV) is the standard sign-error distractor.

White light passes through a sample of cool hydrogen gas and is then dispersed by a prism. Compared with the emission spectrum of hot hydrogen, the resulting absorption spectrum shows:

• bright lines at wavelengths different from the emission lines
• a continuous spectrum with no lines at all
• dark lines at exactly the same wavelengths as the emission lines
• dark lines at wavelengths different from the emission lines

Answer: dark lines at exactly the same wavelengths as the emission lines

Absorption occurs for exactly the photon energies matching the atom's energy-level differences, so the dark absorption lines coincide with the bright emission lines of the same element.

An isotope has a half-life of 5.0 days. What fraction of the original sample remains after 15 days? Give the answer as a decimal.

Answer: 0.125

15 days = 3 half-lives. Fraction remaining = (1/2)^3 = 1/8 = 0.125.

A radioactive sample starts with 800 undecayed nuclei. After 2 half-lives, how many undecayed nuclei remain?

Answer: 200

Each half-life halves the number: 800 -> 400 -> 200. After n half-lives N = N0 × (1/2)^n = 800 × (1/4) = 200.