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Advanced Chemistry Recap – Microscopic World Part 1

Microscopic World Part 1

Student: Vanilla (2026)

Tuesday, 25 August 2026

Isotopes, electronic arrangements, electron-shell filling, periodic-table positions, periodic trends and relative atomic mass calculations.

01

Isotopes and Ions

Nuclear structure and chemical behaviour

Element X has atomic number 12. Two naturally occurring isotopes of X are X-24 and X-26.

  1. Determine the numbers of protons, neutrons and electrons in a neutral atom of each isotope.
  2. Write the electronic arrangement of each neutral isotope.
  3. Explain why X-24 and X-26 are isotopes of the same element.
  4. Explain why the two isotopes have nearly identical chemical properties.
  5. Each atom loses two electrons. State the charge and electronic arrangement of the ions formed.
  6. Explain whether the resulting ions are still isotopic forms of the same element.
  7. Two samples contain equal numbers of atoms. One contains only X-24 and the other contains only X-26. State which sample has the greater mass and explain your answer.
  8. A student claims that X-26 should form a different ion from X-24 because X-26 contains more particles. Evaluate the claim by distinguishing between nuclear particles and electrons.
9 marks
02

Electronic Arrangements

Periodic-table positions and ion formation

Three neutral atoms have the following electronic arrangements:

Atom Electronic arrangement
P 2,8,2
Q 2,8,7
R 2,8,8
  1. Determine the atomic number of P, Q and R.
  2. State the period occupied by each element. Explain your answer.
  3. State the group occupied by each element.
  4. Identify the two atoms most likely to react together.
  5. Explain the electron transfer expected to occur.
  6. Write the electronic arrangements and charges of the ions formed.
  7. Deduce the simplest formula of the compound in terms of P and Q.
  8. Explain why R is much less likely to form an ion.
  9. A student suggests that one atom of P should react with one atom of Q. Explain why this would not produce an electrically neutral compound.
9 marks
03

Error Analysis

Electronic arrangement of potassium

“The third electron shell can hold a maximum of 18 electrons. Therefore, potassium should have the electronic arrangement 2,8,9.”
  1. State the atomic number and number of electrons in a neutral potassium atom.
  2. State the accepted electronic arrangement of potassium.
  3. Explain why the maximum capacity of a shell does not necessarily determine the order in which electrons enter the shells.
  4. Use potassium’s electronic arrangement to determine its period and group.
  5. Predict the ion most commonly formed by potassium.
  6. Write the electronic arrangement of this ion.
  7. Explain why forming this ion gives potassium a more stable electronic arrangement.
  8. Explain why the statement “the third shell is not full” is not sufficient evidence that the nineteenth electron must enter the third shell.
7 marks
04

Periodic Trends

Patterns across a period

A, B, C and D are consecutive elements in Period 3. Their atomic radii are shown below.

Element A B C D
Atomic radius / pm 160 143 118 110
  1. Describe the trend in atomic radius from A to D.
  2. State what happens to the nuclear charge from A to D.
  3. Explain why atomic radius decreases even though the number of electrons increases.
  4. Predict the general change in electronegativity from A to D.
  5. Explain your electronegativity prediction in terms of atomic radius and attraction for electrons.
  6. A student claims that every physical property must change smoothly across a period. Explain why this conclusion is unreliable, using melting point as an example.
  7. All four atoms contain three occupied electron shells. Explain why they do not have the same atomic radius.
8 marks
05

Relative Atomic Mass

Three-isotope calculation

Element Z has three naturally occurring isotopes.

Isotope Relative isotopic mass
Z-63 62.90
Z-65 64.90
Z-67 66.90

The abundance of Z-65 is twice the abundance of Z-67. The relative atomic mass of Z is 64.10.

  1. Let the fractional abundance of Z-67 be x. Write the fractional abundance of Z-65 in terms of x.
  2. Write the fractional abundance of Z-63 in terms of x.
  3. Form a weighted-average equation using the relative atomic mass of 64.10.
  4. Solve the equation to determine x.
  5. Calculate the percentage abundance of each isotope.
  6. Verify your answer by recalculating the relative atomic mass.
  7. Explain why the relative atomic mass is not equal to the simple arithmetic mean of the three isotopic masses.
  8. Explain why writing the answer as 64.10 g would be scientifically incorrect.
  9. Explain what the value 64.10 indicates about the relative abundance of the lightest isotope.
9 marks

Question 1

  1. Isotope Protons Neutrons Electrons
    X-24 12 12 12
    X-26 12 14 12
  2. Both neutral isotopes have the electronic arrangement 2,8,2.
  3. They have the same number of protons but different numbers of neutrons.
  4. They have the same number and arrangement of electrons, including two outer-shell electrons.
  5. Each atom loses two electrons.
    X → X2+ + 2e−
    Electronic arrangement of X2+ = 2,8
  6. Yes. Losing electrons does not change the number of protons or neutrons in either nucleus.
  7. The X-26 sample has the greater mass because each X-26 atom contains two more neutrons.
  8. The claim is incorrect. X-26 contains more neutrons, but ion formation involves electrons. Both isotopes have the same electronic arrangement and both form X2+.

Question 2

    • P: 12
    • Q: 17
    • R: 18
  2. All three are in Period 3 because each atom has three occupied electron shells.
    • P: Group 2
    • Q: Group 17
    • R: Group 18
  4. P and Q.
  5. P loses two electrons. Two Q atoms each gain one electron.
  6. P2+: 2,8
    Q−: 2,8,8
  7. PQ2
  8. R already has a complete outer electron shell.
  9. One P atom forms P2+, while one Q atom forms Q−. A 1:1 combination would have an overall charge of +1. Two Q− ions are required for each P2+ ion.

Question 3

  1. Atomic number: 19; number of electrons: 19.
  2. 2,8,8,1.
  3. Maximum shell capacity and electron-filling order are different. Electrons occupy the available lower-energy levels first.
  4. Potassium is in Period 4 because it has four occupied shells. It is in Group 1 because it has one outer-shell electron.
  5. K+.
  6. 2,8,8.
  7. The ion has a complete outer electron shell.
  8. A shell’s maximum capacity only states how many electrons it can hold. It does not mean that the shell must be completely filled before another shell begins to be occupied. Electron filling depends on energy.

Question 4

  1. Atomic radius decreases from 160 pm to 110 pm.
  2. Nuclear charge increases.
  3. The added electrons enter the same main electron shell while proton number increases. The stronger attraction pulls the electrons closer to the nucleus.
  4. Electronegativity generally increases.
  5. The smaller atomic radius and greater nuclear charge increase the attraction for shared electrons.
  6. Melting point does not necessarily change smoothly because structure and bonding can change across a period. Metallic, giant covalent and simple molecular structures require different amounts of energy to overcome.
  7. The atoms have the same number of occupied shells but different nuclear charges. Increasing proton number strengthens the attraction for the electrons and reduces atomic radius.

Question 5

  1. Fraction of Z-65 = 2x
  2. Fraction of Z-63 = 1 − 3x
  3. 62.90(1 − 3x) + 64.90(2x) + 66.90x = 64.10
  4. 62.90 + 8.00x = 64.10
    8.00x = 1.20
    x = 0.15
  5. Z-67 = 15%
    Z-65 = 30%
    Z-63 = 55%
  6. [62.90(55) + 64.90(30) + 66.90(15)] ÷ 100
    = 64.10
  7. A simple arithmetic mean assumes that all three isotopes are equally abundant. Relative atomic mass is a weighted average.
  8. Relative atomic mass is a ratio and therefore has no unit. 64.10 g is incorrect.
  9. The value 64.10 is closer to 62.90 than to 66.90, indicating that the lightest isotope makes a large contribution to the weighted average. Z-63 is the most abundant isotope.
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