Principles of Chemistry
7 marks · EasyThis question is about atoms, isotopes, and electron shells.
Isotopes of boron are shown by the symbols and . Explain how these two isotopes are alike and how they are different. Refer to subatomic particles in your answer.
State the rule for filling electrons into the shells (energy levels) of atoms, up to the third shell.
Draw a dot-and-cross diagram to show the electronic configuration of a nitrogen atom (atomic number = 7). Show electrons in shells and label each shell.
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(a) —
(b) Electrons fill the lowest available shell first. The first shell holds up to 2 electrons, the second up to 8, and the third up to 8 (for the first 20 elements).
(c) — · must show 1st shell: 2 electrons, 2nd shell: 5 electrons, correct use of dots or crosses, shells shown as circles and labelled
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States what is the same: same number of protons/electrons, same atomic number.
B2 (a) · 1 mark · States what is different: different number of neutrons/mass number.
B3 (b) · 1 mark · States lowest shell/energy level fills first, up to the third shell.
B4 (b) · 1 mark · States 1st holds up to 2, 2nd up to 8, 3rd up to 8 electrons.
B5 (c) · 1 mark · Correct first shell with 2 electrons (dots or crosses).
B6 (c) · 1 mark · Correct second shell with 5 electrons (dots or crosses).
B7 (c) · 1 mark · Shells shown as circles with labels, correct total number of electrons.
Worked solution
a) Isotopes of an element have the same number of protons and electrons, so they have the same atomic number. They differ in the number of neutrons, so their mass numbers are different.
b) The rule for filling electron shells is that electrons fill the lowest available energy level (shell) first. The first shell holds a maximum of 2 electrons, the second holds up to 8, and the third holds up to 8 electrons (for the first 20 elements).
c) Draw three concentric circles to represent shells. Place 2 electrons (dots or crosses) in the first shell, 5 electrons in the second shell. Label each shell as 'first shell' and 'second shell'. Total should be 7 electrons.