Page 13 - Chemistry_Form_2

Chemistry
for Secondary Schools

2. Create two circular loops of varying lengths of string or wire to represent the
electron shells. Attach two black beads (electrons) on the first shell, and four
black beads on the second shell. Ensure they are glued and evenly spaced on
the string loops.

3. Label the model and attach the entire model to a cardboard base for stability.
FOR ONLINE READING ONLY

Questions
1. Why are the electrons arranged in different shells?

2. If one more proton is added to the nucleus of this atomic model, would it still
be a carbon atom? Explain.

3. Why were different colours used for protons, neutrons and electrons in the
model?

Electronic arrangement

Task 1.3

Use a reliable interactive simulation to visualise the arrangements of

electrons in an atom and explain how the electrons are arranged.

In 1913, Niels Bohr suggested that electrons rotate around the nucleus in special
regions called shells or orbits. These shells (also known as energy levels) are
at fixed distances from the nucleus. Each shell can only hold a specific number
of electrons. The maximum number of electrons held within each shell can be
determined by the formula 2n , where n is the position of the shell from the
2
nucleus. According to this formula, the:
first shell can hold (2 × 1 ) = 2 electrons;
2
second shell can hold (2 × 2 ) = 8 electrons; and
2
third shell can hold (2 × 3 ) = 18 electrons.
2
The first four shells are represented by the letters K, L, M, and N, respectively
as shown in Figure 1.5. Each electron in an atom is in a particular shell and the
electrons must first occupy the lowest available shell nearest to the nucleus.

6 Student’s Book Form Two

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