Page 14 - Chemistry_Form_2

Chemistry
for Secondary Schools

FOR ONLINE READING ONLY

Figure 1.5: First four shells of an atom

For reasons beyond the scope of this book, the 3 shell is more stable with
rd
8 electrons. This is why even though the 3 shell can hold up to 18 electrons,
rd
potassium element which has 19 electrons has only 8 electrons in its 3 shell, and
rd
the last electron moves to the 4 shell. This is the same for calcium which has 20
th
electrons, where the last two electrons move to the 4 shell after the 3 shell is
rd
th
completely filled with 8 electrons.

A shell which contains its maximum number of electrons is called a fully-filled
shell. An atom with fully-filled outermost shell is said to be stable. Some atoms
hold a maximum of 2 electrons (helium) or 8 electrons (neon) in their outermost
shells. The elements with a maximum of 2 electrons in their outermost shells

are said to exhibit a duplet state, while those with 8 electrons are said to exhibit
an octate state. Electrons are arranged so that the lowest shells are filled first.
This arrangement of electrons in different shells in an atom is called electronic
configuration. Figure 1.6 shows the diagrammatic electronic configurations of

hydrogen, helium, neon, potassium, and sulfur atoms.

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