Page 62 - Chemistry_Form_2

Chemistry
for Secondary Schools

Questions
1. What does a single stick represent?

2. How do atoms in the models share electrons to achieve stable molecules?
Indicate the number of shared electron pairs in each molecular model.
FOR ONLINE READING ONLY

Valency
Valency refers to the ability of an atom of a given element to combine with other
atoms, and is measured by the number of electrons that the atom will donate,
receive or share to form a chemical bond. It is the combining power/capacity of
an element or a radical. The combining capacity of an atom of a given element is
determined by the number of hydrogen atoms it combines with or displaces. For
example, the valency of chlorine is 1 because one atom of hydrogen combines
with one atom of chlorine to form hydrogen chloride (HCl). The valency of zinc
is 2 because two atoms of hydrogen are displaced from dilute acids by one atom
of zinc.

It is easy to predict the valencies of elements from the periodic table. Group I
elements have one electron in their outermost shells, and so, their valency is 1.
Group II elements have two electrons in their outermost shells, hence their valency
is 2. Valencies are rarely above 4. For elements with more than four electrons
in the outermost shells, the valency number is usually obtained by subtracting
the number of electrons from eight. For example, sulfur with six electrons in the
outermost shell has a valency of 8 – 6 = 2. Those outermost electrons are called
valence electrons. Some elements have more than one valency. For example, iron
has valencies of 2 and 3, copper has valencies of 1 and 2, lead has valencies of 2
and 4, and manganese has valencies of 2, 4 and 7. Table 3.3 shows the valencies of
some elements and radicals.

Task 3.3
Use ball and stick models or coloured beads to represent electrons in different
shells. Build atoms and determine the valency by observing how many electrons
are in the outer shell.

Radicals

Task 3.4

Use a chemistry simulation or software to identify, classify and write different
formulas of radicals.

Student’s Book Form Two 55

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CHEMISTRY FORM TWO NEW 2025 DUMMY.indd 55