4.2
Properties of the elements in period 3
them. Their strength is in the order of S8 > P4 > Cl2. This is the order of their decreasing melting points.
Period 3 element: Argon
silicon
Argon exists as single atoms. The atoms in solid argon are held together by very weak van der Waals forces. They are weaker than those in phosphorus, sulfur and chlorine because argon atoms are smaller. Little energy is needed to break them and argon has the lowest melting point in Period 3.
phosphorus, P4
sulfur, S8
chlorine, Cl2
Figure 9 Silicon has a giant structure (top left), with each silicon bonded to four others in a tetrahedral arrangement (top right). phosphorus, sulfur, and chlorine have molecular structures.
The size of the molecules accounts for the difference between the melting points of phosphorus, sulfur and chlorine. Larger molecules have stronger van der Waals forces, so more energy is needed to break
QUESTIONs 8. Look at the melting points for phosphorus and sulfur in Figure 8. a. Describe the type of bonding in solid phosphorus and solid sulfur. b. Explain the melting points in terms of the forces between molecules.
KEY IDEAS
›› Periodicity is used to describe repeating
patterns of properties at regular intervals in the Periodic Table.
›› The atomic radius of elements decreases across Period 3 as the nuclear charge increases.
›› The overall trend across Period 3 is an increase
increases. The irregular increase across a period is due to the filling of sub-shells.
›› Melting points increase from sodium to silicon, then decrease.
›› Melting points depend on the bonding and structure of the element
in first ionisation energy, as the nuclear charge
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