1. 组员：唐流洋 龚德雁 莫新田

2. Melting When a substance melts, some of the attractive forces holding the particles together are broken or loosened so that the particles can move freely around each other but are still close together. The stronger these forces are, the more energy is needed to overcome them and the higher the melting temperature. Boiling When a substance boils, most of the remaining attractive forces are broken so the particles can move freely and far apart. The stronger the attractive forces are, the more energy is needed to overcome them and the higher the boiling temperature. The basic concept Trends in melting points and boiling points can be used as a convenient measure of the attractive forces between atoms or molecules. trends

3. The melting and boiling points increase down the group because of the van der Waals forces. This trend from gas to liquid to solid is a dramatic example of the increase in attraction and melting points as the halogen atoms get larger and atomic number increase. Melting and Boiling Points: increases down the group Melting and Boiling Points of Halogens (Gas) (liquid) (solid)

4. Alkali metal Melt Point(K) Boiling Point(K) On the other hand, Group IA metals show the opposite trend, with a decr- ease in melting point and boiling points due to the weakening of metallic bonds between atoms as size increase. F < Cl < Br < I < At The size of the molecules increases down the group. This increase in size means an increase in the strength of the van der Waals forces.

5. Periodic Trends Melting points for second period elements increase from left to right for the first four elements, which are solids, and then drop off sharply to low values for the last four elements, which are gases. Notice that carbon has the highest melting point (4100K) for period 2 elements.

6. The diamond form of carbon is the hardest. Its melting point is higher than that of any other element. Silicon, which is directly below carbon in the periodic table, has the highest melting point for elements in the third period. Within a period, then, melting point first increase, then decrease. Moving from left to right within a row, melting point increase sharply as the attractive forces change from strong metallic bonding with loose electrons to solids like carbon and silicon where outer electrons are tied up in complex network. Melting points of the elements

7. Melting point then drop off sharply for the nonmetals, which have very weak forces of attraction. Besides diamond, mentioned previously, tungsten (W) and other transition metals clustered near tungsten in periods 5 and 6 have very high melting points.