The boiling points of group 13 and 14 elements decrease down the group, which is opposite to the trend suggested by van der Waals interactions. The size of the molecules increases down the group. Powered by Create … Generally the melting point of the metals decreases down the group. In group 1 and 2 the melting and boiling point decrease down the group. There is a general decrease in melting point going down group 2. The force of attraction which keep these simple molecules together is temporary dipole-dipole attraction, also termed Van Der Waal’s (VDW) forces of attraction. Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number? However, if you include magnesium, you will see that its melting point is lower than the melting point of calcium, the next element down. If you include magnesium, there is no obvious trend in melting points (see below). F < Cl < Br < I < At. Stronger intermolecular forces/attractions implies that you need to supply more energy hence, an increase in melting and boling points. This increase in size means an increase in the strength of the van der Waals forces. Think of the metal atoms as cations $\ce{M^+}$ and anions $\ce{M^-}$ packed into a … These generally increase with increasing relative molecular mass. IN group 1 and 2 the metal become more reactive down the group and are more readily to give up their electrons to non-metals. This is because as the metal ions get larger the distance between the bonding electrons and the positive nucleus gets larger and reduces the overall attraction between the two. It is seen that more reactive metal have lower melting and boiling points. Melting point. In each case the first member of the set has a significantly higher boiling point than would be expected from the boiling points … Stronger intermolecular forces/attractions implies that you need to supply more energy hence, an increase in melting and boling points. This fact can be best rememberd by the physical states of the halogen molecules. Group 17 elements exist as simple diatomic molecules. Why does volatility decrease down group 7? as u go down a group , with increase in the atomic mass vanderwaal's force of attraction increases, thus increasing the melting and boiling point.Intermolecular forces. Notice how the data for the hydrides of groups 5, 6 and 7 do not follow a smooth trend. Why do melting and boiling points increase down group 7? - Melting points increase - Gas→solid - Volatility decreases. - Size of atoms increases - More electrons - Larger size of molecules - Stronger van der waals forces between molecules - Molecules held together more strongly. 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