Metal to Non-Metal
Most striking trends across Periodic Table are:

metals to non-metals
solid to gas

Na, Mg and Al are clearly metals, but Si is much harder to classify.

Si has the shiny appearance of a metal, but is brittle. It conducts electricity, but very poorly.
Silicon is an in-between element, usually classified as a semi-metal or metalloid.

Trends in Melting and Boiling Points
Although we can identify broad trends just from the appearance of the elements, we can learn more about the bonding and structure of the elements from their melting and boiling points.
Group 1 -> 4 general increase in boiling points
Group 4 to 5 - sharp decrease in boiling points
Group 5 to 8 - comparitely low boiling points

There is a distinct change between Group 4 to 5 in both the physical structure of the elements and the forces holding the structures together. The change is:

from giant structures to simple molecular structures
from strong forces to weak forces

The trend in melting point is similar to the trend in boiling point. There is a sharp decrease between Group 4 to 5 marking the change from giant to simple molecular structures.

The melting and boiling points of the metals Na, Mg and Al in Period 3 increase across the period.

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Undergraduate Chemistry - Part 1 (The Periodic Table) Notiz am Periodicity: Boiling Points, erstellt von siobhan.quirk am 20/05/2013.

Periodicity: Boiling Points