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I missed my lesson on periodicity so had to teach myself, and have always forgotten to ask my teacher to explain to me why these trends are seen, which, unfortunately, the textbooks don't.

Density: the density of an element tends to increase across a period up to group 3, and then starts to decrease and become very low at group 5 or so. Why is this?

Melting and boiling point: they tend to rise up to about group 3 or 4, before falling rapidly after that. I would suggest that this is because group 4 elements tend to have giant covalent structures, so also a higher boiling point, whereas on the right hand side of a period they are simple molecular structures, so have a low boiling point. Am I correct?

Atomic size: atomic size always decreases along a period from left to right. My suggestion is that this is because there are more electrons and more protons as the group increases, so there becomes a stronger attraction between them, pulling the electrons closer to the nucleus. Am I right?

So the only one I have no idea about really is density, and the other two I just need clarification on.

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For 2. I would add, on top of what you already have, that the elements start as metallic elements, hence we have metallic bonding. Across the period, the number of electrons increase, meaning there are more delocalised electrons among the metal cations so that electrostatic attraction increases along the period. For period 2, that's the trend.

For period 3 however, it is a bit more tricky because of the transition metal elements and I think Nicolau explains it better than I could in this answer. But I don't think that you will be required to know those for your level, but it's still interesting for your own knowledge. :)

For 3. I would add that the number of electron shells containing electrons remains constant. If it increased instead, the atomic radius would increase as well.

And you could use 2. and 3. to explain 1:

First, we observe that the atomic mass increases along the period, and that the atomic radius decreases. For giant structures, this generally means that the density increases ($\rho = \dfrac{m}{V}$ and while $m$ increases, $V$ is decreasing slightly, so $\rho$ increases)

When we hit the simple molecular structures, the density sharply falls again, because of the average distance between the atoms of the element has drastically risen, so the volume increases. With $m$ increasing gradually and a sharp rise in $V$, you get a sharp decrease in $\rho$.

These observations change down the groups though. For instance, Astatine is a solid, so you won't expect a huge fall in density across period 6 around group 5. But for the purpose of your syllabus coverage (I'm assuming High School), those should be enough.

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  • $\begingroup$ Your first paragraph needs slightly more detail, as it suggests that melting and boiling points would increase monotonically when going through a period's transition elements. $\endgroup$ – Nicolau Saker Neto May 16 '13 at 1:45
  • $\begingroup$ @NicolauSakerNeto Correctly pointed out. I didn't think much about it since my own syllabus coverage didn't talk much about it due to its complexity at that time ^^; And I looked around a bit and found your own answer to the issue, so I added a link to it in there. $\endgroup$ – Jerry May 16 '13 at 6:55
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When comparing the densities of the elements across the period, we need to take into account the factors that affect the density such as atomic mass, atomic radius and crystal structure. Since each of these factors vary from one element to the next we expect to see a periodic trend in density. As you move from left to right across period 3, there is an increase in the density until a maximum (at Si in Group IV) and begins to decrease moving towards Ar. 

 The atomic radius determines the volume of the elements and the crystal structure tells you how closely packed the atoms in an element are to each other. At the maximum density, the atomic radius is low and the strength of the bonding (metallic or giant covalent bond) is at a maximum. After this point, the densities begin to decrease even though the atomic radii are low and the atomic masses are higher. This decrease in density occurs because the elements on the right side of the period form simple molecular structures which are held together by weak Van der Waals forces.

The trend in the melting point across the period can be linked to the structure of the element. There is an increase in the melting points from Na to Si. Na, Mg and Al are metals and the melting points increase as the strength of the metal bonds increase due to the increased number of delocalised electrons. Si has the highest melting point of the group because of its big molecular structure.

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