Why elements in third and later rows of the periodic table can have expanded octet (have more than 8 electrons)?
This is because elements in third and later rows have empty d-orbital, thus they can use their empty d-orbital to have expanded octet. This can be easily seen through valance shell orbital diagram.
Why is water so special?
Water is a simple molecule consisting of an oxygen atom and two hydrogen atoms. But despite this simplicity, water has fascinating physical and chemical properties. To name a few in terms of chemistry, It can act as both acid and base depending on which compound is present. Another important property of water, that is one of the most important properties of water, is its structure, which makes it a good candidate for both inter-molecular attractions (water molecules have strong affinity for other molecules) and intra-molecular attractions (water molecule have strong affinity for other WATER molecules that are responsible for its high boiling point).
Why are noble gases inert/nonreactive?
Noble gases are inert because they have complete octet (outer shell electrons) therefore they have tendencies for neither accepting nor donating electrons. Bottom line, they are happy and stable as they are and don't need to react!