HELP!! A friend notices that a nail that was left outside for a few months seems larger and heavier than it was before. He says it disproves the law of conser- vation of mass. Explain why he is wrong

It's common for nails to be made of iron, which is a metal that rusts when it comes into contact with air. Exposure to the elements for a long time caused oxygen-iron reactions when there was moisture around. This caused a chemical reaction that made rust, which is made of iron oxide.
Here, the oxygen that mixed with the iron to make rust adds weight to the thing.

They are all full and don't need any more electrons in there outer shell.

Density and pressure