In the beginning of this week's parsha, the sum total of all the metals used for the mishkan is given. Ibn Ezra (38:24) and many other commentaries point out that the total of the gold is given without telling us what the gold was used for. But when the totals of the silver and copper are given, we are told exactly what they are used for.
A number of answers are offered. Ramban answers that the silver and copper were used for parts made completely of silver and copper so their exact weights could be measured out in order to count how much was used. The gold, however, was often used to coat different utensils such as the mizbei'ach or the aron and thus it was not possible to weigh out the amount of gold used for each utensil.
Netziv, in Ha’amek Davar, explains that the gold was used for the more holy parts of the mishkan such as the covering of the aron and the menorah and it would not have been respectful to weigh these items on a scale and thus, none of the gold was weighed.
Meshech Chachmah points out that while all the silver and copper that was used was already mentioned in Vayakheil, the making of the vestments of the kohen gadol were yet to be described and thus, the gold had not yet been finished so the Torah could not yet give a full account of its uses.
R' Moshe Shternbuch explains in Ta'am VaDa'as that the Midrash Tanchuma (7:4) tells us that the fools of the generation were accusing Moshe of taking some of the metals for himself and thus, Moshe gave a full account of all the totals. R' Shternbuch points out that the giver of gold obviously gave with more generosity than the giver of silver. Thus, the really generous givers were so pure-hearted that they didn't demand an account of where their money had gone. Those who gave only silver or copper, however, were more stingy and thus, demanded to know where every last penny (for the copper givers, silver coin for the silver givers) went.
CHAZAK, CHAZAK, veNISCHAZEIK!
Have a good Shabbos.
Mishenichnas Adar marbim be'Simchah!