why is Au a larger atom than Cu

Answer 1

The fact that it has more protons and is heavier doesn't make it larger, if by larger, you mean the atomic radius.

Au has its outer electrons in the 6th energy lever, which is far from the nucleus. Cu has its outer electrons in the 4th energy level which is closer to the nucleus. This is why Au has a larger atomic radius than Cu.

Answer 2

gold has atomic number 79

copper has atomic number 29

Au has 50 more protons than Cu

79>29

Cu has 34 or 36 neutrons and 29 electrons

and about 63.5 atomic mass units

Au has 118 neutrons and 79 electrons

and about 197 atomic mass units

gold is heavier than copper, with more protons, neutrons, electrons and atomic mass units