The mass of piece of sterling silver jewelry is 33.24 g. It contains 92.5% silver Ag by mass. Since, sterling silver is an alloy of Ag-Cu thus, percentage of Cu will be:
% Cu=100-92.5=7.5%
Thus, mass of copper will be:
Molar mass of Cu is 63.546 g/mol, thus, number of moles of Cu can be calculated as follows:
Here, m is mass and M is molar mass.
Putting the values,
Now, in 1 mole of Cu there are 
thus, in 0.03923 mol, number of Cu atoms will be:
Thus, number of atoms of Cu will be
.
Answer:
Explanation:
In photoelectric effect , radiation of some energy is made to fall on metal plate which results in the ejection of electrons by the metal plate . The kinetic energy of electrons comes from the energy of radiation falling on metal plate . Some of energy of radiation falling on metal plate is used in the process of bringing the electron to the surface and it is called threshold energy . The radiation must have at least this energy to see to it that electrons are ejected . and the rest of the energy of radiation is used in imparting kinetic energy to the electron .
The red light radiation has least energy so it is least likely to eject electrons from metal plate and produce photoelectric effect .
Hellium argon neon xenon krypton randon oxygen fluorine chlorine bromine
In order of reactivity: magnesium, iron, copper, silver
