Calculating the Atomic Weight of an Element from Isotopic Abundances

Question

The atomic weight of copper, Cu , is listed as 63.546. Which of the following statements are untrue:
More than one may apply.

 

Not all neutral atoms of copper have the same number of electrons.

All the copper atoms have 29 protons in the nucleus.

The number of electrons in the copper atoms is independent of atomic mass.

The dominant isotopes of Cu must be 63Cu and 64Cu .

Copper is a mixture of at least two isotopes.

Answer 1

All copper atoms have 29 protons in the nucleus: True - The element copper is determined by the number of protons in the nucleus. If there are a different number of protons, it is a different element. 

Not all neutral atoms of copper have the same number of electrons: False. If there are more than 29 electrons, copper will hold a negative charge, if there are fewer than 29 electrons, it will hold a positive charge. Since there will always be 29 protons in copper, there will always be 29 electrons in a neutral atom of copper. 

The number of electrons in the copper atoms is independent of atomic mass: True (ish)- the atomic mass is usually determined by the number of neutrons. Isotopes of an element are determined by the number of neutrons. Neutron to electron mass is 1:1000, therefore changing the number of electrons changes the atomic mass negligably.  

The dominant isotopes of Cu must be 63Cu and 64Cu: False. You can't tell dominant isotopes merely by average atomic mass.  The actual dominant isotopes are 63 and 65.  However, there are also many other forms found.  

Copper is a mixture of at least two isotopes: True: because atomic weight is the average of isotopes and relative abundancies.  63.5 means there must be a mixture of isotopes. 

Good luck