Key Differences:

* Number of Protons: This is the defining characteristic that differentiates one element from another.

* Copper (Cu) has 29 protons.

* Nickel (Ni) has 28 protons.

* Number of Neutrons: The number of neutrons can vary within an element, leading to isotopes.

* The most common isotope of copper (Cu-63) has 34 neutrons.

* The most common isotope of nickel (Ni-58) has 30 neutrons.

* Atomic Mass: The atomic mass is primarily determined by the number of protons and neutrons.

* Copper has an atomic mass of approximately 63.5 amu.

* Nickel has an atomic mass of approximately 58.7 amu.

Similarities:

* Location in the Periodic Table: Both copper and nickel are transition metals located in the same period (Period 4) and close to each other on the periodic table.

* Similar Chemical Properties: Due to their close proximity on the periodic table, copper and nickel share some similarities in their chemical properties.

In Summary:

The nucleus of a copper atom is larger and heavier than the nucleus of a nickel atom due to the presence of one extra proton and typically more neutrons. This difference in nuclear composition accounts for the slightly different chemical properties and atomic masses of the two elements.