Problem Atoms of which two elements have a combined total of 39 protons?
A Nickle, Ni, and copper, Cu
B None of these
CPotassium, K, and calcium, Ca
DNitrogen, N, and oxygen, O
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C) Potassium, K, and calcium, Ca

Explanation:

Given pair of elements:

Ni and Cu

Potassium and Ca

Nitrogen and oxygen

Solution:

We know that number of protons and number of electrons are always equal and number of electron or number of proton is called atomic number.

Number of protons given elements:

Ni number of protons = 28

Copper number of protons  = 29

Combine number of protons = 57

K number of protons  = 19

Ca number of protons = 20

Combine number of protons = 39

Ni number of protons  = 7

O number of protons  = 8

Combine number of protons = 15

Thus,  option C is correct.