Astrontium-90 atom that has a lost two electrons has protons, neutrons, and electrons.

A strontium-90 atom that has a lost two electrons has 38 protons, 52 neutrons, and 36 electrons.

Explanation:
Atomic number of Strontium (Sr) is 38. 
Atomic number = number of protons 
Hence, Strontium has 38 protons.

If the element is in neutral state,
                number of protons = number of electrons. 
Then, neutral Strontium atom should have 38 electrons. 
But the question says Sr has lost 2 electrons. Hence, number of electrons should be 38 - 2 = 36.

Mass number = number of protons + number of neutrons.

The given mass number is 90. Hence, number of neutrons should be 90 - 38 = 52.

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