Answer:
A strontium-90 atom that has a lost two electrons has 38 protons, 52 neutrons, and 36 electrons.
Step-by-step 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.