PS-2.4    Use the atomic number and the mass number to calculate the number of protons, neutrons,  and/or electrons for a given isotope of an element.  
The mass number of any atom is the sum of its protons and neutrons. In other words: 
Mass Number = Protons + Neutrons 
Neutrons = Mass Number - Protons
Protons = Mass Number - Neutrons
# of electrons = # of Protons <-- ( This is true for atoms that are electrically neutral. These atoms are said to be in the ground state or lowest energy state. In this state the valence electrons are in the lowest energy level they can be in.)
Example problem: Suppose an atom has a mass number of 23AMU and it has 12 Protons.
A. What element is it?
B. How many Neutrons does the atom have?
C. How many electrons does this atom have?
Answer: A. This atom is an isotope of the element Magnesium. ( I know this because the atomic number of an isotope is equal to the number of Protons.)
Answer: B. This isotope of Magnesium has 11 Neutrons. ( I know this because Neutrons = Mass Number 23AMU - Protons 12.)
Answer: C. This atom has 12 electrons. ( I know this because in the ground state, electrically neutral atoms have the same number of electrons as Protons.)
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