Protons, electrons, & neutrons (chemistry)...?

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Another science exam review question!

Q: Determine the number of protons, electrons, and neutrons for the following atoms:
• Chlorine
• Calcium
• Magnesium

thanks ♥, xo
 
For these kind of problems you will need a periodic table. Chlorine (Cl)
is atomic number 17 on the periodic table. The atomic number of an element is the number of protons in its nucleus. So chlorine has 17 protons. In a neutral atom, the number of protons=the number of electrons because each proton has a plus charge and each electron has a negative charge. Chlorine therefor has 17 electrons. To determine the number of neutrons, you need to know which isotope of the element you are looking at. If this isn't given then round up the atomic mass of the element and use that. The atomic mass is the number at the bottom of the element's box. For chlorine this is 35.45. So let's call it 35. The atomic mass of an isotope of an element is equal to the number of protons plus the number of neutrons. For chlorine-35 there are 18 neutrons because 35-17=18.
 
You'll need a periodic table for this. If you consider chlorine (Cl) first, you'll notice that there are two numbers written above/below it.

The smaller of these, a whole number, is the 'atomic number' and is the number of protons in the atom. If the atom is neutral, then this number is also the number of electrons in the atom so that the charges balance to zero

The larger of these is the 'mass number', and is equal to the total mass of the atom. Each proton and neutron has a mass of around 1 atomic mass unit, so if you subtract the atomic number from the mass number you get the number of neutrons in the atom

Therefore:
Chlorine - 17 protons, 17 electrons, 18 neutrons
Calcium - 20 protons, 20 electrons, 20 neutrons
Magnesium - 12 protons, 12 electrons, 12 neutrons
 
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