Just remember that Isotopes are atoms that have the same number of protons as they do in their neutral state, but have more neutrons.
Remember this:
Mass number = protons + neutrons
neutrons = mass number - protons
protons = atomic number
electrons = protons
Neutrons and protons are found in the nucleus and the circle around it is an orbit, that is where you put the number of electrons.
Let's take your first question:
a. Carbon: Atomic number is 6 and the mass number is 12.
neutrons = 12 -6 (atomic number = protons)
= 6
So draw 6 neutrons in the nucleus and 6 protons in the middle of the circle. And write 6e electrons outside the circle.
b. a. Carbon: Atomic number is 6 and the mass number is 13
Here you will see something different:
neutrons = 13 -6 (atomic number = protons)
= 7
Notice that the number of neutrons changed. All you do is instead of drawing only 6 neutrons, draw 7 and draw 6 protons.
The name of the atom is Carbon-13 because the number 13 indicates the mass number.
2. The explanation above will lead to the answers for this question. Let's knock out some of those choices.
Compare the 2 carbon atoms, which changed and what stayed the same. The number of protons stayed the same, and so did the atomic number. What changed was the mass number because the number of neutrons changed so your answer would be the third selection.
3.a. Again, take note of the given:
Chlorine: Atomic number is 17 (that stays constant!) the mass number is 35. Name of the atom is Chlorine-35
Neutrons = 35 - 17 17 is the number of protons which is equal to the
= 18 atomic number.
So you will draw 17 protons and 18 protons in the middle of the circle and write 17e outside the circle.
3.b. Name of the atom is Chlorine-37
Chlorine: Atomic number is 17 (This was given already) and mass number is 37.
REMEMBER atomic number = protons
Neutrons = 37 - 17
= 20
So you will draw 20 neutrons and 17 protons inside the circle and on the line of the circle you will write 17e outside the circle.
Why is the electrons the same as protons because there is no sign to indicate that the atoms are ions so they are all neutral which means that the number of electrons is equal to number of protons.
