For each atom: state the protons, neutrons, and electrons, then draw the diagram. Shells hold 2, 8, 8.
| Element | Atomic number | Mass number |
|---|---|---|
| Helium | 2 | 4 |
| Carbon | 6 | 12 |
| Oxygen | 8 | 16 |
| Sodium | 11 | 23 |
| Magnesium | 12 | 24 |
| Chlorine | 17 | 35 |
| Argon | 18 | 40 |
1. Complete the table with the three particle counts for each element.
Answer
Element Protons Neutrons Electrons Shells Helium 2 2 2 2 Carbon 6 6 6 2, 4 Oxygen 8 8 8 2, 6 Sodium 11 12 11 2, 8, 1 Magnesium 12 12 12 2, 8, 2 Chlorine 17 18 17 2, 8, 7 Argon 18 22 18 2, 8, 8 Neutrons = mass number − atomic number, every time.
2. Draw all seven diagrams.
Check yours
A neutral atom has equal protons and electrons. If yours does not, either you have drawn an ion — say so — or there is an arithmetic slip. Count the electrons in your shells and compare with the atomic number.
3. Which of these have a full outer shell? What does that predict?
Answer
Helium (2 in the first shell, which is full at 2) and argon (2, 8, 8).
Both are noble gases, and both are almost completely unreactive. A full outer shell means an atom has no reason to gain, lose, or share anything.
4. Sodium loses one electron; chlorine gains one. Draw both ions and give each charge.
Answer
— 11 protons, 10 electrons, shells 2, 8. One more positive than negative, so the charge is .
— 17 protons, 18 electrons, shells 2, 8, 8. Charge .
Notice what both now have: a full outer shell. That is the whole reason they did it.
5. Magnesium-24 and magnesium-26 are both magnesium. What differs?
Answer
The number of neutrons — 12 against 14. Protons stay at 12, which is what makes it magnesium at all, and electrons stay at 12, so the chemistry is identical. They are isotopes.
Reference: The Bohr-Rutherford Model
Curriculum connection
C2.3
identify the location, relative mass, and charge of subatomic particles within an atom, using the Bohr- Rutherford model
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