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Educated Guess Here!
Since Br-80 does not exist, maybe that means Br-79 or Br-81 have very unequal abundances. For example, Br-79 may have 75% abundance whereas Br-81 may have 25% abundance.
There are 4 significant figures! Start counting after the first non-zero digit :)
Hope this helps.
Answer: 1. Block
2. True ( I'm unsure. He arranged it according to mass, but he is credited for the periodic table)
3. Noble gasses
4. technetium
5. Alkaline earth metals
6. Number of protons
7. False
8. Not true: generally decreases as atomic number increases within a period
9. Argon
10. Four
11. False
12. False
Explanation: