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2. D, because transition metals have a high density.

3. B, because the properties of elements depend on the valence electrons.

4. C, because the atom emits a negatively charged electron, the atom becomes positively charged.

5. D, because hydrogen and helium are both lighter than air, but hydrogen is very reactive, that's why helium is used.

6. A, because it creates caustic soda.

7. C, because only fluorine and argon are gases, but argon is not two-molecular.

8. C, because iodine is either solid or gaseous. Solid iodine is gray and gaseous iodine is purple.

9. C, because all elements of the first main group have an odd number of protons and react violently with water.

10. D, because oxygen is air. Nitrogen is very reactive and helium is lighter than air.

11. A, because carbon has atomic number 6 and magnesium has atomic number 12.

12. B, because table salt is sodium chloride.

13. B, because sodium ions and neon have the same electron configuration.

14. C, because chloride ions and argon have the same electron configuration.

15. C, because calcium ions and argon have the same electron configuration.

16. D

17. C, because silicon has 4 valence electrons and a total of 14 electrons.

18. A

19. B, because it's sodium.

20. Number of protons and electron shells

21. first main group

22. seventh main group

23. cesium

24. fluorine

25. alkali metals

26. alkaline earth metals

27. halogens

28. from metal to nonmetal

29. valence electrons

30. electron shells

31. main group and subgroup

32. semi-metal

33. 18 groups and 7 periods

34. groups

35. periods