Why does atomic radius decrease across a period and increase down a group?

Related questions

1. What are the periodic trends in electronegativity?
2. Where is the highest electronegativity found?
3. What is electron affinity trend?
4. Why is Cl more electronegative than F?
5. Why does electron affinity increase across a period and decrease down a group?
6. What is classification of elements?
7. What are the key features of Mendeleev's periodic table?
8. Does electronegativity increase down a group?
9. What is the trend in melting points across a period of elements?
10. Which is more electronegative C or H?

Tags

1. #radius
2. #increase
3. #decreases
4. #nuclear
5. #valence
6. #strongly
7. #protons
8. #energy
9. #atomic
10. #because
11. #closer
12. #level
13. #period
14. #added
15. #being
16. #nucleus
17. #pulling
18. #attracts
19. #electrons
20. #increasing
21. #charge
22. #across