Why does electron affinity increase across a period and decrease down a group?

Related questions

1. What is the trend for electron shielding across a period?
2. What is the trend in electronegativity from left to right across a period?
3. What are the trends of modern periodic table?
4. Does boiling point increase down a group?
5. What is classification of elements?
6. What are the properties of modern periodic table?
7. What is the trend for density on the periodic table?
8. Why does Z effective increase down a group?
9. What is the trend in melting points across a period of elements?
10. Why does the metallic character increase as you go down a group?

Tags

1. #across
2. #energy
3. #nucleus
4. #valence
5. #periodic
6. #electrons
7. #electron
8. #placed
9. #because
10. #affinity
11. #least
12. #decreases
13. #table
14. #level
15. #likely
16. #right
17. #decrease
18. #groups
19. #higher
20. #periods