13. Why are the first two groups (columns) called the "s block"? 14. Why are the ten groups in the middle called the "d block"? MODEL 4: VALENCE ELECTRONS AND PERIODIC TRENDS Electrons in the outermost principle shell are called valence electrons. The other electrons are called core electrons. The electron configuration of silicon is 1s22s22p63s23p2. 15. What is the value of n for the outermost principle shell of silicon? 16. How many electrons are in n=3 for silicon? 17. How many valence electrons are there for silicon? 18. How many core electrons does silicon have? 19. Recall that the alkali metals (first column of the periodic table) all demonstrate similar chemical properties. How do their electron configurations compare? 20. As you move across a period (row) in the periodic table, do atoms get bigger or smaller? 21. As you move down a group (column), do atoms get bigger or smaller? 22. As you move across a period (row), does the ionization energy generally increase or decrease? 23. As you move down a group (column), does the ionization energy generally increase or decrease? 24. Which atom has the larger ionization energy, Li or F