The Periodic Table

1. The Periodic Table What it Says How to Read It

2. Atoms • Hopefully you have already seen the Atoms slide show • Briefly, all the complicated matter in the Universe is made from atoms • Neutrinos, quarks, and other sub-atomic particles are exceptions, being smaller than atoms themselves • Every atom of an element is the same, and elements are fundamentally different from each other.

3. Aside from hydrogen, some helium, and a little bit of lithium that were made in the Big Bang, all the elements in the Universe (up to Uranium) have been made in the interior of a star • Attempts to organize these 114 elements (as of 2011) into some kind of table was a major task of 19th century chemists and physicists • They did not know all 114 then! • The 1869 Periodic Table of the Elements by Dmitri Mendeleev (men dell A’ ev) serves as the standard

5. http://www.webelements.com/

6. The elements are arranged in rows (periods) and columns (groups) • Rows have increasing electron configurations up to fully populated shells on the right* • Elements in a column have similar chemical properties • Light elements are at the top, heavy sink to the bottom *later

7. How to read each element • Each element block has four pieces of information • The symbol comes from the original name (Latin, German, etc.) for the element; the English name is below

8. The atomic weight, A, is the mass in grams of one mole of atoms of the element • A mole is like a dozen, only bigger • 6.022 X 1023 items per mole • So a mole of Iron has a mass of 55.845 grams • When rounded to the nearest whole number, it can tell you the average number of neutrons by abundance* *hang on

9. The atomic number, Z, is how many protons an element has • Z makes each element unique • It also gives the element a numerical name, e.g. element 26 is Iron • In a neutral element Z also indicates the number of electrons

10. Hydrogen has 1 proton/electron • Helium has 2 protons/electrons • Potassium has 19 protons/electrons • Krypton has 36 protons/electrons

11. Flavors • While each element has a unique number of protons, Z, each can have a range of N • The differing numbers of neutrons makes a different flavor, called an isotope, of the element • Think chocolate: there’s white, Belgium, dark, Swiss, German—but it’s all chocolate! • When Chemists average out how much of each isotope exists, called its abundance, the atomic weight always turns out to be a number with decimal places

12. To find the number of neutrons, N, round the atomic weight A to the nearest whole number and subtract the atomic number Z from it. The remainder is N, the number of neutrons: N = A – Z • 56 – 26 means typical Iron has 30 neutrons

13. Electron Shells • It’s beyond the scope of our course to lay out all the electron configurations in all the atoms • It’s enough to know that the outer shell, or orbital, of an atom can have at most 8 electrons • Chemistry happens because of <8 electrons in that shell • See a Chemist for an explanation!

14. Just if you’re curious… • All the groups (columns) have the same number of electrons in that outermost shell • That’s what makes them chemically similar • All the periods (rows) start on the left with 1 electron in the outer shell and end with 8 electrons in the outer shell on the right • Inner shells get filled up along the way

15. What about those two rows at the bottom? • They are the lanthanide and actinide series, chemically a little different from the others • You can think of it as the Periodic Table would have to be inconveniently wide to fit them in

16. Let’s look at just these, Z > 92 • These transuranic elements* are made only in the laboratory in small, sometimes infinitesimal quantities • They tend to have very short half-lives, meaning they don’t last long before they shed protons and neutrons and become lighter elements * Technetium and Promethium are also laboratory elements

17. That’s pretty much all you need to know here • So, as we go along, discussing nucleosynthesis and other fun things, it might be a good idea to have a handy copy of the PTOE, (or your text open to that page!)