1. Chapter 5: The Periodic Law A brief history of the periodic table (put on your fun hats) : Before 1860, more than 60 elements had been discovered. At that time, there was no way to know the atomic mass or atomic number of the known elements, nor was there a way to know the number of atoms in a particular compound. Different chemists used different atomic masses for the same elements, resulting in different compositions being proposed for the same compounds. Madness! Damn you, chemistry! You do not equal awesome!

2. Chapter 5: But wait! In September of 1860, a group of chemists assembled at the First International Congress of Chemists in Germany to settle these issues once and for all. Italian chemist Stanislao Cannizzaro presented a convincing method for accurately measuring atomic masses, and everyone agreed on it. Partying ensued. Perhaps I was harsh. I’m sorry, chemistry. You’re still not awesome.

3. Trends in Atomic Radii Generally Decreases Generally Increases

4. Trends in Atomic Radii So you could sum up the general periodic trend of atomic radii like so: As you go from the upper-right to the lower-left of the periodic table, the size of the atoms increases.

5. Trends in Atomic Radii Generally Decreases Generally Increases

6. Trends in Atomic Radii (cont’d) Group Trends : Atomic size generally increases as you move down a group of the periodic table because of the additional electrons added as you increase in atomic number. Periodic Trends : Atomic size generally decreases as you move from left to right across a period. The principle energy level remains the same, only another electron is added as you move right. The effect of increasing nuclear charge on the outermost electrons is to pull them closer to the nucleus.

7. Trends in Atomic Radii (cont’d) When an atom gains or loses an electron, it becomes an ion . Ionization Energy : The energy required to overcome the attraction of the nuclear charge and remove an electron from a gaseous atom. Removing an electron(s) results in a positive ion, or cation . Adding an electron(s) results in a negative ion, or anion . Electronegativity : of an element is the tendency for the atoms of the element to attract electrons when they are chemically combined with atoms of another element. Each element is assigned an electro- negativity number in units of Paulings.

8. Trends in Ionic Radius Cationic and anionic radii decrease across periods and increase down groups Cations Decrease Anions Decrease Generally Increases

9. Electronegativity The strength of electronegativity tends to increase as you go up and to the right on the periodic table.