The elements of aluminum, Iron, Oxygen, and Silicon make up about 88 percent of the earth's solid surface. The air is 99 percent nitrogen and oxygen. Hydrogen, oxygen, and carbon make up 97 percent of a person. Thus almost everything you see in this picture is made up of just six elements. Rev

Symbol Ave. Atomic Mass Atomic Number

EXAMPLE How many protons, neutrons and electrons are found in an atom of Cs Atomic number = protons There are 55 protons (and 55 electrons!) Mass number = sum of protons and neutrons 133 – 55 = 78 neutrons (note: if looking at only 1 atom, there is no average, so Ave Atomic Mass = Mass #)

H Hydrogen 1 1 Protons: 1 Neutrons: 0 Electrons: 1

Na Sodium Protons: 11 Neutrons: 12 Electrons: 11

Rhenium Re Protons: 75 Neutrons: 111 Electrons: 75

Rhenium isotope Re Protons: 75 Neutrons: 112 Electrons: 75

Periodic Patterns –The chemical behavior of elements is determined by its valence electrons –Energy levels are quantized. so roughly correspond to layers of electrons around the nucleus. –Each period begins with a new outer electron shell (energy level)

–Each period ends with a completely filled outer shell that has the maximum number of electrons for that shell. –The outer shell electrons are responsible for chemical reactions, and are called valence electrons. –So now…explain WHY atoms get smaller as you move to the right on the periodic table, and what they do as you move up the periodic table, and why (so need to explain 2 mechanisms since they are not the same).

There are 2 stages to an atom’s life: 1. Atoms are “born” on the periodic table with the same number of protons and electrons, so they are neutral. 2. Then they grow up and become ions, so they gain or lose electrons and have a charge.

Ions –When an atom gains or loses an electron it becomes an ion. A cation has a positive charge (the “t” is like a “+”) An anion has has a negative charge

–(Groups 1, 2, or 3). Elements with 1, 2, or 3 electrons in their outer shell tend to lose electrons to get a full outer shell, and become cations. These are the metals which always tend to lose electrons. –Elements with 5 to 7 electrons in their outer shell tend to gain electrons to fill their outer shell and become anions. These are the nonmetals which always tend to gain electrons.

Q: What would the charge be on a sodium ion? EXAMPLE A: Since sodium is in the first Group, it has 1 valence e-, and would want to get rid of it to get a complete valence shell Group 1A elements lose 1 electron So the charge would be +1 *Remember an electron is negatively charged. When you lose them the atom becomes positively charged… when you gain them it becomes negatively charged

Q1: How many TOTAL electrons is beryllium, Be “born” with (when neutral)? Q2: How many valence electrons is beryllium, Be “born” with (when neutral)? Be A1: It is #4 on periodic table, so it has 4 protons and 4 electrons. A2: It is in the 2 nd Group, and the pattern of the periodic table is based on valence e-’s, so it has 2 valence electrons

Q: Will Be want to gain or lose e-’s to become happy? Q: How many valence electrons will Be want to gain or lose? Q: What charge will it have when it becomes happy? EXAMPLE Be A: Lose A: It is in the 2 nd Group so it has 2 valence e-’s. It needs a full valence shell, so it will give away the 2 e-’s which will leave it with the (full) shell underneath. A: losing 2 e-’s makes it +2

Q1: How many TOTAL electrons is oxygen, O, “born” with? Q2: How many valence electrons is oxygen, O, “born” with? O A1: 8 protons, so 8 electrons (it’s neutral). A2: It is in the 6th Group (we don’t count the transition metals), and the pattern of the periodic table is based on valence e-’s, so it has 6 valence electrons.

Q: Will O want to gain or lose e-’s to become happy? Q: How many valence electrons will O want to gain or lose? Q: What charge will it have when it becomes happy? EXAMPLE O A: Gain A: It is in the 6th Group so it has 6 valence e-’s. It needs a full valence shell (8), so it will gain 2 e-’s which will give it 8 valence electrons (8 is great). A: Adding 2 e-’s makes it -2 charge (an anion).

Q1: How many valence electrons does sodium have on the periodic table? A: 1 Q2: How would you write the charge symbol for the sodium CATION? EXAMPLE Na +1