Diagramming Electrons

If we can’t see the electron, and it has no mass, then why is it important? BONDING! Electrons are the only particle that participate in bonding between atoms. How do we know when and how things will bond, though? Diagrams help us to know which electrons will bond.

There are 2 kinds of diagrams that we will discuss – Bohr diagrams show ALL electrons in the atom. – Lewis diagrams show VALENCE electrons in the atom. We will find out about valence electrons later.

Bohr Diagrams Every atom has an ENERGY LEVEL – this is basically the number of rows down the element is found on the periodic table. The number of energy levels tells us the number of rings we need around the atom. Each ring can have up to 8 electrons. – How do we know?

ACTIVITY! We are eating Fruit Loops today. Each person will receive a plate, a worksheet, and a baggie of cereal. In the center of every plate, draw a dot representing the nucleus. The cereal will represent our electrons, so DON’T EAT THEM JUST YET! For each element, fill in the correct number of electrons, starting with the center ring, and moving outward. The center ring only holds 2 electrons. Every other one holds 8. What do you notice?

K7xw K7xw What do these elements have in common? – Why are they different? How might this information be useful? – Can we use this to predict the behavior of other atoms? Which electrons are important in these reactions? – If only there was some way to look at JUST the outermost electrons….

Lewis Diagrams Lewis diagrams, unlike Bohr diagrams, show only the outermost electrons (the outer ring). These electrons are called VALENCE ELECTRONS. Why this ring? Why is it so special? This ring is used in bonding. These electrons are the electrons that interact with other atoms to make SCIENCE happen!

How To: Draw a Lewis Diagram 1.Put the element’s symbol in the center. 2.Find the number of VALENCE ELECTRONS. – How do we find those, again? 3.In clockwise order, add the electrons one at a time. – Why not in pairs yet? N

Let’s look at an argon (Ar) atom. How many VALENCE ELECTRONS does argon have? Ar

Practice! Draw out the electron diagrams (Lewis diagrams) for each of the following atoms: – Aluminum (Al) – Germanium (Ge) – Chlorine (Cl) – Strontium (Sr) – Francium (Fr) – Tin (Sn)

Periodic trends Lets look at the arrangement of the periodic table now. Notice how each element has some similar characteristics with the things around it. What do you see? As it turns out, there are LOTS of trends in the periodic table. But before we begin looking at these trends, why don’t we break the table into categories first?

Categorizing the Periodic Table On your periodic table, follow along as the teacher identifies groups of elements to label. – Metals – Nonmetals – Metalloids – Alkali metals – Alkaline earth metals – Halogens – Noble gases – Group number

Atomic Size As discussed, the nucleus of the atom is VERY small. – As a result, most of the size in the atom comes from the electrons around the nucleus. The more electrons, the bigger the atom, generally speaking. BUT! What about those OCD atoms that REALLY want to pick up one more electron?

Electronegativity Let’s give a name to that OCD tendency of our electrons – electronegativity. ELECTRONEGATIVITY is the tendency for an atom to attract other electrons toward itself. As the atom gets bigger, it gets harder to “hold on” to those electrons, so the trend is the opposite of atomic size.

Bonding Here comes the fun part! Electrons do all the bonding, so let’s see some of it! There are 3 kinds of bonds, but we’re going to talk mainly about IONIC and COVALENT bonds. Remember: – IONIC BONDS are chemical bonds where there is an unequal sharing of electrons. – COVALENT BONDS are chemical bonds where a pair of electrons is shared evenly between two atoms.

Ionic bonds “Unequal sharing of electrons creates an ionic bond.” – What does that even mean? – Remember electronegativity? Sometimes an atom which “likes” electrons more bonds with an atom that “likes” electrons much less than itself. In this case, the electron(s) are pulled literally off of the atom that wants it less and given to the “thirstier” atom.

NaCl Which atom “wants” electrons more? Where does the electron go? What happens to the atoms’ charges? Are they neutral anymore? + -

IONS are charged atoms. They can be either positively or negatively charged. A positively-charged ion is called a CATION. A negatively-charged particle is called an ANION.

How do the electrons move in the following example? LiBr What are their final charges? Which ion is the cation and which is the anion?

Quick Break! – Be the Element In groups of 8 (teacher decides ), you will be given one card apiece. Your number is the number of valence electrons you have. Hold it to your forehead, facing out. Your objective: order yourselves like you would on the periodic table as quickly as possible. The catch: you can’t talk. At all.

Let’s look at a more challenging example: CaO What is different in this example than the previous ones? Do you think this concept carries over to other atoms?

Examples Sodium (Na) + Oxygen (O) Boron (B) + Hydrogen (H) Calcium (Ca) + Chlorine (Cl) Iron (Fe) + Oxygen (O) Why don’t we do D-block metals? That’s why.

Li, BeCl Na, MgS K, CaN Rb, SrBr Cs, BaAl 1.Draw the Lewis structures 2.Show where the electron moves to during a reaction. 3.Write the final charges. 4.Which one is the cation/anion? First partner is the alkali metal. Second partner is the alkaline earth metal.

AlO What would their charges be? How many of each would we need to create a balanced molecule? What is the formula of our balanced molecule? How can we describe this process? This is called a chemical equation – we will be balancing these later. 2 Al O 2-  Al 2 O 3

C H H H H What is the “charge” on carbon? What is the charge on hydrogen? How many hydrogen atoms would it take to satisfy this carbon?

How do we label this molecule? Think about water – H 2 O. Water has 2 hydrogen atoms and 1 oxygen atom. – How should we name this molecule? When we look at the atoms in the previous example, we find that carbon and hydrogen are actually similar when it comes to electronegativity (they both want electrons about the same). This bond isn’t ionic – it’s covalent.

Multiple atoms, not +/- 1 charges, electronegativity rules, examples

Covalent bonds