Understanding Electrons

It is the arrangement of electrons within an atom that determines how elements will react with one another and why some are very reactive while others are not.

Shells Electrons orbit the nucleus in regions called shells. Shells are areas that surround the nucleus like layers around an onion. Each shell is a certain distance away from the nucleus. The first shell is closest to the nucleus; the second is further away.

Shells correspond to the periods: Period 1 has one shell Period 2 has two shells Period 3 has three shells, etc.

Each shell can hold a certain number of electrons and no more. The first shell can hold a maximum of 2 electrons The second shell can hold a maximum of 8 electrons The third shell can hold a maximum of 8 electrons

The First Shell – 2 e-

The Second Shell - 8 e- N

Note Note: when filling up shells with electrons, 1. fill lower shells first (less energy) 2. spread electrons out before doubling them up.

You draw the third shell. Remember, it has 8 electrons as well.

The Third Shell - 8 e- N

Complete Electron Shell Exercise p. 16

Valence Electrons

The outer shell is called the valence shell. Electrons in the outer shell are therefore called valence electrons. Valence electrons are important because they determine the element’s properties and how it will react. It is only the valence electrons that are involved in chemical reactions.

An atom with a complete valence shell is very stable and does not tend to react with other elements. For this reason, the noble gases are very stable. This also explains why the alkali metals and halogens are so reactive.

Electron Dot Diagrams Electron dot diagrams (also called Lewis diagrams) are useful because they show only the valence electrons, since these are the important ones anyways. They include the element’s symbol and dots to represent the valence electrons.

Electron Dot Diagrams H C O Ne

Practice Draw electron dot diagrams for the first 20 elements in the periodic table in prep sheet – p18.

Ions

Atoms are electrically neutral, that is, they have the same number of protons (+) as electrons (-). However, atoms are most stable when their valence shell is full. To obtain a full valence shell, atoms will lose or gain electrons, whichever takes less energy. This upsets the neutral charge and creates atoms with either a positive or negative charge. These are called ions.

Anions (“an-eye-on”) When a neutral atom gains an electron, the resulting negatively charged ion is called an anion. We know from our definition of non-metals that they will gain electrons, and thus usually form anions.

Consider Cl Cl atom has 17 protons and 17 electrons. It has 7 valence electrons. If it gains one more, then it’s outermost shell will be full. Therefore, Cl becomes more stable by gaining an e- 17+

Consider Cl Cl now has 17 protons and 18 electrons, producing a charge of 1–. The resulting anion is written as Cl– – 17+

This gain of e- can be expressed in an equation: Cl + e-  Cl- Oxygen atoms will gain 2 e- to obtain a full outer shell. The equation is: O + 2e-  O2- Anions are named by dropping the ending and adding –ide to the end of the element name Example, Cl- is chloride ion, O2- is oxide ion.

Cations (“cat-eye-on”) When a neutral atom loses an electron, the resulting positively charged ion is called a cation. We know from our definition of metals that they will lose electrons, and thus usually form cations.

Consider Na Na atom has 11 protons and 11 electrons. It has only 1 valence electron. If it loses it, then it’s outermost shell will be the 2nd shell, which is full. Therefore, Na becomes more stable by losing an e- 11+

Consider Na But with 11 protons and 10 electrons, Na will have a charge of 1+. The resulting cation is written as Na+ + 11+

This loss of e- can be expressed in an equation: Na  Na+ + e- Magnesium atoms will lose 2 e- to obtain a full outer shell. The equation is: Mg  Mg2+ + 2e- Aluminum forms a 3+ cation by losing 3 e-. The equation is: Al  Al3+ + 3e-

Important Note Ions are always formed when electrons are either lost or gained. Ions are never formed by changing the number of protons. An atom by itself will not gain or lose e- to form an ion. It has to come in contact with another atom so that a transfer of e- may occur.

Making Predictions Will e- be lost or gained? How many? K Rb Ca Be F Br S

Complete ion table p 22 in prep notes

Orbitals and Electron Configuration Welcome to Chemistry 11 – New Material Please copy this down

Orbitals Electrons spin around the nucleus creating an electron cloud. The electron clouds come in 4 different shapes, called orbitals. The four orbitals are called s, p, d, and f.

Each orbital is capable of holding different numbers of electrons: S (sharp) 2 P (principal) 6 D (diffuse) 10 F (fundamental) 14

Video - Orbitals Video - Orbitals

Energy Level Diagram

Rules for Filling in Energy Level Diagrams Fill lower energy levels before filling higher energy levels Distribute electrons across the orbital before pairing them up Show opposite spins on the electrons with up and down arrows

Example – Oxygen – 8 electrons

Example – Argon – 18 electrons

Practice, Practice Draw energy level diagrams for: Li, N, F, Mg, S, Cl K, Fe, Zn, Br, Ag, U

Electron Configuration and the Periodic Table Groups 1 and 2 represent the s orbital Groups 13-18 represent the p orbital Groups 3-12 represent the d orbital Lanthanides and Actinides represent f orbital

Electron Configurations Electron configurations are a shorthand for writing exactly what was in the energy level diagrams. Electron configuration for O is: 1s22s22p4 # of electrons period orbital Electron configuration for Ar is: 1s22s22p63s23p6

Practice, Practice Write electron configurations for the 12 elements that you just drew energy level diagrams for.

Energy Level Diagram Worksheet Can you identify the element from this energy level diagram?

More Practice Worksheet for q’s 4 and 5 you will need a special periodic table.

Assessment – Electron Configuration and Dot diagrams