# 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.

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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

Bohr Diagrams Fluorine Remember n = mass – p note: neutrons will have decimals

Complete Bohr Diagrams for the First 20 elements of the Periodic Table, write the Symbol of the Element Either Next to the Diagram or in the Center with the Protons and Neutrons (Keep it Neat)

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.

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 HC ONe

Draw Electron Dot Diagrams for the First 20 Elements on the Periodic Table This will be to be handed in.

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: Orbital# of Electrons s2 p6 d10 f14

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

Example - Argon

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: 1s 2 2s 2 2p 4 period orbital # of electrons Electron configuration for Ar is: 1s 2 2s 2 2p 6 3s 2 3p 6

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

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