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F. Schifano, Department of Science Bayonne High School, Bayonne NJ
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Describe electrons in detailWrite electron configurationsUse electron configurations
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The valence electrons of an element are the outermost electrons. Only s and p electrons in an atom’s highest energy level count as valence electrons.
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Write the configuration Identify the highest energy level Add up s and p electrons at that level only
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Each side represents a sub-orbital. s-orbitals get both electrons first. Each p sub-orbital gets one e - before any p- sub-orbital gets two. This is called Hund’s Rule.
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Determine valence # Write chemical symbol Draw dots to represent the valence e
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Determine the number of valence electrons in each of the following elements. Then draw their dot diagrams: Ca F Br Sr N O
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Formation of Covalent Bonds Formation of Covalent Bonds When molecules get close to each other, the valence electrons are attracted to the nucleus of the other atom.
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If trading or sharing electrons would make both atoms more stable, this temporary attraction becomes a full-fledged chemical bond.
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If two elements have the same number of valence electrons, they will react in a very similar way. They need similar changes in their electron configuration to become stable.
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Which ones should have similar properties and react in similar ways? Ca F Br Sr N O
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The most stable configuration of electrons most atoms can have is valence = 8, also known as the octet configuration. In chemical reactions, most atoms just take the simplest path to valence = 8.
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Neon, argon, krypton, radon, and xenon are all called noble gases. They already have valence=8. They don’t benefit from reacting, so they don’t react!
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The periodic table is designed in just such a way that elements with the same valence fall into the same vertical column.
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Alkali Metals Group I s 1 Lose 1 e - Alkaline Earth Metals Group 2 s 2 Lose 2 e - Halogens Group 17 s 2 p 5 Gain 1 e - Noble Gases Group 18 s 2 p 6 No Reaction
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Hydrogen becomes stable at valence = 0 or at valence = 2. It can give away an electron, becoming valence =0 (H+) or it can gain an electron, becoming valence =2 (H-). Helium is already stable at valence =2. Like the other noble gases, helium is already stable and doesn’t react. Boron is stable at valence =6.
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An element’s position on the periodic table tells you the last electron that filled its orbitals: Period (horizontal) = energy level Block = orbital shape Box number = #e - in the orbital. Aufbau principle tells you all the rest!
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Sulfur is in the fourth box of the third row, in the p-block. Its last electron is 3p 4. Aufbau principle says everything under 3p 4 must be filled, so: 1s 2 2s 2 2p 6 3s 2 3p 4.
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If an element is in the d-block its last orbital will really be one energy level down from the row it’s in. Last electron in chromium (Cr) = 3d 4, NOT 4d 4 If an element is in the f-block its last orbital will really be two energy levels down from the row it’s in. Last electron in plutonium (Pu) = 5f 5, NOT 6f 5
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Longer configurations are a chore. Noble gases can be used as starting points for longer configurations.
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Find target element on the Periodic Table Choose a noble gas core Write only electrons between core and target
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Cr =1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 4 Ar = 1s 2 2s 2 2p 6 3s 2 3p 6 ____________________________ So we can write the configuration of Cr as [Ar] plus the difference: [Ar] 4s 2 3d 4
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What are valence electrons?
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