## Some Atoms Can Form Molecules with Expanded Octets [block_0] ## Expanded Octets and Their Connection to the Periodic Table [block_2] Atoms are driven by their quest for stability, often achieved by attaining a noble gas electron configuration, commonly referred to as the **octet rule**. 1. However, some elements in Period 3 and beyond (such as phosphorus, sulfur, and xenon) can exceed this limit. 2. These atoms can accommodate more than eight electrons in their valence shell due to the availability of vacant **d orbitals **in addition to their s and p orbitals. ### Why Only Period 3 and Beyond? [block_4] 1. **Electron Configuration**: 1. Atoms in Period 3 and beyond have a principal quantum number $n \geq 3$, which means they have **d orbitals in their valence shell** (e.g., 3d for Period 3 elements). 2. These d orbitals can **participate in bonding**, allowing the atom to hold more than eight electrons. 2. **Atomic Size**: 1. Larger atoms, like sulfur and phosphorus, have **more space in their valence shell** to **accommodate additional electrons**, reducing electron-electron repulsion. [{"_key":"block_11","_type":"block","asset":null,"children":[{"_key":"f067d9a3c501","_type":"span","markDefs":[],"marks":[],"text":"Elements in "},{"_key":"ed079e832883","_type":"span","markDefs":[],"marks":["strong"],"text":"Periods 1 and 2"},{"_key":"9f280b960ae0","_type":"span","markDefs":[],"marks":[],"text":" cannot expand their octet because they lack d orbitals and have smaller atomic sizes, limiting their ability to accommodate extra electrons."}],"markDefs":[],"style":"normal"}] ## Visualizing Lewis Structures for Expanded Octets [block_12] To understand expanded octets, let’s explore how to draw **Lewis structures **for molecules where the central atom has more than four bonds (or electron domains). ### Steps to Draw Lewis Structures for Expanded Octets: [block_14] 1. **Count Valence Electrons**: 1. Add up the valence electrons of all atoms in the molecule. 2. For ions, adjust for the charge (add electrons for negative charges or subtract for positive charges). 2. **Arrange Atoms**: 1. Place the least electronegative atom in the center. 3. **Distribute Electrons**: 1. Connect atoms with single bonds and distribute the remaining electrons as lone pairs to satisfy the octet rule for peripheral atoms. 4. **Expand the Octet**: 1. Place any leftover electrons on the central atom, allowing it to exceed eight electrons if it belongs to Period 3 or beyond. [{"_key":"680c626fb680","_type":"block","asset":null,"children":[{"_key":"ed35b1e99ed9","_type":"span","markDefs":[],"marks":["strong"],"text":"Phosphorus Pentafluoride (PF₅)"}],"markDefs":[],"style":"h4"},{"_key":"0edfd18e50dd","_type":"block","asset":null,"children":[{"_key":"c99c8a1ffc8f","_type":"span","markDefs":[],"marks":[],"text":"Phosphorus, being in Period 3, can expand its octet."}],"markDefs":[],"style":"normal"},{"_key":"7f37e04f4636","_type":"block","asset":null,"children":[{"_key":"ab2969f61adc","_type":"span","markDefs":[],"marks":["strong"],"text":"Count Valence Electrons"},{"_key":"1a112dd3d564","_type":"span","markDefs":[],"marks":[],"text":": Phosphorus has 5 valence electrons, and each fluorine atom contributes 7. Total = $5 + (7 \\times 5) = 40$."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"72393e85b63c","_type":"block","asset":null,"children":[{"_key":"bb7b77fe47c5","_type":"span","markDefs":[],"marks":["strong"],"text":"Arrange Atoms"},{"_key":"cf41ba1311c5","_type":"span","markDefs":[],"marks":[],"text":": Place phosphorus in the center and bond it to five fluorine atoms."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"61c912e1605e","_type":"block","asset":null,"children":[{"_key":"b96e958e951b","_type":"span","markDefs":[],"marks":["strong"],"text":"Distribute Electrons"},{"_key":"221aaf57dd39","_type":"span","markDefs":[],"marks":[],"text":": Each P–F bond uses 2 electrons, leaving none unused. Phosphorus now has 10 electrons in its valence shell."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] [{"_key":"bcf1ec274c8d","_type":"block","asset":null,"children":[{"_key":"87baa81c850a","_type":"span","markDefs":[],"marks":["strong"],"text":"Sulfur Hexafluoride (SF₆)"}],"markDefs":[],"style":"h4"},{"_key":"57bc5baf9d49","_type":"block","asset":null,"children":[{"_key":"246b75096b69","_type":"span","markDefs":[],"marks":[],"text":"Sulfur, also in Period 3, can expand its octet to accommodate six bonds."}],"markDefs":[],"style":"normal"},{"_key":"398230b00e46","_type":"block","asset":null,"children":[{"_key":"ec5cca8ed602","_type":"span","markDefs":[],"marks":["strong"],"text":"Count Valence Electrons"},{"_key":"49cac48eb381","_type":"span","markDefs":[],"marks":[],"text":": Sulfur has 6 valence electrons, and each fluorine contributes 7. Total = $6 + (7 \\times 6) = 48$."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"0555ade2126c","_type":"block","asset":null,"children":[{"_key":"b6fa8d50dc52","_type":"span","markDefs":[],"marks":["strong"],"text":"Arrange Atoms"},{"_key":"837a2f3300cc","_type":"span","markDefs":[],"marks":[],"text":": Place sulfur in the center and bond it to six fluorine atoms."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"46849f86f470","_type":"block","asset":null,"children":[{"_key":"12d19f730741","_type":"span","markDefs":[],"marks":["strong"],"text":"Distribute Electrons"},{"_key":"23e8eadc5446","_type":"span","markDefs":[],"marks":[],"text":": Each S–F bond uses 2 electrons, leaving none unused. Sulfur now has 12 electrons in its valence shell."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] [{"_key":"block_35","_type":"block","asset":null,"children":[{"_key":"47b875d3f3b0","_type":"span","markDefs":[],"marks":[],"text":"When drawing Lewis structures for expanded octets, always "},{"_key":"f063b9ac58cb","_type":"span","markDefs":[],"marks":["strong"],"text":"prioritize satisfying the octet rule for peripheral atoms"},{"_key":"5a75bd5762d6","_type":"span","markDefs":[],"marks":[],"text":" before expanding the central atom's octet."}],"markDefs":[],"style":"normal"}] ## Predicting Geometry Using the VSEPR Model [block_36] 1. Once the Lewis structure is complete, the **Valence Shell Electron Pair Repulsion (VSEPR) **model helps us predict the molecule's geometry. 2. The shape depends on the number of **electron domains **(bonding pairs and lone pairs) around the central atom. ### Five Electron Domains: **Trigonal Bipyramidal Geometry** [block_38] 1. When a molecule has five electron domains, they arrange themselves in a **trigonal bipyramidal **geometry to minimize repulsion. 2. This geometry consists of: 1. **Three equatorial bonds **at 120° angles in the same plane. 2. **Two axial bonds **at 90° angles to the equatorial plane. #### Molecular Geometries for Five Domains: [block_43] 1. **Trigonal Bipyramidal**: All five domains are bonding (e.g., PF₅). 2. **Seesaw**: Four bonding domains and one lone pair (e.g., SF₄). 3. **T-shaped**: Three bonding domains and two lone pairs (e.g., ClF₃). 4. **Linear**: Two bonding domains and three lone pairs(e.g., XeF$_2$) ![](https://cdn.sanity.io/images/w7j7fz60/production/a0c8a4c930cf096bad8ad06f1a5a2b52cf3b4e05-2862x855.png) ### Six Electron Domains: **Octahedral Geometry** [block_50] When a molecule has six electron domains, they adopt an **octahedral **geometry with 90° angles between all domains. #### Molecular Geometries for Six Domains: [block_52] 1. **Octahedral**: All six domains are bonding (e.g., SF₆). 2. **Square Pyramidal**: Five bonding domains and one lone pair (e.g., BrF₅). 3. **Square Planar**: Four bonding domains and two lone pairs (e.g., XeF₄). ![](https://cdn.sanity.io/images/w7j7fz60/production/e6809609a41f836ebe33495406fc0c37e45a2a5f-2301x861.png) [{"_key":"block_59","_type":"block","asset":null,"children":[{"_key":"30ded1e3d763","_type":"span","markDefs":[],"marks":[],"text":"Do not confuse"},{"_key":"305f451fa8d0","_type":"span","markDefs":[],"marks":["strong"],"text":" electron domain geometry"},{"_key":"345e95759ee7","_type":"span","markDefs":[],"marks":[],"text":" (which includes all electron pairs) with "},{"_key":"9c3ac7a40faa","_type":"span","markDefs":[],"marks":["strong"],"text":"molecular geometry"},{"_key":"f263d5f81037","_type":"span","markDefs":[],"marks":[],"text":" (which considers only bonding pairs)."}],"markDefs":[],"style":"normal"}] [{"_key":"block_77","_type":"block","asset":null,"children":[{"_key":"rDwF3A94nQhRe8FKU2smI9","_type":"span","markDefs":[],"marks":[],"text":"Can you draw the Lewis structure of BrF₅ and predict its molecular geometry using the VSEPR model?"}],"markDefs":[],"style":"normal"}] ## [block_68]