Covalent Model and Related Topics

This lesson develops Covalent Model and Related Topics as part of Measurement and Data Processing. It uses the source curriculum to organize the important facts, relationships, examples, and vocabulary into a focused Stemosphere teaching sequence.

In Stemosphere terms, covalent Model and Related Topics

In Stemosphere terms, covalent bonding explains how many non-metal atoms join to form molecules and giant covalent structures. In acovalent bond, electrons are shared rather than transferred. This sharing lower s the overall energy because the bondingelectrons are attracted to more than one nucleus. The topic becomes more complex when we move beyond single bondsand simple molecules, because we thenmustthink about Lewisstructures, shapes, polarity, resonance, formalcharge, sigmaand pi bonding, and hybridization. These ideas are linked, so understanding one part helps with the rest.

In Stemosphere terms, a covalent bond is the electrostatic attraction between a shared pair of electrons and the nuclei of the atoms involved inthe bond. Covalent bonding usually occurs between non-metals. Each atom contributes at least one electron to theshared region, although in acoordinatebondboth electrons may come from the same atom.

In Stemosphere terms, when atomic orbital soverlap, a molecular orbital is formed, and the electron density is concentrated between the two nuclei. This pulls thenuclei together andcreatesa stable bond.

In Stemosphere terms, covalent bonding is directional, which is why molecules have definite shapes. In simple molecules, themain featurestoconsider arethe number of shared pairs, lone pairs, and the relative electronegativity of the atoms involved. In giantcovalent structures, the same basic idea of strong shared electron pairs applies, but the bonding extends through a verylarge network instead of existing as separate molecules.

• Covalent bonding happens because shared electrons are attracted to two nuclei.
• Single, double, and triple bonds differ in the number of shared electron pairs.
• The octet rule is a useful starting point, but there are important exceptions such as incomplete octets and expandedoctets.
• The electrons in a covalent bond should be thought of as a charge cloud, not as a fixed pair sitting still between the atoms.

In Stemosphere terms, lewis structures are simplified diagrams that show valence electrons, bonding pairs, lone pairs, charges, and sometimesformalcharges. They are the starting point for predicting shape and polarity. The usual method is to count total valenceelectrons, draw the skeletal structure, place one pair of electrons in each bond, complete octets on outer atoms, and thendecide whether multiple bonds are needed.

In Stemosphere terms, hydrogen only needs two electrons, while some central atoms can havefewer than eight or more than eight depending on the molecule.

In Stemosphere terms, vSEPR stands for Valence Shell Electron Pair Repulsion theory. It says that electron domains around a central atomarrange themselves as far apart as possible tominimizerepulsion.

In Stemosphere terms, lone pairs repel more strongly than bonding pairs, sotheycompress bond angles. Multiple bonds count as a single domain for shape prediction, although they can still slightlyaffect actual bond angles because electron density is higher.

In Stemosphere terms, a coordinate covalent bond, also called a dative bond, forms when both bonding electrons are donated by the sameatom. This usually happens when one species has a lone pair and another species is electron-deficient and has an emptyorbital or an incomplete shell.

In Stemosphere terms, after the bond forms, it behaves like any other covalent bond; the difference is only in howthebondformed.A classic example is the formation of NH4+ from NH3 and H+. Nitrogen in ammonia has a lone pair. The hydrogen ionhas no electrons and can accept a pair. Nitrogen donates its lone pair to H+, forming the fourth N-H bond.

In Stemosphere terms, a single bond contains one shared pair of electrons, a double bond contains two shared pairs, and a triple bond contains

In Stemosphere terms, three shared pairs. For the same pair of atoms, bond strength increases from single to double to triple, while bond length

In Stemosphere terms, decreases from single to double to triple. This is because greater electron density between the nuclei produces stronger

In Stemosphere terms, attraction and pulls the atoms closer together.

In Stemosphere terms, for carbon-carbon bonds, a single bond is the longest and weakest, a double bond is shorter and stronger, and triplebonds arethe shortest and strongest. This is important in both structure and reactivity. For example, the strong C≡C bond in

In Stemosphere terms, bond polarity depends on electronegativity difference. If two atoms have the same electronegativity, the bonding

In Stemosphere terms, electrons are shared equally and the bond is non-polar. If one atom is more electronegative, the electron density is pulled

In Stemosphere terms, toward that atom, producing a polar covalent bond. The more electronegative atom becomes partially negative (delta

In Stemosphere terms, minus), and the less electronegative atom becomes partially positive (delta plus).

In Stemosphere terms, examples include H-Cl, O-H, and C-F. The idea is not that electrons are fully transferred, as in ionic bonding, but that theyare shared unequally. Bond polarity is one step in predicting how a molecule behaves, but bond polarity alone is notenough to decide if the whole molecule is polar.

In Stemosphere terms, molecular polarity depends on both bond polarity and molecular shape. A molecule is polar when the individual bonddipoles do not cancel out, so there is a net dipole moment.

In Stemosphere terms, a molecule is non-polar wheneither thebonds are non-polaror the polar bonds are arranged symmetrically so that their dipolescancel.

In Stemosphere terms, cO2 is a good example of a non-polar molecule with polar bonds. Each C=O bond is polar, but the molecule is linear, sothe dipoles point in opposite directions and cancel.

In Stemosphere terms, h2O is polar because its O-H bonds arepolarand the bent shapemeans they do not cancel.

In Stemosphere terms, nH3 is also polar because the trigonal pyramidal shape and lone pair create an unevenelectron distribution.

In Stemosphere terms, bF3 is non-polar even though B-F bonds arepolar, because its trigonal planar shape is symmetrical.

In Stemosphere terms, rule for predicting molecular polarity

• Draw the Lewis structure.
• Use VSEPR to find the shape.
• Work out whether the bonds are polar.
• Decide whether the bond dipoles cancel or produce a net dipole.

In Stemosphere terms, covalent network structures, also called giant covalent structures, are made of atoms joined by strong covalent bondsthroughout a giant lattice. There are no separate molecules. This gives themvery differentproperties from simplemolecularsubstances.

In Stemosphere terms, diamond consistsof carbon atoms, each bonded tetrahedrally to four other carbon atoms. It is extremely hard, has avery highmelting point, and does not conduct electricity because all valence electrons are used in sigmabondsand there arenomobile charged particles. Graphite also consists of carbon, but each carbon is bonded to three others in layers.

In Stemosphere terms, thefourthvalence electron on each carbon isdelocalizedwithin the layer, so graphite conducts electricity along the layers.

In Stemosphere terms, silicon dioxide is another giant covalent structure, with a three-dimensional network of Si and O atoms and a high meltingpoint.

In Stemosphere terms, the reason these substances have high melting points is that melting requires many strong covalent bonds to be broken,not just weak intermolecular forces. Graphite is soft despite strong covalent bonds because the layers can slide over eachother; only weak forces act between layers.

In Stemosphere terms, intermolecular forces are attractions between molecules. They are much weaker than covalent bonds within molecules,but they strongly affect meltingpoint, boilingpoint, volatility, viscosity, and solubility.

In Stemosphere terms, the main types relevant hereare Londondispersion forces, dipole-dipole interactions, and hydrogen bonding.

In Stemosphere terms, london dispersion forces arise from temporary dipoles caused by momentary uneven electron distribution. They arepresent in all molecules andatoms, butare the only intermolecular force in non-polar substances. They become strongeras electron cloud size, number of electrons, and surface area increase.

• Hydrogen bonding > dipole-dipole > London dispersion (for similar-sized molecules).
• Intermolecular forces are weaker than covalent bonds, so they affect phase changes but not usually chemical bond
• Larger molecules often have higher boiling points because London forces increase with size.

strongly to a polar stationary phase and therefore move less. Rf values can be calculated using:

The formal charge formula is: