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Covalent Bonding

Section 8.1. Covalent Bonding. Are electrons only transferred?. While studying ionic and metallic bonding, electrons have been moving between atoms.

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Covalent Bonding

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  1. Section 8.1 Covalent Bonding

  2. Are electrons only transferred? • While studying ionic and metallic bonding, electrons have been moving between atoms. • Atoms have lost or gained electrons as they formed ionic bonds, or atoms have so loosely held their valence electrons that they were free to travel from atom to atom – essentially forming a “sea of electrons” in which metal cations existed. - +

  3. What else is possible? • Instead of transferring electrons, atoms may stay neutral (not form ions) yet still bond together by sharing electrons – this is known as a covalent bond. • This frequently happens when two nonmetals form a bond, such as hydrogen and chlorine. • That pair of electrons is shared between the H and Cl atoms.

  4. What’s it called? • When the bond(s) holding the atoms together in a substance are covalent bonds, the result is a molecule(which results in a neutral group of atoms). • Sometimes atoms of the same element will bond together:

  5. What’s it called? • The molecules shown below have two atoms of the same type forming a molecule – they are called diatomic molecules. • It will be very important in the future to remember the diatomic elements – N, O, F, Cl, Br, I and H (upside down L, plus hydrogen).

  6. Compounds • M0lecules are frequently formed from different atoms covalently bonding together: • These examples shown above are called molecular compounds – the compound arises from covalent bonding (not ionic bonding).

  7. Compounds • Note that nitrogen monoxide is a diatomic molecule, but the rest consist of 3 or more atoms – they are said to be polyatomic. • Every molecule of water is the same: it has 2 hydrogen atoms bonded to an oxygen atom. This gives rise to a molecular formula: H2O

  8. Formulas • Compounds can be either ionic or molecular – it depends upon the type of bonds they contain. • Remember that the formula for an ionic compound is called a formula unit (the simplest ratio of the ions in the compound). • Molecular formulas accurately represent how many of each type of atom is bonded together to form the molecule – you do not simplify the ratio.

  9. Why the difference in Formulas? • Notice that in an ionic compound (such as salt) the ions are attracted to all oppositely- charged ions that are nearby. • However, molecules (such as water) exist as discreet units. It is easy to tell where one molecule of water ends and another begins.

  10. Molecular Formulas • Remember that a subscript written after an element symbol in a chemical formula tells how many atoms of the preceding element will be found in one unit of the substance. • Water is not the only substance composed of just oxygen and hydrogen atoms. Hydrogen peroxide is too:

  11. Molecular Formulas • Although a molecular formula tells you what atoms make up a molecule, they don’t tell you how they are connected. • For that you need structural formulas.

  12. Telling the Difference • Note that both ethanol and dimethylether have 2 carbons, 1 oxygen and 6 hydrogens (C2H6O). However, they are very different compounds due to the differences in the arrangement of the atoms in the molecules.

  13. Depicting a Molecule • Molecules can be represented with different types of models to help you to understand their basic structure. • Here are some of the types of models shown for a molecule of ammonia:

  14. Is it ionic or molecular? • How do we know whether a bond is covalent or ionic? We compare the electronegativities. • If the difference in electronegativity for the two bonded atoms is 2.0 or greater, then the bond is classified as an ionic one. • If the difference is less than 2.0, then the bond will be considered to be covalent. • These cutoff values were arbitrarily determined by scientists.

  15. Property Comparison • Because of the strong electrostatic attraction between ions, ionic bonds are very strong and form in multiple directions. This makes ionic compounds have very high melting and boiling points. • Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds. (We will discuss why this is the case in section 4 of chapter 8.) Realplayer\8.1 Covalent bonds overview.flv

  16. References • http://projectshum.org/wp-content/uploads/Covalent-bond.png • http://3.bp.blogspot.com/-0lmakBuwdIc/TVvctuCdf8I/AAAAAAAAADc/TKzKB8OzE5E/s1600/ionic.bond.jpg • http://www.maryrose.org/learning/science/images/thumb2.jpg • http://images.yourdictionary.com/images/science/AScovale.jpg • http://apbrwww5.apsu.edu/thompsonj/Anatomy%20&%20Physiology/2010/2010%20Exam%20Reviews/Exam%201%20Review/compounds_molecules.jpg • http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter03/Text_Images/FG03_00-03UN2.JPG • http://www.vias.org/genchem/img/valel_dimethylether.png • http://apchemcyhs.wikispaces.com/file/view/Ethanol-structure.png/36853805/Ethanol-structure.png • http://img.tfd.com/mgh/ceb/thumb/Structural-formula-for-x3b1-D-glucose.jpg • http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Hydrogen_peroxide.png/220px-Hydrogen_peroxide.png • http://1.bp.blogspot.com/-OlnUXIHKtDg/Teh5ekJUTjI/AAAAAAAAAD8/UIgoC2JK2PI/s1600/770px-Water-2D-flat.png • http://3.bp.blogspot.com/-pnWFaI3vfzM/TWZlR_l5JFI/AAAAAAAAABA/Z-7-i-nqkW4/s1600/vvvv.GIF • http://0.tqn.com/d/chemistry/1/0/S/S/1/Ammonia.jpg • http://www.gcsescience.com/cyclohexane.gif • http://2.bp.blogspot.com/-csFzqlWDqco/TWJ9yljDbPI/AAAAAAAAAAM/oZnOmrMw9Yw/s748/Ethanol.gif • http://worldbridge.com/images/800px-Dimethyl-ether-3D-balls.png • http://thumbs.dreamstime.com/thumb_2/11011211137LQGhL.jpg • http://www.youtube.com/watch?v=UR4eG60jjQQ

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