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Chapter 9 Bonding of atoms

Chapter 9 Bonding of atoms. A model of bonding. All atoms have attractive and repulsive forces between them. A bond forms only if the attractive forces are greater than the repulsive.

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Chapter 9 Bonding of atoms

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  1. Chapter 9 Bonding of atoms

  2. A model of bonding • All atoms have attractive and repulsive forces between them. A bond forms only if the attractive forces are greater than the repulsive. • If you know the bonding, then you can predict the many physical properties. One may know the properties, but predicting the bonding maybe difficult due to the lack of a clear cut division between ionic and covalent compounds

  3. Electronegativity • The measure of the ability of an atom to attract electrons. • It increases as you move from left to right on the periodic table (number of proton increase with the same energy level distance.. • It decreases at you move down a column due to the shielding effect (inner energy levels block the attraction of the nucleus for the valence electrons)

  4. Ionic • The greater the difference between electronegativities of the bonding atoms, the unequally the electrons are shared. • Delta EN (difference in ElectroNegativity) is calculated by subtracting the smaller electronegativity form the larger so the delta EN is always positive. • Difference greater than 2.0 the sharing is so unequal, we assume that the electron is transferred to the more electronegative atom

  5. Calculating delta EN • NaCl: Cl – 3.0, Na - .9 delta EN (3-.9 = 2.1) • LiF : F – 4, Li – 1 delta EN (4-1= 3) • KBr: Br – 2.8, K - .8 delta EN (2.8 - .8 = 2) • LiCl – • KF-

  6. Covalent • Electronegativity different is less than or equal to .5, the molecular is considered a covalent bond. Slight unequal sharing doesn’t have much effect on the properties. • Pure covalent – when delta EN is equal to 0. The same atoms together (diatomic) and different atoms if same EN. • F2 : F-4 (4-4=0)

  7. Polar Covalent • Delta EN is between .5 and 2 • The electrons are shared but unequally with the more electronegative having them more. This gives it a partial charge of negative (delta minus symbol). • The less electronegative has a partial positive charge (delta positive symbol).

  8. water • H2O : O -3.5, H – 2.1 (3.5 – 2.1 = 1.4) • Water has polar bonds, gives it higher boiling point compared to similar molecules with more molecular weight.

  9. Metallic bonding • Bonding in metals doesn’t result in the formation of compounds, but an interaction that holds the atoms together. Metals release their valence electrons into a sea of electrons shared by all of the metal atoms. The bond that results from this shared pool is a metallic bond. • This gives the properties of malleable, ductile, and conductivity

  10. Shapes of molecules • Models can help one visualize the 3-D structures of molecules. • Diatomic molecules – Linear (lie in a straight line). They can only fit one way.

  11. water • Water – H bent at 105* angle (off from 109.5) • Valence Shell Electron Pair Repulsion (VSEPR) – hybridization (mixing) of oxygen’s 2s and 2p orbitals. The 4 orbitals combine to form a tetrahedron shape. (2-filled, 2 – ½ filled) • Electron dot – non bonding pairs of electron are called lone pairs. They play a major role in determining the shape of a molecule. Tetrahedron keeps lone pair repulsion at min.

  12. Other molecules • Carbon Dioxide: Triatomic molecule but linear due to double bonds (keeps the electron clouds the farthest apart, less repulsive action) • Ammonia – Triangular pyramid (N-H) with bond angle of 107* due to lone pair in top position. • Methane – tetrahedral arrangement, bond angles of 109.5*

  13. Other molecules con’t • Ethane (C2H6) – 2 connecting tetrahedral arrangements around the 2 carbon atoms • Ethene (C2H4)– double bond Carbons. Flat, triangular arrangement. • Ethyne (C2H2) – (acetylene) triple bond carbons. Linear

  14. Polar bonds and geometry • Water is a polar molecule with a positive and negative pole. A polar molecule is also called a dipole. • Ammonia has negative on N and positive on the H. • Carbon dioxide – nonpolar – polar bonds oppose each other and cancel each other effects.

  15. Physical properties • Polar water has higher boiling and melting point than nonpolar methane. • Ionic – high melting points • Polar covalent – depends on bond strength and arrangement. • Pure covalent – usually gases

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