Kimia Umum (6)

Indikator Menjelaskan Konsep Pembentukan ikatan kimia dan Peranan Elektron Valensi dalam Pembentukan Ikatan Kimia Menjelaskan aturan Oktet dan struktur Lewis serta memberikan contohnya Menjelaskan pengertian ikatan ion, kovalen, kovalen koordinat dan logam Menjelaskan bentuk resonansi Menjelaskan teori VSEPR 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 4

The Octet Rule The Octet Rule All noble gases except He has an s2p6 configuration. Octet rule: atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons (4 electron pairs). Caution: there are many exceptions to the octet rule. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 6

Exceptions to the Octet Rule Central Atoms Having Less than an Octet Relatively rare. Molecules with less than an octet are typical for compounds of Groups 1A, 2A, and 3A. Most typical example is BF3 (Boron triflouride). Formal charges indicate that the Lewis structure with an incomplete octet is more important than the ones with double bonds. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 7

Exceptions to the Octet Rule Central Atoms Having More than an Octet ,[object Object]

Atoms from the 3rd period onwards can accommodate more than an octet.

Drawing Lewis Structures Follow Step by Step Method Total all valence electrons. [Consider Charge] Write symbols for the atoms and guess skeleton structure [ define a central atom ]. Place a pair of electrons in each bond. Complete octets of surrounding atoms. [ H = 2 only ] Place leftover electrons in pairs on the central atom. If there are not enough electrons to give the central atom an octet, look for multiple bonds by transferring electrons until each atom has eight electrons around it. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 10

1. Decide on the central atom; never H. Central atom is atom of lowest affinity for electrons. In ammonia, N is central 2. Count valence electrons H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons or Building a Dot Structure 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 12 Ammonia, NH3 4 pairs

•• H H N H 3. Form a sigma bond between the central atom and surrounding atoms. 4. Remaining electrons form LONE PAIRS to complete octet as needed. 3 BOND PAIRS and 1 LONE PAIR. Note that N has a share in 4 pairs (8 electrons), while each H shares 1 pair. Building a Dot Structure 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 13

Sulfite ion, SO32- Step 1. Central atom = S Step 2. Count valence electrons S = 6 3 x O = 3 x 6 = 18 Negative charge = 2 TOTAL = 6 + 18 + 2 = 26 e- or 13 pairs Step 3. Form sigma bonds 10 pairs of electrons are left. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 14

06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 15 •• O • • • • •• •• O O S • • • • •• •• •• Sulfite ion, SO32- (2) Remaining pairs become lone pairs, first on outside atoms then on central atom. Each atom is surrounded by an octet of electrons. NOTE - must add formal charges (O-, S+) for complete dot diagram

Carbon Dioxide, CO2 1. Central atom = __C____ 2. Valence electrons = _16_ or _8_ pairs 3. Form sigma bonds. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 16 This leaves __6__ pairs. 4. Place lone pairs on outer atoms.

Carbon Dioxide, CO2 (2) 4. Place lone pairs on outer atoms. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 17 5. To give C an octet, form DOUBLE BONDS between C and O. The second bonding pair forms a pi (p)bond.

Sulfur Dioxide, SO2 1. Central atom = S 2. Valence electrons = 6 + 2*6 = 18 electrons or 9 pairs 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 19 3. Form pi () bond so that S has an octet — note that there are two ways of doing this.

Sulfur Dioxide, SO2 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 20 Equivalent structures called: RESONANCE STRUCTURES The proper Lewis structure is a HYBRID of the two. A BETTER representation of SO2 is made by forming 2 double bonds Each atom has - OCTET - formal charge = 0 O = S = O

Urea (NH2)2CO 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 21 1. Number of valence electrons = 24 e- 2. Draw sigma bonds. Leaves 24 - 14 = 10 e- pairs. 3. Complete C atom octet with double bond. 4. Place remaining electron pairs on oxygen and nitrogen atoms.

Sulfur Tetrafluoride, SF4 Central atom = S Valence electrons = 6 + 4*7 = 34 e- or 17 pairs. Form sigma bonds and distribute electron pairs. 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 22 5 pairs around the S atom. A common occurrence outside the 2nd period.

Formal Charge ,[object Object]

There are 4 valence electrons (from periodic table).

In the Lewis structure there are 2 nonbonding electrons and 3 from the triple bond. There are 5 electrons from the Lewis structure.

There are 5 valence electrons.

an atom may lose or gain enough electrons to acquire a filled valence shell and become an ion. An ionic bond is the result of the force of attraction between a cation and an anion.

an atom may share electrons with one or more other atoms to acquire a filled valence shell. A covalentbond is the result of the force of attraction between two atoms that share one or more pairs of electrons.

Ionic Bonding Between atoms of metals and nonmetals with very different electronegativity Bond formed by transfer of electrons Produce charged ions all states. Conductors and have high melting point. Examples; NaCl, CaCl2, K2O 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 27

Electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions. Ionic Bonding 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 29

Covalent Bonding A chemical bond in which 2 atoms share a single of electron to form one bond. Two nonmetal atoms form a covalent bond because they have less energy after they bonded H+H H: H = HH = H2 hydrogen molecule 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 31

Covalent Bonding Between nonmetallic elements of similar electronegativity Formed by sharing electron pairs Stable non-ionizing particles, they are not conductors at any state Examples; O2, CO2, C2H6, H2O, SiC 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 33

Double Covalent Bonding 2 pairs of electrons are shared between 2 atoms Example O2 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 35       O  + O  O::O      double bond

Type of Covalent Bonding ,[object Object]

Shapes of molecules lead to additional properties of covalent compounds

When electrons are not shared equally between two atoms

Nonpolar Covalent Bonding ,[object Object]

Polar Covalent Bonding ,[object Object]

Diatomic Elements Elements that are naturally in molecules with 2 atoms each. HONClBrIF (pneumonic) Existing as diatomic molecule yields a stable octet Gases that exist as diatomic molecules are H2, F2, N2, O2, Cl2, Br2, I2 Examples Fluorine & Bromine 06/01/2011 © 2010 Universitas Negeri Jakarta | www.unj.ac.id | 42

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