concepts of chemical bonding ( konsep-konsep dalam ikatan kimia )

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Concepts of Chemical Bonding

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Chemical BondsThree types:– Ionic

• Electrostatic attraction between ions

• Gaya tarik menarik antar ion-ion

– Covalent• Sharing of electrons• Pemakaian elektron bersama

– Metallic• Metal atoms bonded to several other atomsatom2 logam terikat pada beberapa atom lain

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Lewis symbols• A convenient way to keep track of the valence electrons in an atom or moleculecara yang mudah untuk menelusur elektron valensi dlm atom/molekul

• Lewis dot symbol

Each dot is one valence electrontiap titik = elektron valensi

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Covalent Bondingikatan kovalen

• There are several electrostatic interactions in these bonds:beberapa interaksi elektrostatis dlm ikatan ini:– Attractions between electrons and nucleitarik-menarik antara elektron-inti

– Repulsions between electronstolak-menolak antara elektron

– Repulsions between nucleitolak-menolak antara inti

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Polar Covalent Bonds• Although atoms often form

compounds by sharing electrons, the electrons are not always shared equally.Meski atom2 membentuk senyawa dgn pemakaian elektron bersama, tetapi elektron tidak dibagi dengan sama tepat.

• Fluorine pulls harder on the shared electrons than hydrogen does.Fluor menarik lebih kuat elektron yg dipakai bersama drpd hidrogen

• Therefore, the fluorine end has more electron density than the hydrogen end.Fluor memiliki rapat elektron drpd hidrogen (dlm molekul H-F)

• But how do you know who pulls hardest?

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Electronegativity:skala Pauli

• The ability of atoms in a molecule to attract electrons to itself.Kemampuan atom dlm molekul untuk menarik elektron

• On the periodic chart, electronegativity increases as you go…– …from left to right across a row.– …from the bottom to the top of a column.

Electr

onegat

ivity

increa

ses

Electronegativity

increases

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Polar Covalent Bondsikatan kovalen polar

• When two atoms share electrons unequally, a bond dipole results.2 atom berbagi-pakai elektron dgn tidak setara dipole ikatan

• The dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated:momen dipole, yg dihasilkan 2 muatan yg berlawan dan terpisah dgn jarak r, dihitung sbg:

= Qr• It is measured in debyes (D).

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Polar Covalent BondsThe greater the difference in electronegativity, the more polar is the bond.Semakin besar beda elektronegatifitas semakin polar ikatan

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Menuliskan Struktur Lewis

G.N. Lewis

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Lewis Structures

Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.Struktur Lewis menggambarkan molekul dgn menunjukkan seluruh elektron, baik yang dipakai berikatan dan tdk.

Lines correspond to 2 electrons in bond

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Writing Lewis Structures

1. Find the sum of valence electrons of all atoms in the polyatomic ion or molecule.Jumlahkan seluruh elektron valensi dari seluruh atom dalam molekul / ion banyak atom– If it is an anion, add

one electron for each negative charge.Jika anion +1elektron utk tiap muatan negatif

– If it is a cation, subtract one electron for each positive charge. Jika kation -1elektron utk tiap muatan positif

PCl3

5 + 3(7) = 26

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Writing Lewis Structures

2. The central atom is the least electronegative element that isn’t hydrogen.Atom “pusat” atom dgn elektronegatifitas terendah kecuali Hidrogen Connect the outer atoms to it by single bonds.Hubungkan atom “luar” ke atom “pusat” dgn ikatan tunggal

Keep track of the electrons:hitunglah jumlah elektron26 6 = 20

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Writing Lewis Structures

3. Fill the octets of the outer atoms.Penuhi aturan “oktet” bagi atom2 “luar”

Keep track of the electrons:

26 6 = 20 18 = 2

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Writing Lewis Structures

4. Fill the octet of the central atom.Penuhi aturan oktet bagi atom “pusat”

Keep track of the electrons:

26 6 = 20 18 = 2 2 = 0

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Writing Lewis Structures5. If you run out of

electrons before the central atom has an octet…jika kehabisan elektron sebelum atom pusat memenuhi “oktet”…form multiple bonds until it does.Bentuklah ikatan rangkap hingga aturan oktet terpenuhi...

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Writing Lewis Structures• Then assign formal charges.

Tandailah “muatan formal”– For each atom, count the electrons in lone pairs and half the electrons it shares with other atoms.= ATiap atom, hitung elektron PEB dan separuh jika berbagi-pakai dng atom lain = A

– Subtract that from the number of valence electrons for that atom: The difference is its formal charge.Eleketron valensi tiap atom = B.Muatan formal = B-A

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Writing Lewis Structures• The best Lewis structure…

– …is the one with the fewest charges.... muatan paling rendah

– …puts a negative charge on the most electronegative atom....meletakkan muatan negatif pada atom yg paling elektronegatif

-2 0 +1 -1 0 0 0 0 -1

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Latihan• Tuliskan struktur Lewis untuk molekul berikut:CH4, CHCl3, CH3COOH

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ResonanceDraw the Lewis structure for ozone, O3.

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ResonanceDraw the Lewis structure for ozone, O3.

But why should one O be different from the other?

-

+

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Resonance• It is at odds with the true, observed structure of ozone,

tidak sesuai dgn struktur ozone yg teramati – …both O—O bonds are the same length.O-O = sama panjang

– …both outer oxygens have a charge of 1/2.muatan O “luar” = -1/2

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Resonance• One Lewis structure cannot accurately depict a molecule such as ozone.Satu struktur Lewis tdk bisa menggambarkan dgn akurat molekul seperti ozone

• We use multiple structures, resonance structures, to describe the molecule.Multi struktur, struktur resonansi digunakan

+- -

+

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ResonanceJust as green is a synthesis of blue and yellow…semisal warna hijau = hasil perpaduan warna biru dan kuning

…ozone is a synthesis of these two resonance structures.

It is not jumping between the two.

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ResonanceDraw resonance structure for:HCO2

-

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ResonanceDraw resonance structure for:HCO2

-

O C O

H

.... ......-

But why would the two oxygens be different?

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Resonance• In truth the electrons that make up the double bond are not localized, but rather are delocalized.Elektron yang membuat ikatan rangkap dua tidak terlokalisasi delokalisasi

O C O

H

..........

-O C O

H

..........

-

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Resonance• Draw the Lewis structure of NO3

-

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Resonance• Draw the Lewis structure of NO3

-

N

O

OO ............ ..

..--

+N

O

OO ........ .... ..

..--

+ N

O

OO ........

.. ....-

+

..

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Resonance• The organic compound benzene, C6H6 is a hexagon of carbon atoms with 6 H/s Draw the Lewis structure for benzene.

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Resonance• The organic compound benzene, C6H6, has two resonance structures.Bensen memiliki dua struktur resonansi

• It is commonly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.Lazim digambarkan sbg segienam dgn lingkaran di dalamnya elektron terdelokalisasi