CHAPTER 3 CHEMICAL BONDING BY DR. VIDHI GROVER

Hey! I find your electrons attractive! Chemical Bonding Get lost, loser! Hey! I find your electrons attractive!

Valence Electrons – electrons in the outermost occupied energy level Valence Electrons – electrons in the outermost occupied energy level. (s and p electrons outside the core) Valence electrons can be represented by “dots” drawn around the atom.

Atoms and electron changes Atoms can obtain completely full outer electron shells by either gaining or losing electrons when they react with other atoms. When this happens, atoms become ions. Unlike atoms, ions have an electrical charge because they contain an unequal number of protons and electrons. Atoms that lose electrons have more protons than electrons and so have a positive charge. They are called positive ions or cations. Atoms that gain electrons have more electrons than protons and so have a negative charge. They are called negative ions or anions.

COVALENT BOND

Define covalent bond The mutual sharing of electrons between two or more atoms . Eg : H2O

Chlorine molecule Cl2 Electrons shared Each outer shell has 8 electrons Forces (bonds) between atoms in the molecule very strong Forces between molecules very weak Molecular or covalent compounds are usually gases or liquids – they have low melting points and low boiling points Molecules have no overall electric charge

H H

Electron-dot formula for Methane (CH4) Here is a Carbon atom (4 val e-’s) and four Hydrogen atoms (1 val e- each) H

Each H atom “feels” like a stable “He” atom with 2e-s Electron-dot formula for Methane (CH4) H Now they have formed a stable molecule. Each C atom “feels” like it has a stable octet. H C H Each H atom “feels” like a stable “He” atom with 2e-s H

Electron-dot formula for Ammonia (NH3) Here is a Nitrogen atom (5 val e-’s) and three Hydrogen atoms (1 val e- each) H

Electron-dot formula for Ammonia (NH3) “N” now feels like it has a stable octet Each “H” feels like it has 2 e- like Helium. H N H H

Write the electron-dot formula for CF4 Because “F” is a halogen, it has 7 valence e-s, so you must show all 7 red dots around each “F” atom! C F

Write the electron-dot formula for H2S The two H’s MUST be at right angles to each other!!

IONIC BOND

Lewis Structures (Electron-dot formulas) for Ionic Compounds. Remember, in an ionic compound, the metal loses e-’s and the non-metal gains. There is no sharing. Here is the e-dot formula for sodium chloride (NaCl) Cl Na+

DEFINE IONIC BOND It is defined as the complete transfer of valence electrons from one atom to another. Eg : NaCl

When the electronegativities of two atoms are quite different from each other: One atom loses an electron (or electrons) The other atom gains an electron (or electrons) This results in an Ionic Bond.

+ The Sodium atom has 1 Electron in it’s outer shell. Na e.c. 2,8,1 Atom + The Sodium loses 1 electron to leave a complete outer shell. It is now a Sodium ion with a charge of 1 + (Na +) e.c. (2,8)+ Ion

- The Chlorine atom has 7 electrons in it’s outer shell. Cl e.c. 2,8,7 Atom - (Cl - ) The Chlorine gains 1 electron to gain a complete outer shell. It is now a Chlorine ion with a charge of 1 - e.c. (2,8,8)- Ion

+ - The Ionic Bond Sodium atom Na Chlorine atom Cl Sodium ion (Na +) Chlorine ion (Cl -) The sodium atom loses one electron to attain a complete outer shell and become a positive ion (Na +). The Chlorine atom gains one electron to attain a complete outer shell and become a negative ion (Cl –). Strong electrostatic forces attract the sodium and chlorine ions.

Ionic Compounds Held together by strong electrostatic forces Opposite charges attract High melting points and boiling points Regular structures – giant ionic lattices When melted or dissolved ionic compounds conduct electricity

Differences IONIC BOND COVALENT BOND It’s formed by complete transfer of valence electron It has high melting & boiling point. Strong force of attraction. Soluble in polar organic solvents Give ionic reactions It conduct electricity in aqueous state Eg : NaCl It’s formed by mutual sharing of valence electrons Low melting &boiling point Weak force of attraction In soluble in organic solvents Give molecular reactions Do not conduct electricity in aqueous state Eg : H2O

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