Section 2 of Chapter 2

Bonding of Atoms

Properties of electrons

Electron Shells: Electrons encircle the nucleus in discrete orbits, called electron shells. Each shell can contain only a fixed number of electrons.

1st shell holds 2 electrons2nd shell holds 8 electrons3rd shell holds 8 electrons

Octet rule: Except for the 1st shell, each

electron shell holds up to 8 electrons

* Lower shells are filled first.

HeliumAtomic number = 2Atomic weight = 4

(2 electrons fill the 1st electron shell)

CarbonAtomic number = 6Atomic weight = 12

(The first 2 electrons fill the inner shell, and the remaining 4 electrons are placed the 2nd

electron shell).

Ions

Ions are atoms that readily gain or loose electrons

Cation: an ion that looses electrons

• Cations are positively charged ions

Anion: an ion that gains electrons

•Anions are negatively charged ions

Example of a cation

Sodium (Na)

atomic number = 11

atomic weight = 23

Na+ = Sodium cation

Only 1 lone electron sits in the outer shell. This electron is unpaired and is easily lost, forming the sodium cation.

Example of an anion

Chlorine (Cl)

atomic number = 17

atomic weight = 35 Cl - = Chloride anion

7 electrons fill the outer shell of chlorine, leaving room for 1 more electron. Chlorine readily accepts one electron, creating the chloride anion.

Ionic Bond

Ionic bonds are formed when the oppositely charged particles attract.

Figure 2.4 (a) An ionic bond forms when on

atom gains and another atom looses electrons,

and then (b) oppositely charged ions attract.

Covalent Bonds

Covalent bonds are formed when atoms share electrons.

Example: A hydrogen molecule (H2) is formed when two hydrogen atoms share their single electron.

H + H H2

Covalent Bonds of water

Water consist of oxygen covalently bonded to two hydrogen atoms.

Structural Formula: depicts the covalent bonds of a molecule as lines.

Molecular Formula: is a shorthand notation for representing molecules.

Oxygen joined to two hydrogen atoms by single bonds

Two oxygen atoms joined by a double bond.

A Carbon atom joined to hydrogen by a single bond and to nitrogen by a triple bond.

Nonpolar covalent bonds

Nonpolar covalent bonds occur when the atoms share the

electrons equally, so the molecule has no overall charge.

Two hydrogen atoms share their electrons equally. Thus, the hydrogen molecule has no overall charge and is nonpolar.

Slightly negative end

Polar covalent bonds

Polar bonds have an unequal distribution of electrons.

One portion of the atom has a higher affinity for electrons than the rest of the molecule

(electronegative).

Slightly positive end

Water is a polar molecule because the oxygen atom (with 8 protons) tends to pull the electrons away from hydrogen. The oxygen end has a slight negative charge, while the hydrogen end has a slight positive charge.

Hydrogen bonds

Occur when the slightly positive (hydrogen) end of a polar molecule

weakly attracts to the slightly negative end of another molecule.

Hydrogen Bonds:

• Form weak bonds at room temperature, but are

strong enough to form ice

• Stabilize large proteins, DNA, and RNA• Stabilize large proteins, DNA, and RNA

End of Section 2, Chapter 2