section 2, chapter 2

Section 2 of Chapter 2Bonding 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.

Octet rule: Except for the 1st shell, each electron shell holds up to 8 electrons

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

* Lower shells are filled first.

Examples of filling electron shells

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

Na+ = Sodium cation

Sodium (Na)atomic number = 11atomic weight = 23

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 = 17atomic weight = 35

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

Cl - = Chloride anion

Na+ + Cl- → NaCl

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.

Ionic bonds do not form molecules

Figure 2.4 (c) salt crystal formation occurs because of the ionic bonds of sodium and potassium.

Cations & anions attract in all directions, forming organized arrays, such as crystals. They do not form molecules.

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

OH H

H2O

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.

Types of covalent bonds

A Single Bond occur when two atoms share one pair of electrons

A Double Bond occurs when atoms are joined by two pairs of electrons

A Triple Bond occurs when atoms are joined by three pairs of electrons

Two oxygen atoms joined by a double bond.

Oxygen joined to two hydrogen atoms by single bonds

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

Slightly positive end

Polar covalent bonds

Polar bonds have an unequal distribution of electrons.

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.

One portion of the atom has a higher affinity for electrons than the rest of the molecule (electronegative).

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

End of Section 2, Chapter 2