Electrons in atoms are arranged in discrete shells around the nucleus. The first shell holds 2 electrons and subsequent shells hold up to 8 electrons each following the octet rule. Ions form when atoms gain or lose electrons to fill or empty their outer shell, becoming negatively or positively charged. Ionic bonds occur when oppositely charged ions attract. Covalent bonds occur when atoms share electrons in molecular orbitals. Nonpolar covalent bonds share electrons equally while polar covalent bonds have an unequal electron distribution making one end partially negative and the other partially positive. Hydrogen bonds form between the partially positive hydrogen of one polar molecule and the partially negative end of another.

1. Section 2 of Chapter 2
Bonding of Atoms

2. 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 electrons
2nd shell holds 8 electrons
3rd shell holds 8 electrons
Octet rule: Except for the 1st shell, each
electron shell holds up to 8 electrons
* Lower shells are filled first.

3. Helium
Atomic number = 2
Atomic weight = 4
(2 electrons fill the 1st electron shell)
Carbon
Atomic number = 6
Atomic weight = 12
(The first 2 electrons fill the inner shell, and
the remaining 4 electrons are placed the 2nd
electron shell).

4. 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

5. 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.

6. 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.

7. 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.

9. 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

10. 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.

11. 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.

12. 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.

13. 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 negative end
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.

14. 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