Terms and their definitions to keep in mind:
chemical bond; ionic bond; covalent bond; Lewis (dot) structure/symbol; electronegativity; dipole moment; lone pair (of electrons); VSEPR

There are ~100 elements, but millions of compounds with different chemical and physical properties; and chemistry is the tool for the synthesis and study of these compounds.

Example: How many different ways for 5 C atoms and 12 H atoms to bind together? (Hint: Each C can have 4 "bonds", but H can have only one.)

Atoms in compounds are held by "chemical bonds". Depending upon the properties of the atoms, different chemical bonds are formed.

Molecule: an electronically neutral group of tightly bound atoms that acts as one single unit. Its composition can be shown by a chemical formula, e.g., oxygen gas as O₂, sugar as C₆H₁₂O₆, the amino acid glycine as C₂H₅NO₂. <<What about NaCl?>>  The chemical formula of a molecules gives no clue about the structure of the molecule.

Review: Valence electrons (VEs): the electrons in the outermost principal energy level of the atoms of an element (which determine the chemistry of the element). Elements in the same group have the same number of VEs, e.g., Mg: [Ne]3s², Ca: [Ar]4s², and Sr: [Kr]5s²

Electron dot structures (Lewis dot structure): A "pictorial" representation of valence electrons
—Dots (which are the valence electrons) are placed around 4 sides of the symbol of an element, following the Hund's rule).

                  • • 
such as :F •  with 7 dots/VEs (1s² 2s² 2p⁵),
                  ^{• •}

•Be• with 2 dots/VEs (1s² 2s²),

         • •
and  :O  with 6 dots/VEs (1s² 2s² 2p⁴).
         ^{• •}
Ions: Formed from atoms by gaining or losing electrons

Cation— a "+" charged ion; anion— a "–" charged ion

The easier an atom can form a cation, the easier the electron(s) can be removed from the atom.

Ionization energy of an atom: The energy required to remove an electron from the atom in the gaseous state.

Inert gases have the highest IEs, thus do not form ions easily.
In other words, inert gases are very stable, thus take relatively higher energy to reach excited state and ionized.
Conversely, alkali metals are very active (unstable), thus take relatively less energy to become ionized.

Cations (cf. Tables 3.1 & 3.3)
For example: Na, 1s² 2s² 2p⁶ 3s¹         Ne, 1s² 2s² 2p⁶

                    (Very active)                     (Very inert)

       Na (Na•) ---> loses VE (from 3s) ---> Na⁺ + e^–

                                                            1s² 2s² 2p⁶   (a Ne-like electron configuration with 8 VEs)

How about Mg, 1s² 2s² 2p⁶ 3s²?

**Transition metals can lose 2 e^– from an s orbital, and more e^– from the d orbitals, e.g., Fe²⁺, Fe³⁺, and Fe⁴⁺; Cr²⁺ to Cr⁶⁺; etc.
Fe, 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶;Cr, ...4s² 3d⁴

Anions
For example: Cl, 1s² 2s² 2p⁶ 3s² 3p⁵         Ar, 1s² 2s² 2p⁶ 3s² 3p⁶

                            Very active                         Very inert
                         • • 
                  :Cl •  ---> gains 1 e^– ---> Cl^– ,
                           ^{• •}
                                    1s² 2s² 2p⁶ 3s² 3p⁶   (an Ar-like electron configuration with 8 VEs)

How about O, 1s² 2s² 2p⁴?

Ions can be formed from more than one atoms (Table 3.2): NH₄⁺, NO₃^–, OH^–, CN^–, CO₃^2–, PO₄^3–, SO₄^2–, etc.

Ionic bonds—formed by the attraction between oppositely charged ions, e.g., Na⁺ and Cl^–, Mg²⁺ and PO₄^3–, and so on.

Ionic compounds
    —containing ions
    —electronically neutral
    —may not have equal number of cations and anions
    —Anios and cations in some ionic compounds can be "separated" by water molecules when dissolved in water.
    —Can conduct electricity when dissolved in water or melted.

Examples (P. 68–72):
LiF (Li gives 1 e^– to 1 F to form Li⁺ and F^–.)
MgF₂ (Mg gives 2 e^–‘s to 2 F’s, one each, to form Mg²⁺ and F^–.)
AlF₃ (Al gives 3 e^– to 3 F’s to form Al³⁺.)
Al₂O₃ (2 Al’s gives total of 6 e^–‘s to 3 O’s to form 2Al³⁺ and 3O^2–.)

Covalent bonds—formed by electron sharing between atoms,

e.g., hydrogen gas, oxygen gas, nitrogen gas, etc.

H• + •H ---> H₂ (H:H or H–H)
  ..        ..           ..     ..
:F •  + •  F:  ---> :F : F : (F₂ or F–F)
  ^{• •                • •                      • •      • •}
**Sharing of one electron pair (single bond) and a few unshared pairs not shown (lone pairs)

  ..        ..            ..        ..
:O •  + •O:  ---> :O :: O :  (O::O or O=O)
   •                  •
 Sharing of two electron pairs — a double bond

How about N + N ---> N₂? <<a "triple bond"!!>>

Heteronuclear molecules can be formed in a similar way
such as, H₂O, NH₃, CO₂, CH₄ (methane), and so on.

Coordinate covalent bonds (PP. 83.84)—the shared electron paired between two atoms/ions is contributed by only one atom,
For example,
   .        ..                     ..
• C •  + •O:  ---> :C :: O :   (This is not correct! Why not?)
   •                  •
   .        ..                   
• C •  + •O:  ---> :C ::: O :   (This is correct! Why is it correct and how does it formed?)
   •                  •
 
H⁺ + :NH₃ ---> H:NH₃⁺ (NH₄⁺)

Some "rules" about ions with respect to the periodic table)

1. IA elements form 1+ cations; IIA, 2+, and IIIA, 3+.
2. VIIA elements form 1– anions, and VIA elements, 2–.
3. IVA elements form 4 covalent bonds; VA, 3; VIA, 2.
4. Depending on their difference in electronegativity, different atoms can form either ionic (large difference) or covalent (small) bonds, e.g., The Na–Cl bond is ionic, while the C–Cl bond is covalent.

Naming compounds (cf. Tables 3.1, 3.2, and 3.4 and example 3.6)

Representation of covalent bonds: use a "bar", —, to represent a covalent bond, such as H₂ as H–H and O₂ as O=O, etc.

Sections 3.7 and 3.8 show some step-by-step descriptions about how to draw Lewis structures of both simple and more complicated molecules.

Dipole interaction—interactions between polar molecules,
    such as: H^d+—F^d– •••••• H^d+—F^d–

H-bond: a kind of dipole interaction, in which a hydrogen atom is involved
    e.g., >N^d–—H^d+ •••• O^d–=C^d+< in proteins; the bonds that hold the two helices of DNA together

VSEPR (Valence-Shell Electron-Pair Repulsion Theory) and the shape of molecules: The non-bonding electron pairs in a molecule is taken into consideration for the determination of molecular shapes

What are the possible and correct shapes for the different HnX molecules? How do H₂O and NH₃ interact with H⁺?
        HCl         H₂O         NH₃                             CH₄
       linear        bent        trigonal pyramidal        tetrahedral     (But why?)