Bonding: Covalent and Ionic Bonds

A covalent bond is formed when atoms share electrons to satisfy the octet rule.
Carbon example: carbon atom has four electrons in its outer shell, but it would be much more stable with eight electrons in that shell.
To gain the additional four electrons, carbon can share with other atoms, resulting in a completed outer shell.
When one carbon atom shares electrons with four hydrogen atoms, carbon achieves an octet of electrons. The overall formula for this sharing arrangement is $CH_4$ .
Hydrogen-specific rule: each hydrogen in this compound achieves two electrons in its outer shell (a duet), because hydrogen’s valence shell can hold a maximum of two electrons.
The statement “This is a stable configuration of the first electron shell” refers to the stability provided by filled electron shells in general; the first electron shell can hold up to two electrons, contributing to overall stability, while additional stability in the valence shell is achieved by completing the octet for second-period and heavier elements.
Key takeaway: Covalent bonding involves sharing electrons to reach more stable electron configurations (octet for most elements, duet for hydrogen).

An ionic bond forms when two atoms are held together by the attraction between opposite charges (cations and anions).
Example: sodium (Na) and chlorine (Cl) form an ionic bond.
Sodium’s tendency: sodium has one electron in its outer (third) shell and tends to give up that electron.
After losing that electron, sodium’s remaining outer shell contains eight electrons (the second shell becomes filled, giving a stable configuration).
Resulting charge: the loss of an electron gives sodium a slightly positive charge, forming Na extsuperscript{+}.
Chlorine’s tendency: chlorine’s outer shell already has seven electrons and tends to gain one electron to complete its octet.
After gaining an electron, chlorine becomes slightly negative, forming Cl extsuperscript{−}.
Ions of opposite charge attract each other and form ionic bonds.
Compound formed: the attraction between Na extsuperscript{+} and Cl extsuperscript{−} forms sodium chloride, commonly known as table salt (NaCl). Note: in solid form, NaCl forms a lattice structure; in simple discussions, it is often described as NaCl.

Sodium: electron configuration before bonding is $ext{Na}: [ ext{Ne}] \, 3s^1$
Sodium ion: after donating one electron, $ext{Na}^+: [ ext{Ne}]$
Chlorine: electron configuration before bonding is $ext{Cl}: [ ext{Ne}]\, 3s^2\, 3p^5$
Chlorine ion: after gaining one electron, $ext{Cl}^-: [ ext{Ne}]\, 3s^2\, 3p^6$
These ion configurations illustrate why Na becomes Na extsuperscript{+} and Cl becomes Cl extsuperscript{−} and how their electrostatic attraction leads to bond formation.

Covalent bonds involve electron sharing to satisfy the octet rule; hydrogen follows the duet rule (2 electrons in its outer shell).
Ionic bonds involve electron transfer and subsequent attraction between positively charged cations and negatively charged anions.
Examples from transcript:
- Covalent: CH_4 (carbon with four hydrogens) achieving octet for carbon and duets for hydrogens.
- Ionic: Na + Cl → NaCl, with Na forming Na^+ and Cl forming Cl^−, resulting in an ionic bond.
Real-world relevance: CH_4 is a representative covalently bonded molecule; NaCl is a common ionic compound widely used as table salt.
Foundational principle connections: stable electron configurations (filled shells) underpin both octet/duet goals and overall chemical stability.

Covalent bond concept: atoms share electrons to satisfy octet rule; hydrogen follows duet rule.
Methane example: $CH_4$
Ionic bond concept: transfer of electrons creating oppositely charged ions that attract.
Sodium configuration before bonding: $ext{Na}: [ ext{Ne}]\, 3s^1$
Sodium ion: $ext{Na}^+: [ ext{Ne}]$
Chlorine configuration before bonding: $ext{Cl}: [ ext{Ne}]\, 3s^2\, 3p^5$
Chloride ion: $ext{Cl}^-: [ ext{Ne}]\, 3s^2\, 3p^6$
Ionic compound example: $ext{NaCl}$ (table salt)