Noble Gases and the Octet Rule

Noble Gases

• Noble gases are extremely stable (inert) elements and do not typically form compounds.
• Helium is less dense than air, making it suitable for filling blimps.
• Hydrogen is also a light gas but is highly flammable, as demonstrated by the Hindenburg disaster.
• Argon and krypton extend the lifespan of bulb filaments in incandescent lights.
• Neon is used in colourful lighting displays and vacuum/television tubes due to its chemical inertness.

Octet Rule

• Elements with full valence shells exhibit special stability.
• In the first 18 elements, a full valence shell (except the first shell) contains 8 electrons.
• Noble gas atoms have 8 valence electrons (except helium, which has 2). This is a stable octet.
• Atoms of other elements tend to achieve this octet configuration by sharing, losing, or gaining electrons.
• Losing or gaining electrons forms an ion (positive or negative).

Formation of Ions

• Metals located to the left of the periodic table's staircase tend to lose valence electrons to form cations (positive ions).
• Sodium (Group 1, alkali metals), with one valence electron, loses it to form Na+ (sodium ion).
• Aluminum, with 3 valence electrons, forms Al3+ ion.
• Non-metals on the right side of the periodic table are more likely to gain electrons to form anions (negative ions).
• Chlorine (halogen family) gains one electron to form Cl– (chloride ion).
• Sulfur gains 2 electrons to form S2– (sulfide ion).

Naming Ions

• Cations are named the same as their element (e.g., sodium ion, Na+).
• Anions are named by replacing the element's ending with "-ide" (e.g., chloride ion, Cl–; sulfide ion, S2–).

Multivalent Elements

• Some elements can form more than one type of ion (multivalent).
• Transition metals often display this.
• Copper (Cu) can form Cu+ and Cu2+ ions.
• Traditional naming system uses Latin names and suffixes like "-ous" and "-ic" (e.g., ferrous, ferric).
• IUPAC naming system uses Roman numerals to denote the ion's charge (e.g., copper(I), copper(II)).

Polyatomic Ions

• Polyatomic ions are ions composed of more than one atom.
• Examples include phosphate (PO43–), nitrate (NO3–), and nitrite (NO2–).
• These ions occur naturally and are important for health (e.g., calcium phosphate in bones).
• They have industrial applications (e.g., sodium nitrate in fertilizers).
• Polyatomic ions are mainly anions.

Ions in the Human Body

• About 99% of the human body is composed of 6 elements (oxygen, carbon, hydrogen, nitrogen, calcium, phosphorus).
• Many elements exist as ions dissolved in water in the human body.
• Sodium ions are crucial for nerve impulse transmission, muscle contraction, and water balance in the body.
• Excessive sodium intake can lead to health problems (e.g., hypertension).