(3009) CAIE A-Level Chemistry | Chemical periodicity - physical & chemical properties of period 3 | Lesson

Introduction to Periodicity

In this lesson, we focus on the chemical periodicity in relation to Group 2 and Group 17 elements, structured into three main parts. The first part examines general periodicity, particularly within the bounds of the third period in the periodic table. The second and third parts delve into the specific trends observed in Group 2 and Group 17, respectively. Today's topics include atomic radius, cationic and anionic radii, melting point trends, reactions with oxygen and chlorine, their behavior in water, and acid-base characteristics of metal oxides.

General Periodicity

When discussing periodicity, particularly in the third period from sodium to argon, students are not required to memorize every element, as a periodic table will be available during exams. Understanding trends within this periodic window is crucial since they often echo across periods, even if slight variations exist. Therefore, maintaining a conceptual grasp of these trends is essential for answering exam questions effectively.

Atomic Radius

Atomic Radius Across the Period

The trend of atomic radius in the third period mirrors that of first ionization energy. The atomic radius is defined as the distance from the nucleus to the outermost valence shell. Several factors influence this measurement, including nuclear charge, the distance of valence electrons from the nucleus, and electron shielding. As one moves across the period from sodium to argon, the atomic radius decreases due to an increase in nuclear charge while the electron shielding remains relatively constant. This results in a stronger nuclear attraction pulling the valence electrons closer to the nucleus.

Atomic Radius Down the Group

Conversely, when examining the atomic radius down a group, the radius increases despite an increasing nuclear charge. This increase is attributable to the addition of principal quantum shells, which leads to greater electron shielding and an increased distance of the outer electrons from the nucleus.

Cationic and Anionic Radius

Cationic Radius Trends

For cations in the third period, consider sodium (Na+), magnesium (Mg2+), and aluminum (Al3+). The increasing nuclear charge combined with the loss of electrons results in a decreasing cationic radius as one moves across the period. This phenomenon can be explained by the stronger effective nuclear charge that acts on the remaining valence electrons, leading to a tighter pull from the nucleus.

Anionic Radius Trends

In contrast, the anions in the third period include phosphide (P3-), sulfide (S2-), and chloride (Cl-). Here, the nucleus's increasing positive charge must also attract an additional number of electrons, resulting in a weaker effective nuclear charge for the negatively charged ions. Consequently, as one moves across the period, the anionic radius increases owing to the enhanced nuclear charge while the electron shielding remains unchanged.

Melting Point Trends

A notable trend in the melting points across the period reveals an initial increase from sodium to magnesium and aluminum, followed by a dramatic rise with silicon, and then a significant drop from sulfur to chlorine and argon. Metals inherently possess higher melting points due to their robust metallic structures, which necessitate greater energy for atomic separation. Silicon's elevated melting point is attributed to its complex tetrahedral giant covalent structure, requiring substantial energy to disrupt the extensive covalent bonds.

Reactions with Oxygen and Other Elements

Reactions with Oxygen

Elements in the third period react with oxygen in distinguishable ways:

• Sodium reacts vigorously, producing sodium oxide (Na2O) and exhibiting yellow flames.

• Magnesium burns with bright white flames to form magnesium oxide (MgO).

• Aluminum produces aluminum oxide (Al2O3), observed as a glow.

• Silicon reacts slowly to yield silicon dioxide (SiO2).

• Phosphorus forms phosphorus pentoxide (P4O10) with a yellow-white flame.

• Sulfur reacts to produce sulfur dioxide (SO2) with a gentle blue flame.

• Chlorine typically does not react with oxygen under standard conditions.

Reactions with Chlorine

The reactions with chlorine produce notable products: sodium (Na) with chlorine gas yields sodium chloride (NaCl), and magnesium (Mg) reacts to form magnesium chloride (MgCl2). Aluminum can form aluminum chloride (AlCl3) or a dimeric form, Al2Cl6, depending on its oxidation state.

Reactions with Water

Sodium's reaction with water produces sodium hydroxide (NaOH) and hydrogen gas (H2), generating visible effervescence. In comparison, magnesium reacts with water in different manners depending on conditions: it generates magnesium hydroxide (Mg(OH)2) at cooler temperatures and magnesium oxide (MgO) at elevated temperatures. Phosphorus reacts to create phosphoric acid (H3PO4), while sulfur dioxide (SO2) in the presence of water forms sulfurous acid (H2SO3). Sodium chloride (NaCl) and magnesium chloride (MgCl2) dissolve in water, dissociating into their constituent ions to yield neutral to mildly acidic solutions.

Acid-Base Characteristics of Metal Oxides

The acid-base characteristics of metal oxides reveal that both magnesium hydroxide (Mg(OH)2) and sodium hydroxide (NaOH) are bases because they produce hydroxide ions (OH-) upon dissolution in water. Additionally, aluminum oxide (Al2O3) is amphoteric, displaying dual characteristics by functioning as both an acid and a base. This duality allows it to neutralize acids and bases, leading to salt and water formations as products of these reactions.

Conclusion

In conclusion, today’s lesson offered a comprehensive overview of the periodic trends across Group 2 and Group 17, encompassing aspects such as atomic and ionic radii, melting points, oxidation reactions, and acid-base characteristics. Understanding these trends is critical for future chemistry endeavors and exams. Students are encouraged to ask questions in the comment section below if any concepts need clarification.