Problem 4 - General Chemistry: Principles and Modern Applications

Question

The following melting points are in degrees Celsius. Show that melting point is a periodic property of these elements: \(\mathrm{Al}, 660 ; \mathrm{Ar},-189 ; \mathrm{Be}, 1278 ; \mathrm{B}, 2300 ; \mathrm{C}\) \(3350 ; \mathrm{Cl},-101 ; \mathrm{F},-220 ; \mathrm{Li}, 179 ; \mathrm{Mg}, 651 ; \mathrm{Ne},-249 ; \mathrm{N}\) \(-210 ; \mathrm{O},-218 ; \mathrm{P}, 590 ; \mathrm{Si}, 1410 ; \mathrm{Na}, 98 ; \mathrm{S}, 119.\)

Step-by-Step Solution

Answer

The melting point of the given elements is a periodic property. Evidence for this includes the general increase and then decrease in melting points as you move from left to right across each period in the Periodic Table. The trends can be explained by differences in atomic structure and electron configuration.

Step 1: Sorting the elements

First, sort the elements according to their position in the Periodic Table. Here is how they will look arranged from left to right, top to bottom: Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar.

Step 2: Organizing melting points

Next, list the melting points of the elements in the same order as Step 1: 179, 1278, 2300, 3350, -210, -218, -220, -249, 98, 651, 660, 1410, 590, 119, -101, -189.

Step 3: Identifying trends

Now examine this list for patterns. As you move across each period (row) from left to right, the melting points generally increase, then decrease. This trend repeats in the second period. This is evidence that melting point is a periodic property of these elements.

Step 4: Explaining the trends

The trend in melting points can be explained by atomic structure. Elements on the left of the period (metals) have loosely held valence electrons which require less energy (lower temperature) to break free, hence lower melting points. At the right end are the noble gases with full electron shells, with low melting points due to weak interatomic forces. Between these two are elements with more tightly bound electrons, hence higher melting points.

Key Concepts

Melting Points

Melting points indicate the temperature at which an element transitions from solid to liquid. For various elements, this temperature can vary greatly. In the context of the Periodic Table, the melting point is influenced by an element's position, which is a reflection of its atomic structure.

• Elements on the left, such as alkali metals, usually have lower melting points. They have fewer electrons loosely bound to their nucleus.
• As we move to the right towards the center of the table, the melting points generally increase. This happens because elements like carbon and boron form strong covalent bonds, requiring more energy to change states.
• Conversely, on the far right are noble gases and non-metals, which often have very low melting points due to weak intermolecular forces.

Observing these trends within the table reveals that melting point is indeed a periodic property, meaning it varies in a predictable manner across each period.

Periodic Properties

Periodic properties are characteristics of elements that vary in a recurring pattern across periods (rows) or groups (columns) on the Periodic Table. The melting point is a great example of such a property.

• As elements fill each row or period, new trends in physical and chemical properties emerge.
• For instance, each period begins with metals having relatively low melting points and ends with noble gases and non-metals generally with much lower melting points.
• Patterns of periodic properties are a direct result of an element’s electron configuration and the nuclear charge of their atoms which increases progressively across a period.
• Therefore, these periodic trends help us predict the behavior of an element based on its location on the table.

By understanding periodic properties like melting points, scientists can make informed predictions about chemical behavior.

Atomic Structure

The atomic structure of an element is a crucial factor in determining its periodic properties, including its melting point.

• An atom consists of a nucleus with protons and neutrons, and electrons occupying energy levels around the nucleus.
• The arrangement of these electrons, particularly the valence (outermost) electron shell, influences how an element interacts and bonds with others, which in turn affects its physical properties.
• Elements on the left side of each period generally have fewer valence electrons which are more easily removed, explaining their lower melting points.
• In contrast, elements in the right tend to have nearly full valence shells, contributing to their distinct properties such as non-reactivity and lower melting points.

Thus, the periodic variability in melting points can be understood by analyzing the atomic structure and electron configurations across the Periodic Table.