Physical Properties of Boron Family

The element belongs to Group 13 of the periodic table, called the Boron Family. The elements belonging to the group are boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). These elements under consideration are compositionally odd upon physical properties, but find a lot of applications that are, in fact, very essential as part of the industry but of end-use.

JEE Main 2025: Chemistry Formula | Study Materials | High Scoring Topics | Preparation Guide

JEE Main 2025: Syllabus | Sample Papers | Mock Tests | PYQs | Study Plan 100 Days

NEET 2025: Syllabus | High Scoring Topics |  PYQs

This Story also Contains

1. Electronic configuration
2. Atomic radii
3. Electronegativity
4. Density
5. Melting and boiling points
6. Physical Properties of Group 13 - 1
7. Some Solved Examples
8. Summary

For instance, the lightweight yet solid nature of aluminum is the reason this element is at the heart of the aeronautical industry, while the low melting point of gallium sees this element mostly applied in high technologies, among which are semiconductors. Suffice it to say, that the physical properties of the Boron Family are very much a blend of metallic and non-metallic.

Electronic configuration

The outer electronic configuration of these elements is ns²np¹. A close look at the electronic configuration suggests that while boron and aluminum have noble gas cores, gallium, and indium have noble gas plus 10 d-electrons, and thallium has noble gas plus 14 f-electrons plus 10 d-electron cores. Thus, the electronic structures of these elements are more complex than the s-block elements. This difference in electronic structures affects the other properties and consequently the chemistry of all the elements of this group.

Atomic radii

On moving down the group, for each successive member, one extra shell of electrons is added and, therefore, the atomic radius is expected to increase. However, a deviation can be seen. The atomic radius of Ga is less than that of Al. This can be understood from the variation in the inner core of the electronic configuration. The presence of an additional 10 d-electrons offers only a poor screening effect for the outer electrons from the increased nuclear charge in gallium. Consequently, the atomic radius of gallium (135 pm) is less than that of aluminum (143 pm).

Electronegativity

Down the group, electronegativity first decreases from B to Al and then increases marginally. This is because of the discrepancies in the atomic size of the elements.

Density

Density increases from boron to thallium. However, boron and aluminum have comparatively low values. This is due to their lower atomic masses as compared to gallium, indium, and thallium.

Melting and boiling points

The elements of this group do not show a regular change in their melting points with an increase in atomic number. The melting point decreases from B to Ga and then increases. The high melting point of boron is due to the fact that it exists as a giant covalent polymer in both solid and liquid states. The elements Al, In, and Tl all have close-packed metal structures. Gallium has an unusual structure. It consists of only Ga₂ molecules. It has thus low melting point. It exists as liquid up to 2000^oC and is hence used in high-temperature thermometry.

Physical Properties of Group 13 - 1

Atomic and Ionic Radii

Both the atomic and ionic radii of the Group 13 elements share the trend common with several other groups in the periodic table.

The atomic radius increases the group down due to extra electron shells. One, in fact, must always remember that since boron is the smallest of all then it should have the smallest atomic radius, and as thallium is the largest hence it should have the largest atomic radius. However, there is an exception in the comparison of atomic radii between gallium and aluminum. The atomic radius of Ga is slightly smaller than that of Al because of the presence of d-electrons in Ga which are ineffective in shielding the nuclear charge. It forms ions of that type generally. They lose three electrons to have an oxidation state of +3. B³⁺ can be considered the smallest, and Tl³⁺ shows the largest ionic radius. This trend will have an impact on the chemical reactivity and bonding nature of these elements.

Ionization enthalpy
The ionization enthalpy values as expected from the general trends do not decrease smoothly down the group. The decrease from B to Al is associated with an increase in size. The observed discontinuity in the ionization enthalpy values between Al and Ga, and between In and Tl are due to the inability of d- and f-electrons, which have low screening effect, to compensate for the increase in nuclear charge. The order of ionization enthalpies, as expected, is $\mathrm{\Delta }{H}_1<\mathrm{\Delta }{H}_2<\mathrm{\Delta }{H}_3$. The sum of the first three ionization enthalpies for each of the elements is very high. The effect of this will be apparent when you study their chemical properties.

Oxidation states
It is in group 13 that we first encounter elements possessing more than one oxidation state. As s²p¹ grouping is present in the outermost energy shell of the elements of the group IIIA, the expected oxidation states are +3 and +1. Boron shows a +3 oxidation state in all its compounds. Other members show +3 and +1 oxidation states. The stability of the +1 oxidation state increases from aluminum to thallium and the stability of +3 is a more important oxidation state for Al, Ga, and In whereas the +1 oxidation state is more important for Tl.

Electropositive character
The elements of group 13 are less electropositive as compared to the elements of groups 1 and 2. This is due to their size and high ionization energy. The electropositive character increases from boron to aluminum and then decreases from aluminum to thallium. Boron having very high ionization energy is considered to be as a semimetal. It is closer to non-metals. Aluminium is a metal and is most electropositive. The increase in electropositive nature from B to Al is due to increases in atomic size. The remaining three elements Ga, In, and Tl are less electropositive and less metallic than aluminium and there is a decrease from Ga and Tl.

Melting and Boiling Points

These elements have no marked general trend in their boiling or melting points.

Boron is different in this respect, in that it has a very high melting point. This comes about because it is a covalently bonded network solid and the resulting bond energies are those for a refractory material. All of the aluminum, gallium, indium, and thallium have relatively low melting points; gallium is particularly interesting—it will melt in the palm of your hand (at 29.76°C). The boiling point of the elements in this group generally decreases from boron to thallium, except for the disruption at gallium. Some of the most important parameters that help decide these particular trends are metallic bonding and lattice structures. Boron with a high melting point will be used accordingly for high-temperature applications, while gallium with a low melting point will find its applications in temperature-sensitive applications.

Densities

Group 13 has their densities that are very different from each other.

Boron has quite low density, which is because of its style of covalent nature of bonding and rigid lattice structure. Aluminum is only moderately dense, which finds wide application in the aerospace and automobile industries. Generally, the density increases from aluminum to Thallium moving down the group. It is noticed to be maximum for thallium and is attributed to the increase in both the atomic size and mass.

Complex formation
Group IIIA elements form complexes much more readily than the s-block elements, because of their smaller size, increased charge, and availability of vacant orbitals.

Nature of compounds

• The tendency of the formation of ionic compounds increases from B to Tl.
• Boron forms only covalent compounds.
• Aluminum forms both ionic as well as covalent compounds.
• Gallium forms mainly ionic compounds except anhydrous GaCl₃ which is covalent.

Electrical Conductivity

Electrical Conductivity Electrical conductivity varies in the Boron Family: it is very different. Boron is a semiconductor. Its semiconductor trait increases in conductibility as temperature goes up. Aluminum, gallium, indium, and thallium are all metals with high electrical conductivity. That is why aluminum is applied so widely for making electrical transmission lines. Gallium expands when it is solidified, and this makes it quite useful in semiconductors and thermometers.

Relevance and Application

From the physical properties belonging to Group 13, some very important practical applications become imperative. The practical applications to be noted are the high melting point and hardness of boron make it feasible for tank armor and bulletproof vests to have boron carbide. Also, boron is used for making semiconductors in electrical engineering because of its property of semiconducting especially when it has to operate at high temperatures.
Aluminum is lightweight and very conductive; thus, it has widely been applied in the aerospace, automobile, and electrical industries. It is the material for aero-plane use in both aerospace and car industries. Other uses include supply cables the naming above strategies make it a very vital multifunctional metal.

Another distinct property this element has is its very low melting point, and it increases in volume upon solidifying. These combined will now sum up to forming compounds with their semiconductor properties and now could be synthesized through modern crystal growing in high purity for single crystals. These promise applications as well for thermometers, barometers, and semiconductor technology. That is gallium arsenide, which is written as GaAs.

In the view of the literature, both indium and thallium are mentioned very rarely; however, concerning their importance, they hold the importance of their elements. Indium, on the other hand, is part of the element of the touch screens and LCDs. Its alloys have shown to be of worth to modern technology. Thallium is part of the elements that are toxic in themselves, but its importance goes to another high end; it makes the application highly specialized for the highest form of infrared detectors for low-temperature thermometers.

Recommended topic video on (Physical Properties of Boron Family)

Some Solved Examples

Example 1
Question: Which of the following has the highest density?

1. B
2. Al
3. Ga
4. In (correct)

Solution: The density of the Boron Family increases on moving down the group. This is evident from the density data given below:

- B: 2.35 g/cm³
- Al: 2.70 g/cm³
- Ga: 5.90 g/cm³
- In: 7.31 g/cm³

Hence, the answer is option (4).

Example 2
Question: The decreasing order of melting points in Group 13 elements is:

1. B > Al > Ga > In > Tl
2. Tl > In > Ga > Al > B
3. B > Al > Tl > In > Ga (correct)
4. B > Al > In > Tl > Ga

Solution: The elements of this group do not show a regular change in their melting points with an increase in atomic number. The high melting point of boron is because it exists as a giant covalent polymer in both solid and liquid states. The melting point data is given below:

- B: 2453 K
- Al: 933 K
- Ga: 303 K
- In: 430 K
- Tl: 576 K

Hence, the answer is option (3).

Example 3
Question: Anomalous melting point of boron is due to:

1. Small size
2. High electropositive character
3. Giant Covalent Polymer (correct)
4. High electronegativity

Solution: Boron has a very high melting point due to its existence as a giant covalent polymer in both solid and liquid states. Boron forms a giant$B_{12}$ icosahedral structure due to which its melting point is very high.

Hence, the answer is option (3).

Summary

The physical properties of elements in the Boron Family are very diverse and interesting. Specifically, from atomic radius to ionic radii, melting and boiling points, density, electric conductivity, etc., every element of Group 13 has special features that boost its applicability in industry. The high melting point of boron due to its small size, conductivity with the lightness of aluminum, the low melting point of gallium, and the special uses of indium and thallium directly relate these elements of practical importance. An appreciation of these properties deepens our insight into chemistry and also opens innovations in material science and technology.