Properties of Boron Family

Physical Properties of Boron Family

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:13 PM IST

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.

This Story also Contains

Electronic configuration
Atomic radii
Electronegativity
Density
Melting and boiling points
Physical Properties of Group 13 - 1
Some Solved Examples
Summary

Physical Properties of Boron Family

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).

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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 $Δ H_{1} < Δ H_{2} < Δ H_{3}$ . 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.

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.

Frequently Asked Questions (FAQs)

1. How does the atomic size change as we move down the boron family?

The atomic size increases as we move down the boron family (from boron to thallium). This is due to the addition of new electron shells with each period, which increases the distance between the nucleus and the outermost electrons.

2. What is the trend in atomic radius for the boron family elements?

The atomic radius increases as we move down the boron family. This is due to the addition of new electron shells with each period, which increases the overall size of the atom. The effect of increased nuclear charge is outweighed by the addition of new shells, resulting in a net increase in atomic radius.

3. What is the trend in covalent radius for the boron family elements?

The covalent radius generally increases as we move down the boron family. This trend is similar to the atomic radius trend and is due to the addition of new electron shells with each period. However, the increase in covalent radius may not be as pronounced as the increase in atomic radius due to the effects of bonding on electron distribution.

4. How does the atomic volume change as we move down the boron family?

The atomic volume generally increases as we move down the boron family. This is primarily due to the addition of new electron shells with each period, which increases the overall size of the atom. However, the rate of increase may not be uniform due to factors such as electron configuration and nuclear charge.

5. What is the trend in polarizability for the boron family elements?

The polarizability of boron family elements generally increases as we move down the group. This is because larger atoms with more electrons are typically easier to distort in an electric field. The outer electrons of heavier elements are farther from the nucleus and less tightly bound, making them more susceptible to polarization.

6. How does the density of boron family elements change as we move down the group?

The density of boron family elements generally increases as we move down the group. This is because the atomic mass increases more rapidly than the atomic volume, resulting in more mass packed into a given volume for the heavier elements.

7. Why does boron have a lower density compared to other elements in its family?

Boron has a lower density compared to other elements in its family because of its unique crystal structure and lighter atomic mass. Boron forms a covalent network structure with large interatomic spaces, resulting in a lower mass per unit volume compared to the more compact metallic structures of the heavier elements.

8. Why does gallium expand upon solidification, unlike most other elements?

Gallium expands upon solidification due to its unique crystal structure. When liquid gallium freezes, it forms a less dense orthorhombic crystal structure with more space between atoms compared to its liquid state. This unusual property is shared by only a few other elements, such as water, and is related to the specific arrangement of atoms in the solid phase.

9. How does the crystal structure of boron differ from other elements in its family?

Boron has a unique crystal structure compared to other elements in its family. It forms a three-dimensional covalent network structure, while the heavier elements (aluminum, gallium, indium, and thallium) typically adopt metallic crystal structures. This difference in crystal structure contributes to boron's distinct physical properties, such as its high melting point and hardness.

10. How does the thermal conductivity trend compare to the electrical conductivity trend in the boron family?

The thermal conductivity trend in the boron family is similar to the electrical conductivity trend, generally increasing as we move down the group. This correlation exists because both thermal and electrical conductivity in metals are primarily due to the movement of free electrons. As the metallic character increases down the group, both types of conductivity tend to improve.

11. How does the electronegativity change among the boron family elements?

The electronegativity generally decreases as we move down the boron family. Boron has the highest electronegativity in the group, while thallium has the lowest. This trend is due to the increasing atomic size and the decreasing attraction between the nucleus and the valence electrons in larger atoms.

12. How does the metallic character change as we move down the boron family?

The metallic character increases as we move down the boron family. Boron is a metalloid with more non-metallic properties, while aluminum and the elements below it exhibit increasingly metallic characteristics. This trend is due to the decreasing electronegativity and the increasing tendency to form metallic bonds in the heavier elements.

13. What is the trend in first electron affinity for the boron family elements?

The first electron affinity generally becomes more negative (indicating a higher tendency to gain an electron) as we move down the boron family. However, the trend is not as consistent as in other groups due to the complex interplay of factors such as atomic size, electron configuration, and shielding effects.

14. How does the reactivity with acids change as we move down the boron family?

The reactivity with acids generally increases as we move down the boron family. Boron is relatively unreactive with acids, while aluminum and the elements below it react more readily. This trend is due to the increasing metallic character and the greater ease of losing electrons in the heavier elements, allowing them to react more easily with acids to form salts and hydrogen gas.

15. How does the reactivity with water change as we move down the boron family?

The reactivity with water generally increases as we move down the boron family. Boron does not react with water, aluminum reacts slowly, and gallium, indium, and thallium react more readily. This trend is due to the increasing metallic character and the decreasing strength of the metal-metal bonds in the heavier elements.

16. Why do boron family elements tend to form covalent compounds rather than ionic ones?

Boron family elements tend to form covalent compounds because they have three valence electrons, which is not enough to easily lose or gain electrons to form stable ions. Instead, they often share electrons with other elements to achieve a stable electron configuration, resulting in covalent bonding.

17. Why does boron have a lower electrical conductivity compared to other elements in its family?

Boron has a lower electrical conductivity compared to other elements in its family because it has fewer free electrons available for conduction. Boron's electron configuration and covalent bonding in its solid state result in most of its electrons being tightly bound in covalent bonds, unlike the more metallic elements below it that have more freely moving electrons.

18. What is the trend in the energy required to form M3+ ions for boron family elements?

The energy required to form M3+ ions (where M is a boron family element) generally decreases as we move down the group. This is because the outermost electrons become easier to remove in larger atoms due to increased shielding and distance from the nucleus. However, the trend may not be perfectly linear due to factors such as electron configuration and the inert pair effect in heavier elements.

19. How does the magnetic susceptibility change as we move down the boron family?

The magnetic susceptibility of boron family elements generally increases as we move down the group. Boron and aluminum are diamagnetic (weakly repelled by magnetic fields), while gallium, indium, and thallium exhibit increasing paramagnetic behavior (weakly attracted to magnetic fields). This trend is related to the increasing number of unpaired electrons in the heavier elements' atomic structures.

20. How does the tendency to form complexes change among the boron family elements?

The tendency to form complexes generally increases as we move down the boron family. Boron forms fewer complexes compared to the heavier elements like aluminum, gallium, indium, and thallium. This trend is due to the increasing availability of empty orbitals for bonding and the larger atomic size, which allows for higher coordination numbers in the heavier elements.

21. What is the trend in melting points of boron family elements?

The melting points of boron family elements generally decrease as we move down the group, with boron having the highest melting point and thallium having the lowest. However, there is an exception with aluminum, which has a higher melting point than gallium due to its stronger metallic bonding.

22. Why does boron have a much higher melting point compared to other elements in its family?

Boron has a much higher melting point because it forms strong covalent bonds in its solid state, creating a three-dimensional network structure. This structure requires more energy to break compared to the metallic bonding in other elements of the family, resulting in a higher melting point.

23. Why is gallium liquid at room temperature despite being below aluminum in the periodic table?

Gallium is liquid at room temperature (melting point: 29.76°C) due to its unique electron configuration and weak metallic bonding. Unlike aluminum, which has stronger metallic bonds, gallium's atoms are held together by weaker forces, allowing it to melt at a much lower temperature than expected for its position in the periodic table.

24. How does the boiling point trend compare to the melting point trend in the boron family?

The boiling point trend in the boron family is similar to the melting point trend, generally decreasing as we move down the group. However, the difference between boiling and melting points tends to decrease for heavier elements, indicating a narrower liquid range for elements like gallium and indium compared to boron and aluminum.

25. Why does aluminum form an oxide layer on its surface, and how does this affect its properties?

Aluminum forms a thin, protective oxide layer (Al2O3) on its surface when exposed to air. This process, called passivation, occurs because aluminum is highly reactive with oxygen. The oxide layer protects the underlying metal from further corrosion, making aluminum more resistant to environmental degradation and affecting its surface properties.

26. Why are boron family elements called metalloids?

Boron family elements (Group 13) are called metalloids because they exhibit properties of both metals and non-metals. They have some metallic characteristics like luster and electrical conductivity, but also share some non-metallic properties like brittleness and lower melting points compared to typical metals.

27. How does the electrical conductivity change among the boron family elements?

The electrical conductivity generally increases as we move down the boron family. Boron is a poor conductor, while aluminum and the elements below it are good conductors of electricity. This trend is due to the increasing metallic character and the availability of more free electrons for conduction in the heavier elements.

28. How does the hardness of boron family elements change as we move down the group?

The hardness of boron family elements generally decreases as we move down the group. Boron is the hardest due to its strong covalent bonds, while the other elements become progressively softer due to their increasing metallic character and weaker interatomic forces.

29. What is the oxidation state trend in the boron family, and why does it occur?

The most common oxidation state for boron family elements is +3. However, as we move down the group, the stability of the +1 oxidation state increases. This is due to the inert pair effect, where the outermost s electrons become more difficult to remove in heavier elements, making the +1 state more favorable.

30. What is the trend in ionization energy for the boron family elements?

The ionization energy generally decreases as we move down the boron family. This is because the valence electrons are farther from the nucleus in larger atoms, making them easier to remove. However, there are some exceptions due to the effects of electron configuration and shielding.

31. How does the color of boron family elements change as we move down the group?

The color of boron family elements changes as we move down the group. Boron is typically brown or black, aluminum is silvery-white, gallium is silvery-blue, indium is silvery-white with a slight yellow tinge, and thallium is silvery-white with a bluish tint. These color differences are related to the elements' electronic structures and how they interact with light.

32. Why does aluminum have a higher specific heat capacity compared to other elements in its family?

Aluminum has a higher specific heat capacity compared to other elements in its family due to its relatively low atomic mass and strong metallic bonding. The specific heat capacity is inversely related to atomic mass, so lighter elements like aluminum can absorb more heat per unit mass before their temperature rises significantly. Additionally, aluminum's strong metallic bonds require more energy to vibrate, contributing to its higher heat capacity.

33. How does the solubility of boron family elements in water change as we move down the group?

The solubility of boron family elements in water generally increases as we move down the group. Boron is practically insoluble in water, while the solubility increases for heavier elements. This trend is related to the increasing metallic character and the ability to form hydroxides or oxides that can dissolve in water. However, the actual solubility also depends on the specific compounds formed and their interactions with water molecules.

34. Why does boron have a higher ionization energy compared to other elements in its family?

Boron has a higher ionization energy compared to other elements in its family because it has a smaller atomic size and a higher effective nuclear charge. The valence electrons in boron are closer to the nucleus and more tightly bound, requiring more energy to remove. Additionally, boron's electron configuration (2s²2p¹) makes it more difficult to remove an electron compared to the more metallic elements below it.

35. How does the tendency to form hydrides change among the boron family elements?

The tendency to form hydrides decreases as we move down the boron family. Boron forms a variety of stable hydrides (boranes), while aluminum forms some hydrides, and the heavier elements form fewer and less stable hydrides. This trend is due to the decreasing strength of the element-hydrogen bond and the increasing stability of the +3 oxidation state in heavier elements.

36. What is the trend in the first ionization energy for the boron family elements?

The first ionization energy generally decreases as we move down the boron family. This trend is due to the increasing atomic size and the greater distance between the nucleus and the outermost electrons in larger atoms. However, there may be some irregularities in the trend due to electron configuration effects and the increasing stability of the +1 oxidation state in heavier elements.

37. How does the ability to form catenated structures change as we move down the boron family?

The ability to form catenated structures (chains or rings of atoms of the same element) generally decreases as we move down the boron family. Boron has the strongest tendency to form catenated structures, while the heavier elements show less propensity for this behavior. This trend is due to the decreasing strength of element-element bonds and the increasing metallic character in the heavier elements.

38. Why does gallium have a lower melting point than both aluminum and indium?

Gallium has a lower melting point than both aluminum and indium due to its unique crystal structure and weak metallic bonding. Gallium atoms are arranged in a way that requires less energy to break the bonds between them, resulting in a surprisingly low melting point (29.76°C). This anomaly in the melting point trend is related to gallium's electron configuration and the specific interactions between its atoms in the solid state.

39. How does the tendency to form oxides change among the boron family elements?

The tendency to form oxides increases as we move down the boron family. Boron forms a covalent oxide (B2O3), while the heavier elements form increasingly ionic oxides. This trend is related to the increasing metallic character and the greater ease of electron loss in the heavier elements, allowing them to form more ionic bonds with oxygen.

40. What is the trend in the stability of the +1 oxidation state for boron family elements?

The stability of the +1 oxidation state increases as we move down the boron family. Boron and aluminum rarely exhibit a +1 oxidation state, while gallium, indium, and thallium show an increasing tendency to form compounds in the +1 state. This trend is due to the inert pair effect, where the outermost s electrons become more difficult to remove in heavier elements, making the +1 state more favorable.

41. How does the ability to form alloys change as we move down the boron family?

The ability to form alloys generally increases as we move down the boron family. Boron has limited alloying ability due to its covalent nature, while aluminum and the heavier elements have an increasing tendency to form alloys with other metals. This trend is related to the increasing metallic character and the ability to share electrons in metallic bonds.

42. Why does boron have a higher electronegativity compared to other elements in its family?

Boron has a higher electronegativity compared to other elements in its family because of its smaller atomic size and higher effective nuclear charge. The valence electrons in boron are closer to the nucleus and more strongly attracted to it, resulting in a greater tendency to attract shared electrons in chemical bonds.

43. How does the ability to form coordination compounds change among the boron family elements?

The ability to form coordination compounds generally increases as we move down the boron family