Melting and boiling points are very important physical properties in chemistry when we study about elements and compounds. Different elements and compounds have different melting and boiling points. From studying values of melting and boiling points of elements, we can get a understanding of structure of elements, intermolecular forces between molecules or atoms and more.
In this tutorial, we will cover following sections.
Written by: Heshan Nipuna, updated: 2020/01/05
Melting and boiling point variations are not clear (do not have uniform pattern) throughout the periodic table (this means we cannot see similar trend always. You will understand this when you finish reading this tutorial).
But we can see, some elements have higher melting points and boiling points while some elements have less. In
first part of this tutorial, we learn melting and boiling points of
s, p, d blocks elements and their compounds and followed by organic compounds.
IVAth group elements (carbon and silicon) show high melting and boiling points in second and third periods respectively because they have covalent gigantic lattice structures.
In first three periods, there is a clear variation of melting and boiling points (has a clear trend). Melting and boiling points increase upto the IVA group with when going from left to the right. (As an example from sodium to argon in third period). IVA group has the highest melting and boiling point element. Then it starts to decrease melting and boiling points from VA group to noble gases (VIIIA).
There are many reasons to effect for melting and boiling points of elements and compounds. One or several of things may effect to melting and boiling points.
Inert gasses have the lowest melting and boiling points element in period because their form only van der waals' forces are they are very weak to form a strong intermolecular force between atoms.
Now, we are going to learn about, how melting and boiling points of elements vary in group by considering taking each group separately.
S block contains group IA and group IIA and most elements of they are metals without hydrogen. (hydrogen has both alkali metal properties and halogen properties.)
Alkali metals (Li, Na, K, Rb, Cs) are soft and have low melting and boiling points. Alkali metals have only one valence electron per metal atom and therefore, the energy binding the atoms in the crystal lattice of the metal is low. Therefore, the metallic bonds in these metals are not very strong. So melting and boiling points decreases on moving down from lithium to cesium.
Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) have low melting and boiling points when compared with d block metals. But their melting and boiling points are higher than corresponding alkali metals in the same period due to comparatively smaller size. But melting and boiling points do not show regular trends in alkali earth metal group.
Both alkali and alkali earth metals are in s block. We know alkali metals have only one valence electron per metal atom.
But alkaline earth metals have two valence electrons per metal atom.
Also alkali earth metals are small in size than alkali metals.
When number of valence electrons in the lattice increases, the metallic bond is strong. Also when atomic radius decreases, metallic bond become strong. Also Therefore metallic bonds of alkali earth metals are much stronger than alkali metals.
Therefore melting and boiling points of alkali metals are less than melting and boiling points of alkali earth metals
Beryllium is the smallest size atom of the group 2 elements. So its ionic lattice is more stronger than other alkaline earth metals.
Now we are going to discuss about variations of melting and boiling points of p block elements.
In the p block, there are varies types of elements including metals, non-metals and their physical states are also very different. At room temperature, some of they are in solid state, and some are at gaseous state. Bromine is at liquid state too.
P block contains the highest melting points element (carbon) and lowest melting point element of the periodic table (helium).
Now, we are going to look melting and boiling points of p block elements from group 13 to group 18.
Halogen and noble gases are located in p block of the periodic table. Halogens exist as diatomic molecules such as F2, Cl2, Br2, I2 and noble gases exist as monoatomic molecules.
When going down the each group, molecular mass increases which can be reason to have a higher melting and boiling points. Also, all halogen and inert gas molecules form only van der waals' forces which are weakest intermolecular forces and does not effect much to melting and boiling of halogen and noble gases. Molecular mass of molecules have the greatest effect to increment of melting and boiling points. So Melting and boiling points of both halogen and inert gases increase along the group.
Boron, Aluminum, Gallium, Indium, Thallium are the elements of group 13 elements. Melting and boiling points decrease on moving down the group. However, the decrease in melting point is not as regular as in boiling points. Gallium has very low melting point (303K).
The atoms of this group form covalent bonds with each other and therefore, there are strong binding forces between their atoms in both solid and liquid states.
Melting and boiling points of group 14 elements are much higher than group 13 elements.
When moving down the group, the melting and boiling points decreases.
Melting and boiling points of 3d metals are generally higher than s block elements.
Vanadium has the highest melting point and zinc has the lowest melting point.
But melting and boiling points do not show regular trends.
Zinc has a stable electrons configuration, 3d10 4s2. Therefore zinc does not contribute much electrons to the metallic lattice like other 3d metals. Hence strength of metallic lattice is lower than other 3d metals lattices. So zinc has the lowest melting point in 3d metal series.
Electrons configuration of manganese is 3d5 4s2. That electrons configuration has some stability because all five d orbits are half filled (each d orbit has one electron.) So contribution of electrons to the metallic lattice is limited in manganese. Therefore lattice is not much strong. That is the reason why manganese has a sudden drop in melting point.
Melting and boiling points of alkali metal
decreases with the increase in atomic mass of the halides as:
F- > Cl- > Br- > I-
Ex: Melting point of NaCl is higher than NaBr
For given halide ion, melting and boiling points of LiX is always less than NaX.
Thousands of organic compounds are discovered so far by scientists in the world. With discovering lot of compounds, organic chemistry was born. In this chapter we are going to discuss melting and boiling points of organic compounds.
Following facts are important when we studying melting and boiling point values of organic compounds.
ethyne (alkyne compound) has the highest melting and boiling point.
Halogen atom is more electro negative than carbon atom. So C-X bond is polarized. There are dipole dipole interaction between alkyl halide compounds. These interactions are much stronger than intermolecular forces between alkanes,
Now we discuss some problems by comparing different elements and compounds which have different melting and boiling points. These problems very important in examinations. Study them carefully.
First we will look, what are the p block metals and what are the s block metals. You know when we discuss about melting points of metals, their metallic lattice is so important. So now you know what should we find out to compare melting points of p block metals and s block metals.
When metallic lattice of a metal is strong, that metal has a great chance to has a higher melting point.
As examples, two metals, sodium and aluminium are taken to compare. Sodium is s block metal and aluminium is a p block metal. But both are located in 3rd period of the periodic table.
Due to release of three electrons and less radius, metallic lattice of aluminium is much stronger than sodium. So melting point of aluminium is greater than sodium.
d block elements can contribute more electrons to the metallic lattice. As an example vanadium can contribute 5 electrons.
Contributing more electrons to the metallic lattice will increase the strength of metallic bonds.
Due to more strong metallic bonds, d block elements have higher melting values.
Tungsten (W). From metals, tungsten has the highest melting point in periodic table. It is located in d block. 3,422 0C is the melting point of tungsten.
Beryllium has the highest melting point from s block metals. It is about 1,287 0C
Mercury (Hg) has the lowest melting point (-38.83 0C) because mercury has a very weak metallic lattice.
Helium (He) is the element which has lowest melting point (-272.2 0C). Helium exist as atoms. It does not form compounds and no intermolecular force between He atoms. Also relative molecular mass (1) is very low.
We know, elements in periodic table are in solid state, liquid state and gaseous state. Intermolecular forces, relative molecular mass are factors which decides melting and boiling point of element.
Let's consider two metals. One metal has very high melting point than other one. In metals, metallic lattice is the major factor of deciding melting and boiling point. More strong metallic lattice have higher melting point.
We can understand about elements' intermolecular forces, relative molecular masses from studying melting and boiling points.
We can list several flammable gases and their melting and boiling points of them.
Alkanes are easily flammable. As an example, consider methane. Methane's melting and boiling points are -182.40C and -161.50C respectively.
Melting point depends on their molecular mass and intermolecular forces between elements or molecules. Different elements have different molecular mass and intermolecular forces. So their melting point values are different from other elemets and compounds.
Lattice strength of calcium is greater than potassium due to two reasons.
Due to these two reasons, metallic lattice of calcium is much greater than potassium.
Yes. There are. Some compounds are not stable to heat. When heating a such compound, they decompose to another substance.
Metallic lattice of magnesium is much strong than sodium. So melting point of Mg is higher than Na.
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