Melting and boiling points are very important physical properties in chemistry. Different elements and compounds have different melting and boiling points. From values of melting and boiling points of elements, we can get a understanding of structure of elements, intermolecular forces between molecules and more.
IN THIS TUTORIAL,
Melting and boiling point variations are not clear (do not have uniform pattern) in periodic table. But we can see, some
elements have higher melting and boiling points and some have less. Here we study melting and boiling points of s, p, d blocks
IVAth group elements (C,Si) show high melting and boiling points because they have covalent gigantic lattice structures.
Yes. There are. Some compounds are not stable to heat. When heating a such compound, they decompose to another substance.
In first three periods, there is a clear variation. Melting and boiling points are incresed upto the 4th group with when going to the right. Then it starts to decrease upto noble gases.
There are many reasons to effect for melting and boiling points of elements.
Inert gasses have the lowest melting and boiling points element in period because their van der waals' forces are very weak.
Melting and boiling points of both halogen and inert gases increases along the group.
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 Li to Cs.
Alkaline earth metals(Be, Mg, Ca, Sr, Ba, Ra) have low melting and boiling points. 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.
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
B, Al, Ga, In, Tl 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 boiler points. Ga 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.
Boiling points increases on moving down from F to I
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 Zn does not contribute more electrons to the metallic lattice like other 3d metals. Hence strength of metallic lattice is less than other 3d metals lattices. So Zn has the lowest melting point in 3d metals.
Manganese electrons configuration is 3d5 4s2. That electrons configuration has some stability. So contributing electrons to the lattice is limited in Mn. Therefore lattice is not much strong. That is the reason why Mn has a 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.
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.
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.
Metallic lattice of magnesium is much strong than sodium. So melting point of Mg is higher than Na.
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