Sodium chloride (NaCl) lewis dot structure, polar or nonpolar, crystal or lattice structure, unit cell, molecular structure

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Crystal structure of NaCl

Sodium chloride composed of two ions Na+ and Cl- is an ionic compound having the chemical formula NaCl. It appears as a white crystalline solid and is usually odorless in nature. The most common use of sodium chloride is in the Solvay process for the production of sodium carbonate and calcium chloride.

In this article, we will discuss sodium chloride (NaCl) lewis dot structure, crystal or lattice structure, is it polar or nonpolar, shape, unit cell, etc.

Sodium chloride is commonly used as a condiment and food preservative, and in various industrial processes for the production of various chemicals. Also, it has a role as an emetic and a flame retardant.

Properties of Sodium chloride

  • It has a molar mass of 58.443 g/mol.
  • It appears as a white crystalline solid.
  • It has a melting point of 800.7 °C and a boiling point of 1,465 °C.
  • Sodium and chloride ions represent in 1:1 in NaCl chemical formula.
  • It has a coordinate geometry of octahedral for Na+ and Cl-.
Name of MoleculeSodium chloride
Chemical formulaNaCl
Coordination number6:6
Unit cells4
Crystal structureFace-centered cubic
NatureIonic
Total Valence electron for NaCl lewis structure8

How to draw lewis structure for NaCl (Sodium chloride)?

NaCl lewis structure is unique and very interesting to draw because it is an ionic compound formed from metal(Na+) and nonmetal(Cl-). The lewis dot structure of NaCl contains one positive charge on sodium metal and one negative charge on chlorine nonmetal. We have to represent them by putting brackets around them.

The lewis diagram of an ionic compound is formed with a different approach than the normal procedure we used for drawing the compounds like NH3, BF3, BrF5, etc.

Steps to draw electron dot structure or lewis dot structure of NaCl

Step 1: In the first step, we have to count the valence electron available for drawing the lewis structure of NaCl. For knowing the valence electron you have to remember a periodic group of each atom.

As sodium belongs to the 1st periodic group and chlorine belongs to the 17th group, hence, the valence electron of sodium is 1 and for the chlorine atom, it is 7.

⇒ Sodium valence electron = 1

⇒ Chlorine valence electron = 7

∴ Total valence electron available for drawing the lewis dot structure of NaCl = 1 + 7 = 8 valence electrons

Total valence electron available for drawing the lewis structure of NaCl (sodium chloride)

Step 2In ionic compounds, the metal atom is the electron donator and the nonmetal atom is the electron acceptor. So, in the case of the NaCl compound, the sodium(Na) is the metal that wants to give the one-electron away and forms Na+ ion whereas chlorine is the nonmetal that will accept the one-electron and forms Cl- ion.

Na metal losing electron in NaCl lewis structure

chlorine gaining electron in NaCl lewis structure

Sodium chloride (NaCl) lewis structure

As you see in the above figure, Na metal donated its one electron to the chlorine nonmetal, hence, Na forms Na+ cation and Cl forms Cl- anion.

The lewis diagram is all about completing the octet of atoms and achieving stability.

For the Na atom to obtain an octet, it must lose an electron; for the Cl atom to gain an octet, it must gain an electron. 

So, in the above NaCl lewis structure, the chlorine ion gets 8 electrons represented as dots in its outermost shell, hence, achieved stability because it completed the octet.

Whereas sodium atom has only one valence electron in its outermost shell, so giving it up would result in the same electron configuration as neon(noble gas).

Hence, both chlorine and sodium ions have the electron configurations of noble gases, with a full valence shell.

⇒ Sodium loses the electron, hence it gains a positive charge.

⇒ The chlorine atom accepts the electron, hence it gains a negative charge.

The positive and negative charge ions of sodium and chloride will attract each other to forms an ionic bond.

Sodium chloride (NaCl) lewis dot structure

In the NaCl lewis dot structure, we put the bracket around sodium and chlorine atoms to show their positive and negative charges.

That’s all, we have completed the lewis structure of the NaCl (sodium chloride) compound.

Electron dot structure of NaCl

What is the crystal or lattice structure of NaCl and its unit cell?

A crystal structure is simply a three-dimensional arrangement of the atoms in space whereas lattice is a geometrical concept that is used to describe the structure of the crystal.

“When a group of molecules tends to arrange each unit repeatedly at a lattice point, a crystal is made.”

In a crystal structure, there is a pattern of arranging the atoms or units. These patterns are located upon the points of a lattice. These lattice points are arranged in a three dimensionally ordered manner.[differ]

Now, what is the lattice or crystal structure of NaCl? NaCl has a cubic crystal system and face-centered lattice type, this can also be said, NaCl has a crystalline structure of face-centered cubic.

NaCl is a face centred cubic unit cell that has four cations and four anions.

“Unit cells are the simplest basic units which repeat itself in a certain pattern to form a complete crystal lattice.”

The crystal structure of NaCl salient feature:

  • NaCl has two ions (Na+ and Cl-) basis, “A basis is defined as the atoms or group of atoms that are associated with each lattice point in the crystal system.
  • The lattice of sodium chloride contains 8 total ions in one unit cell, four are Na+ ions and the other four are Cl- ion.
  • Sodium-ion is present at the body centre and edge-centres while chloride is present at corners and face centres. It can also be interchanged.
  • Every Na+ ion is surrounded by 6 Cl- ions whereas every Cl- ion is also surrounded by 6 Na+ ions. So, we can say NaCl structure has 6:6 coordination because coordination number defines the number of atoms or ions immediately around a particular atom or ion.

The corner site of the NaCl crystal structure is shared by the 8 unit cells (1 corner equal to 1/8 of ion or atom) and the 6 face site is shared by 1/2 atoms or ions.

So, the total number of ions or atoms present in face-centred cubic of NaCl unit cell = 8(1/8) + 6(1/2) = 4

∴ There are 4 atoms or ions present in each unit cell of a face centred cubic structure, hence, the number of NaCl units present in a unit cell of NaCl is 4

Crystal or lattice structure of NaCl (sodium chloride)

Image source: Google image

NaCl has a cubic unit cell that can be represented as a face-centred cubic array of anions with an interpenetrating fcc cation lattice (or vice-versa). This unit cell of NaCl looks the same whether we start with cations or anions at the corners. Each of these ions is 6-coordinate and has a local octahedral geometry.(source)

Sodium chloride polarity: is NaCl polar or nonpolar?

There is so much confusion for the students on whether NaCl is polar or nonpolar in nature?

As we know, NaCl is an ionic compound made up of one metal and nonmetal. It contains two ions Na+ and Cl- that are attracted by the electrostatic force generated between them.

If we go with the true definition of polar and nonpolar in chemistry, it is said that polar molecules have some net dipole moment and it occurs where the large difference of electronegativity between the atoms exists whereas nonpolar molecule is just opposite to it.

In the NaCl compound, Na+ has an electronegativity of 0.93, and chlorine ion has an electronegativity of 3.16, the difference of electronegativity between them is greater than 2.

As per Pauling scale

⇒ When the electronegativity difference between the atoms is greater than 2, then the compound is said to be ionic in nature.

⇒ When the electronegativity difference between the atoms is less than 0.5, then the compound is said to be nonpolar covalent in nature.

⇒ When the electronegativity difference between the atoms lies between 0.5 to 2, then the compound is said to be polar covalent in nature.

So, in the case of the NaCl compound, the difference of electronegativity between atoms is greater than 2, hence, it definitely is an ionic compound.

But Is NaCl polar or nonpolar? NaCl is neither polar nor nonpolar because the bond formed between the Na+ and Cl- is ionic, the polar and nonpolar usually occur where the covalent bonds between the atoms exist.

Ionic bonds occur when there is a complete transfer of electrons from one atom to another and they are connected by the electrostatic force of attraction whereas a covalent bond is sharing of electrons between the atoms.

Even NaCl atoms are separated by positive and negative charge, and the difference of electronegativity between the Na and Cl is high, still, we can’t say NaCl is polar or nonpolar.

Polar and nonpolar terms are reserved for molecular compounds like NCl3, NH3, H2O, etc. – It is not applied to the ionic compounds like NaCl.

If you ignore the term “covalent character” in compounds, then yes, you can say NaCl is a polar molecule on the basis of the electronegativity difference between Na+ and Cl-.

Also, polar molecules have a major covalent character and very minor ionic character, and in NaCl compound, it has a very large ionic character, so, we can’t say it is either polar or nonpolar.

FAQ

Why does sodium ion transfer the electron in sodium chloride (NaCl) lewis dot structure?

In ionic compounds, one metal transfers all its electrons to the nonmetal, and metal-induced the positive charge whereas nonmetal gains the negative charge.

So, in the case of NaCl lewis dot structure, Na atom is metal and Cl is a nonmetal, so, in order to complete the octet and gains the stability, Na metal transfer one electron to the chlorine atom.

After transferring the electron, the sodium atom gains the positive charge(Na+) whereas the chlorine atom gains the negative charge(Cl-).

Both ions(Na+ and Cl-) lie in the zone of noble gas configuration with full valence shell, hence, gains stability.

Therefore, in order to complete the octet and gain stability, the transformation of electrons from one metal to nonmetal takes place.

How many molecules of NaCl are present in each unit cell?

The corner site of the NaCl crystal structure is shared by the 8 unit cells (1 corner equal to 1/8 of ion or atom) and the 6 face site is shared by 1/2 atoms or ions.

So, the total number of ions or atoms present in face-centred cubic of NaCl unit cell = 8(1/8) + 6(1/2) = 4

∴ Each unit cell has 4 atoms of each Na+ and Cl-. Hence, there is a total of 4 molecules of NaCl in each unit cell.

What are the lattice type and crystal systems present in the NaCl compound?

NaCl has a Face centered lattice type and cubic crystal system.

Or you can say “The Bravais lattice of NaCl is a face-centered cubic lattice”.

Why is NaCl compound neither polar nor nonpolar in nature?

This is because the bond that occurs between the Na+ and Cl+ is ionic, not covalent.

For polar and nonpolar nature, there must be a high covalent character in molecule and atoms should be shares electrons with a covalent bond.

But in the case of the NaCl compound, the Na atom transfers the electron completely to the Cl atom, hence, there is no sharing of electrons between these like a covalent bond.

Therefore, we can say, the presence of high ionic character in NaCl leads to neither polar nor nonpolar in nature.

Summary

  • The total valence electron is available for drawing the Sodium chloride (NaCl) lewis structure is 8.
  • NaCl is a face-centered cubic unit cell that has four cations and four anions.
  • In the NaCl lewis dot structure, the sodium atom completes its octet by transferring one electron to the chlorine atom.
  • NaCl has a Face centered lattice type and cubic crystal system.
  • Sodium chloride (NaCl) compound is neither polar nor nonpolar, it is an ionic compound with very high ionic character. If you ignore the term “covalent character or covalent bond” from the polar or nonpolar definition, then on the basis of electronegativity difference between Na+ and Cl-, you can say NaCl is a polar molecule.
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