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Why Is Graphite Softer Than Diamond?

Why Is Graphite Softer Than Diamond?

Did you know that both diamonds and graphite are formed from the same element: carbon? Even though both of them are made out of pure carbon they drastically differ from one another in their physical properties.

As allotropes of the same element, one would expect these two to share a lot of similarities, but that simply isn’t the case. So, why is graphite softer than diamond?

The sole reason for graphite being softer than a diamond is the difference in their crystal structures. Graphite is a crystalline form of carbon that has carbon atoms ordered in a hexagonal structure making it extremely soft.

On the other hand, diamond features a tetrahedral crystal structure which gives it its unparalleled hardness.

You might be wondering, what’s the connection between the two? And why are they so different from one another? In this article, we’ve covered answers to these and many more questions so without further ado,

Let’s dive in.

About Diamonds

Diamond is, as you may already know, the hardest naturally existing element and it is an allotrope of carbon. Diamonds have four carbon atoms that are covalently bonded to one atom, making these stones extremely complex substances. 

Separating carbon atoms within a diamond is almost impossible due to the covalent bonding. So, the fact these gems are the hardest natural materials adds to their reputation instinctively.

The bond that connects the four carbon atoms is sp3 hybridization. Since diamonds have four electrons linked together to one carbon atom, they showcase a tetrahedral structure. Diamonds are actually crystal lattices created of symmetry-arranged atoms in a 3D structure. 

Furthermore, diamonds have an impressive mix of physical, chemical, and mechanical characteristics, including:

  • Hardness
  • Low coefficient of friction 
  • Low thermal expansion coefficient
  • Thermal conductivity
  • Electrical resistance

Diamonds are also chemically resistant and they reflect ultraviolet and infrared light.

Due to their extreme durability and famous luster, diamonds are widely used in jewelry. Additionally, because of diamonds’ hardness, they’re utilized to cut, drill, or grind other materials. 

About Graphite

Graphite is an allotrope of pure carbon which is layer structured. It is primarily found as a grey mineral that occurs in rocks. Three carbon atoms are connected via a sigma bond in graphite. 

Since graphite’s chemical bond looks this way, it’s very soft and it can be easily broken. In other words, because of the Van der Waals forces, the covalent bonds between carbon atoms are easy to break, making graphite a soft material. 

The properties of this carbon allotrope include:

  • High melting point
  • Slippery greasy feel
  • Insolubility in water and other organic substances 
  • Opaque and black appearance 

Besides that, graphite is used in pencils, and because of its high conductivity, graphite is also used in electronic products such as batteries and solar panels, but more on this topic later.

Types of Graphite

Graphite can be found in several forms, so we’ve covered different types of graphite down below:

Natural Graphite

A mineral form of graphitic carbon, also known as natural graphite, varies quite a bit in crystallinity. Generally, commercial graphite is mined and it can contain other minerals.

After graphite is excavated, it typically requires mineral processing such as froth flotation to concentrate the graphite. Furthermore, this natural graphite comes in three different forms, all of which are processed from naturally sourced graphite.

Each of these three forms has distinct properties that make them well-suited for various applications.

Crystalline Graphite

Crystalline graphite is the rarest and highest quality form of natural graphite and thus the most valuable. Solid graphitic carbon’s direct deposition from underground high-temperature fluids results in the formation of crystalline graphite. This type of graphite has 95-99% carbon purity without refining.

Amorphous Graphite

This type of graphite is formed when an anthracite coal seam and a metamorphic agent engage in contact metamorphosis. As a result of this reaction, the microcrystalline graphite is formed, which is commonly known as amorphous graphite. 

This graphite type is the most limited one among natural graphite. Based on the geological environment, the graphite content can be between 25% and 85%.

Flake Graphite

Exposing carbon material to high pressure and temperature results in the creation of natural flake graphite. Even though most flake graphite found on the market is derived from organic deposits, the carbon material can also be inorganic.

The carbon purity varies between 5% and 40% and it can be found in certain metamorphic rocks such as gneiss, schists, and limestone.

Synthetic Graphite

Synthetic graphite is produced by using coke and pitch. However, this graphite type isn’t as crystalline as its natural counterpart.

Synthetic graphite contains mainly graphitic carbon that has been obtained by chemical vapor deposition from hydrocarbons at extremely high temperatures or heat treatment of non-graphitic carbon.

What Are Allotropes?

Allotropy describes one element’s ability to exist in several different forms while being in the same physical state with different atom arrangements. These different atom forms are called allotropes of the given element.

Let’s say you have 36 balls that you can order in any number of patterns to achieve mutually-visually geometrical shapes. These imaginary balls represent atoms, and the various shapes they make are allotropes.

Allotropes of the same chemical element have different bonding arrangements, which leads to different physical and chemical properties of the substance.

In addition, different allotropes can differ in the molecule occurrence in the number of atoms.

Allotropes Of Carbon

Carbon, being the rockstar in the world of allotropes, has the ability to create many allotropes, due to its chemical structure. Carbon’s atomic number is 6, meaning it has 4 electrons in its valence shell.

Until now, there have been identified 8 allotropes of carbon, and the scientific research for discovering even more carbon allotropes is on.

However, the most popular carbon allotropes are definitely diamond and graphite. These two carbon allotropes are made of nothing but carbon but they visually appear incredibly different. 

Although their composition is identical, they come with different physical and chemical properties, because of the carbon atoms arrangement within their structures.

Why Is Diamond Hard But Graphite Is Soft?

The answer to this question boils down to one single factor: geometry.

The carbon atom arrangement within diamonds follows a tetrahedral fashion. What this means is that each atom is bonded to 4 other atoms, creating rigid covalent bonds.

This arrangement is very favorable in terms of energy and it grants characteristics such as strength and durability. Thanks to this crystal arrangement, diamond is the hardest naturally-occurring substance on Earth.

On the other hand, graphite has a completely different atom arrangement. Its carbon atoms are ordered in two-dimensional sheets, whereas each atom is connected to three other atoms to create hexagonal rings. 

Even though the atom bonding within each layer is covalent and thus pretty strong, the bonds between each layer are weak due to the Van der Waals forces.

As a result, the layers slide over one another. Also, they can detach from one another easily. These weak links between the several layers of carbon atoms make the graphite that is used in pencils flake off on paper, enabling you to write.

Additionally, graphite has a much lower density compared to diamond.

Related Read: Why Does Diamond Not Turn Into Graphite?

Graphite Vs. Diamond

Both diamond and graphite are crystalline types of carbon – both of them are made out of pure carbon. While they are identical in chemical structure, they vary quite a bit in their physical make-up. 

On the Moh’s scale of mineral hardness, diamond scores a perfect 10, making it the hardest naturally occurring mineral, whereas graphite rates 1, meaning it’s the softest.

Therefore, based on the physical structure, diamonds and graphite are utilized for various functions. Also, due to the rarity, diamonds are far more expensive than graphite.

Let’s take a look at how diamonds and graphite can be differentiated from one another:

Molecular Structure

Diamonds feature a giant covalent structure, with еасh atom covalently bonded to fоur other atoms in a tetrahedral order tо create a strong structure.

Graphite also has a giant covalent structure, but еасh carbon atom is covalently bonded to three other аtоmѕ in a hexagonal manner.

Hardness

Diamonds, scoring a 10 on Moh’s scale of mineral hardness, are extremely hard. This is because of rigid, tetrahedral carbon аtоmѕ arrangement.

On the other hand, graphite is extremely sоft. Sheets оf hexagonally ordered carbon аtоmѕ саn slide оvеr each other and can even detach from one another.

Electrical Conductivity

Diamonds are insulators with mobile electrons being аbѕеnt. All fоur valence electrons of carbon atoms are uѕеd in covalent bonds.

Graphite is a conductor. Three vаlеnсе electrons are used in covalent bonds with other carbon аtоmѕ. The rest of valence electrons can be scattered across the carbon atom planes.

Physical And Chemical Properties 

Thе physical and chemical properties of diamonds and graphite аrе аѕ follows:

Diamond

  • Colorless transparent substance with extrordinary brilliance thanks to its high refractive index
  • It’s quite heavy
  • It’ѕ the hardest known naturally occurring substance
  • Doesn’t conduct electricity 
  • It features high thermal conductivity as well as a high melting point

Graphite

  • It’s lighter than diamond and it slippery to touch
  • It’s a greyish black and opaque material
  • It’s good conductor or electricity but bad heat conductor

Diamond And Graphite Uses

In this section, we’ll cover the main uses of these two materials.

Diamond Uses

The first thing that comes to mind when you hear the word diamond is jewelry – diamonds are the most popular and sought-after gemstones used in jewelry.

In addition, diamonds have numerous industrial applications due to the fact that they are very hard. Diamonds are used in various tools that cut glass and pierce hard rocks. They can also be used as abrasives to cut and also polish other materials and even other gemstones.

Graphite Uses

The first thing that comes to mind when you hear the word graphite is pencils. “Lead” pencil cores are actually a mix of graphite and clay. Cleaved graphite flakes mark the paper or any other surface, and the clay serves as a binding material.

In addition, graphite is one of the main components in lubricants such as grease. Graphite reacts with water vapor to create a thin layer over adjacent surfaces reducing the friction between them.

Paints that guarantee wall protection have graphite in them. It’s no secret that factories mix powdered graphite in their paints to produce authentic protection for the walls.

Furthermore, the graphite’s capacity to absorb fast-moving neutrons is pretty high, therefore, this mineral is in use to stabilize or even neutralize the neutrons reactions.

Lastly, crystalline flake graphite is utilized in carbon electrodes, brushes, and plates manufacturing which are needed in the electrical industry. Interestingly enough, natural graphite is also processed into synthetic graphite which is useful in lithium-ion batteries.

Conclusion – Why Is Graphite Softer Than Diamond?

Diamond and graphite are chemically identical, both made up of carbon atoms. However, they have completely different crystal structures.

In diamond, the carbon atoms have rigid covalent bonds, making a diamond extremely hard. In graphite, carbon atoms are arranged in two-dimensional layers and each carbon atom is linked to three other atoms to create hexagonal rings making graphite soft and weak. 

Despite their differences, diamonds and graphite both have their own use. Diamonds are famous for being one of the most sought-after gemstones in jewelry and are also used in various types of industrial tools due to their hardness.

Graphite finds its use in pencils and lubricants, is added to paints as a means to protect the walls, and is even used in various electrical components used in the electrical industry.