Mechanical Materials for Beginners 4: Interatomic Bonds (Metallic Bonds, Ionic Bonds, Covalent Bonds, Molecular Bonds)

In the previous explanation, we described the structure of atoms and the important ionic states of atoms.

This time, we will explain how atoms bond together to form molecules.

Originally, atoms are,It is extremely rare for atoms to exist in isolation; most of the time, they exist as substances (molecules) formed by the bonding of multiple atoms.

For atoms that are particularly prone to becoming ions, the electron shells may be stable, but they are not electrically balanced, making them extremely unstable.

そのElectrical imbalance has a significant impact on atomic bonding.

Furthermore, the way in which materials bond determines the general properties of a substance, so this is a very important topic that I hope you will make sure to understand.

The basic types of bonds between atoms include metallic bonds, ionic bonds, and covalent bonds.

Let's start by explaining the bonding of metals, which is frequently used in mechanical materials.

metal bond

The term "metallic bonding" suddenly appeared, but its meaning is simple: it's simply a type of bonding commonly found in metals, hence the name.

Before explaining the mechanism of metallic bonding, some of you may be wondering, "What exactly is a metal?" So, let's briefly review: the materials shown in the following diagram are typical metals used in mechanical materials.

Looking at the periodic table of chemistry, the elements enclosed in the blue box in the following diagram are often called metallic atoms (the definition of metal in chemistry is difficult and I didn't fully understand it).

To explain metallic bonding, let's consider iron (Fe, atomic number 26) and aluminum (Al, atomic number 13) as representative metals.

First, as a recap from last time, the number of electrons in iron and aluminum is 13 for aluminum (Al) and 26 for iron (Fe).

In each atom, since the electron shells are not yet closed, electrons exchange with other electrons to form an ionic state.

Aluminum (Al) has 13 electrons, so the electron arrangement is 2 in the K shell, 8 in the L shell, and 3 in the M shell. As it tries to close off in the L shell, it throws out 3 electrons, so the ionic state becomes the positive ion state $Al^{3+}$.

On the other hand, iron (Fe) has 26 electrons, so as explained last time, it has 2 in the K shell, 8 in the L shell, 8 in the M shell, and 8 in the N shell. However, iron (Fe) has a somewhat special electron configuration, and in fact, once 2 electrons enter the N shell, the electron shell stabilizes and electrons begin to enter the M shell.

Due to its properties, the electron configuration of iron consists of 2 electrons in the K shell, 8 in the L shell, 8 in the M shell, and 2 in the N shell, with an additional 6 electrons filling the M shell.

Here, something unusual happens: the M shell becomes closed with 8 + 6 = 14 electrons.

Therefore, the outermost electron orbital is the N shell, and there are about two extra electrons, which try to be released elsewhere, so the ionic state of iron is $Fe^{2+}$.

It's a bit complicated, but elements with a special electron configuration like iron are called transition elements, and in the periodic table, they are the atoms enclosed in the red box below.

The laws governing the electron configuration of these transition elements can be explained by a basic understanding of quantum mechanics, which divides electron shells such as K, L, M, etc., into further sub-shells such as s, p, d, etc., and how the electrons are arranged within those shells. However, I will omit the detailed explanation for now (I will do so if there is demand).

I feel that chemistry has a lot of exceptions, and that's why I have a hard time with it.

In any case, as exemplified by aluminum (Al) and iron (Fe), many metal atoms become positively charged ions.

When metal atoms enter an ionic state and are near other metal atoms of the same type, an interesting phenomenon occurs.

That is, Because metals become stable when they enter an ionic state,Each metal atom experiences a reaction that attempts to release excess electrons elsewhere.

At that time, the electron released elsewhere shares an electron shell with electrons from other atoms of the same metal.They begin to do so.

What's even more interesting isIt shares electrons not only with neighboring metal atoms but also with all metal atoms in the entire material.

These electrons share electron shells, causing the metal atoms to bond together and form a substance.

This is the mechanism of metallic bonding.

Characteristics of metallic bonding

A key characteristic of this bond is that electrons share electron shells throughout the entire material, so even if part of the bond is broken, it can easily recombine.

This propertyThis relates to the ductility properties of metals.

Furthermore, the fact that metals do not easily split when processed with cutting tools, and possess good workability (moderate bonding strength), is also due to this bonding property.

Conversely, because of its tendency to recombine easily, it must be designed with appropriate settings.Be careful, as sliding identical metal parts together will cause them to stick together (adhesion).

Other important characteristics include:Because electrons share electron shells throughout the entire material, they can move freely throughout the material. This allows electricity to flow easily when a voltage is applied (good conductivity).

Furthermore, when heat is applied, the thermal energy is converted into the kinetic energy of electrons and transferred, so it often exhibits very good thermal conductivity.

Furthermore, when you look at metallic materials with an electron microscope, you can see that many electrons are moving around in shared electron shells, making the entire material appear as if it's covered in a cloud.

These clouds reflect specific wavelengths of light, making metallic materials appear shiny and glossy.

in this wayMetallic bonds possess properties that make them very easy for humans to handle.

Incidentally, the bonding strength of metallic bonds is relatively weaker compared to ionic bonds and covalent bonds, which will be explained later.

Next, we will explain ionic bonds, which are relatively common in nonmetals, and covalent ionic bonds.

Ionic Bonds

Ionic bonding is very easy to understand.

First, when an atom becomes an ion, it gains or loses electrons, disrupting its electrical balance and causing it to acquire positive or negative charges.

At this timeIf the positive and negative charges of two atoms are equal, an electrostatic force acts between them, causing them to stick together.

このBonding caused by electrostatic force is called ionic bonding.

This type of ionic bond is frequently observed when bonding metal atoms to nonmetal atoms.

For example, a typical example of ionic bonding is when sodium (Na, atomic number 11), a metal atom, and chlorine (Cl, atomic number 17), a nonmetal atom, become ionic. Sodium becomes $Na^+$ and chlorine becomes $Cl^-$, and the atoms of each become electrically positive and negative, and electrostatic force acts to bond them together.

The substance formed by this bonding is sodium chloride (NaCl), which is the salt you are all familiar with.

Characteristics of ionic bonds

A characteristic of this ionic bond is that, because it is electrically balanced, it generally does not have good conductivity.

However, dissolve it in a solution like water.When a substance is dissolved in an aqueous solution, it is more likely to enter an ionic state, allowing electrons to move freely, thus resulting in good electrokinetic properties in aqueous solutions.

A typical example of this mechanism is why saltwater, such as seawater, conducts electricity well (pure water does not conduct electricity very well).

Next, we will explain covalent bonding, which is a type of bonding in nonmetals.

共有結合

The last of the basic types of bonding to be explained is covalent bonding, which is a type of bonding in which electrons are shared between atoms, thereby closing off electron shells.

Before explaining covalent bonding in detail, let's explain the properties of electrons. Electrons basically have the property of forming a stable combination of two.

In other words, electrons exhibit behavior that involves filling their electron shells and simultaneously trying to form pairs.

When each atom attempts to become an ionic state, the electrons it exchanges with other atoms pair up (2 electrons) with electrons from other atoms, and electrons are shared in order to close the electron shell.

このA bond formed by a force that attempts to share electrons is called a covalent bond.

At this timeThe resulting force is quite powerful and creates a very strong bond.

For example, when two nonmetals bond, silicon (atomic number 14) and oxygen (atomic number 8) become ionic, silicon becomes $Si^{4+}$ and oxygen becomes $O^{2-}$.

In this case, if there is only one oxygen atom, there are not enough electrons (8 electrons for stability from the octated electron shell) necessary for the M shell of the silicon atom to become stable, so two oxygen atoms are prepared.

In this process, electrons from the silicon atom and oxygen atom pair up to close their shells, and they share electrons to close their respective electron shells, resulting in silicon dioxide (SiO2).

This material is a representative example of glass, which you are all very familiar with.

Incidentally, since it would be very difficult to draw all the electrons in an atom every time, in the world of science, electrons other than those in the outermost electron shell are omitted and drawn as follows.

In the case of a covalent bond, electrons are shared between pairs, so the bond state is represented as follows:

Other representative molecules with covalent bonds, such as hydrogen (H2) and oxygen (O2), can be depicted as follows.

Characteristics of covalent bonds

The characteristics of this covalent bond are,Due to the strong bonding, it tends to form a very hard and strong substance.

Furthermore, its strong bonding results in a high melting point.

InsteadBecause electrons are tightly bound together and have difficulty moving, electrical conductivity and thermal conductivity are poor.

Also, instead of a strong bondMany materials are brittle because they lack the good recombinability of metallic bonds.

Glass is a good example of this characteristic; everyone knows that while it's hard and strong, it's also brittle and breaks easily.

Furthermore, because glass does not conduct electricity and has poor thermal conductivity, it is sometimes used as an insulating material (such as double-glazed windows for cold regions).

However,When ceramics or silicon are processed appropriately, they can be transformed into semiconductors with special properties, such as being able to conduct electricity or not.

This is an important characteristic that underpins modern society.

Finally, I will introduce molecular bonds, which are not bonds between atoms but are important bonds that connect molecules together.

molecular bond

The molecular bonds I will explain next are, as the name suggests, bonds that connect molecules together.

This molecular bond is often described in the same breath as the metallic, ionic, and covalent bonds we've discussed so far, but in reality, it's a completely different type of bond.

I, too, was greatly misled during my high school days.While metallic bonds, ionic bonds, and covalent bonds are bonds between atoms, molecular bonds are bonds between molecules that are already bonded together by atoms.

This is a point that requires considerable attention.Note that intermolecular bonding deals with molecules, not atoms.

Now, let's explain molecular bonding in detail.

We have explained that both ionic and covalent bonds, as described so far, involve atoms exchanging electrons to form a bond.

In other words, as a result of proper electron exchange, molecules, which are a bonded state between atoms, are electrically balanced.

However, upon closer observation of the molecule, it appears that the molecule as a whole is electrically balanced.Upon closer inspection, localized electrical imbalances are present.

Taking water (H2O) as an example, the nonmetallic atoms are basically bonded together by covalent bonds, and the overall structure is electrically balanced.

However, looking at it in more detail, the bond angle between oxygen (O) and hydrogen (H) is 104.45°, as shown in the diagram.It has an asymmetrical planar structure.

When we observe the electron distribution at this time, we see that a positive charge bias occurs towards the direction with two hydrogen atoms (H), and a negative charge bias occurs towards the direction with oxygen atoms (although the overall balance is maintained).

When more water molecules are brought closer to these water molecules, each has an uneven charge distribution.A coupling occurs locally due to the balance of electricity (a coupling due to electrostatic force).

In this way, many water molecules combine to form water, the substance you are all familiar with.

Such a combinationThis is called a molecular bond, and a type of bond formed by an imbalance in charge caused by hydrogen, such as in water, is called a hydrogen bond.

Molecules that have an electrical bias, such as water molecules, are called polar molecules, while molecules that do not have an electrical bias are called nonpolar molecules.

A typical example of a nonpolar molecule is carbon dioxide (CO2).

A characteristic of these nonpolar molecules is that they do not form hydrogen bonds, so they are poorly soluble in solvents (liquids) such as water.

A familiar and practical example is carbonated water, where carbon dioxide is forcibly dissolved in water, but as soon as you open the container, the carbon dioxide escapes—something many people have experienced (like how beer continues to foam).

In addition to this hydrogen bond,Some substances bond together due to an imbalance in electrical charge within their molecules.

Although it may not be a very familiar term, a typical example is its occurrence in hydrocarbons (such as CnH2n and CnH2n+2, where n is a natural number).

This is a fairly common substance and is a basic component of organic materials, which we discussed in the second lesson of the Mechanical Materials course.

For example, in the CnH2n+2 system, when n=1, CH4 becomes ethane, C2H6 becomes methane, and C3H8 becomes propane.

Ethane may be an unfamiliar substance to many, but methane is the main component of flatulence, and propane is used by a considerable number of people as a combustion gas.

As an example, the bonding state of these three substances can be depicted in a planar diagram as shown in the following figure.

This diagram shows that while the overall electrical balance is maintained, a closer look reveals an electrical imbalance, leading to the formation of molecular bonds.

like thisSome of the organic materials that come together and bond are called polymers, and these are the plastics that you are all familiar with.

That concludes the explanation of the molecular bonding mechanism; now, I will describe the characteristics of molecular bonding.

Characteristics of molecular bonding

Molecular bonds are basically electrically balanced.The bonding is caused by electrostatic forces resulting from the uneven distribution of electricity within the molecules, so it's not a bond formed by very strong forces.

Because it's simply a slight imbalance in electricity, the electrostatic force (the difference between positive and negative is small) is relatively weak.

However, even if the electrical bias is small, if the number of atoms that make up a molecule becomes enormous, the electrical bias will increase due to that enormous number.

For example, ethane (CH4) has five constituent atoms, but ipsyl (C20H82), which has an extremely large number of constituent atoms, has as many as 102, resulting in a greater imbalance of electrical charge within the molecule.

As the electrical imbalance increases, the electrostatic force also increases, which in turn strengthens the molecular bonding force..

In other words, a key characteristic is that the bonding strength changes depending on the number of constituent atoms (the more constituent atoms there are, the stronger the bond).

This concludes the explanation of typical bonding types.

Summary

This has turned out to be quite long, but let's summarize.

become.

Next, let's summarize the characteristics of each bond.

This concludes our summary of atomic bonding.

Next, we will summarize the bonds between molecules, which are formed by the bonding of atoms.

Becomes

This has become a bit long, but this concludes the explanation of atomic bonding and molecular bonding.

This content is something that is often memorized in the early stages of high school chemistry,I want you to understand the mechanism of each bond, rather than just memorizing it.

If we understand the mechanism of bonding, then naturally we willYou will also be able to understand the characteristics of each type of bond.

Specifically, in the field of mechanical design, relying on memorization can lead to selecting combinations of identical metals for sliding parts, which then stick together due to heat during operation because of their good recombination properties.

Furthermore, if you select materials without considering conductivity, water coming into contact with the machinery due to rain or other sources can spontaneously conduct electricity, leading to corrosion.

As you can see, the content of this article is surprisingly...This forms the basis for material selection in practical machine design.This is important information, so I really hope you understand it.

This time, we explained the basics of the four types of bonds learned in high school chemistry: metallic bonds, ionic bonds, covalent bonds, and molecular bonds. However, since metals are frequently used in mechanical materials, metallic bonds become extremely important.

So, next time, we'll delve into the mechanisms and characteristics of metallic bonding, which are what make it truly a mechanical material.

To those who found this article helpful in understanding design:

While there is basically no textbook covering this content, and it is my own original work, I will introduce the textbook that I have been using since I was a student.