Electrons are responsible for most of the chemical behavior of atoms, but not all electrons participate in chemical bonding in the same way. Factors such as their energy, arrangement, and position from the nucleus impact how they interact with other atoms.
Electron shells provide a simplified way to describe how electrons orbit the nucleus of an atom. The valence shell is the outermost electron shell, which contains valence electrons. Valence electrons are primarily responsible for ordinary chemical bonding because their behavior helps to decide whether atoms share, gain, lose, or redistribute electron density upon interaction.
These processes impact the structure of adhesive polymers, the polarity of molecules, the reactions involved in curing, the oxidation of metal surfaces, and the chemical interactions between the adhesive and the substrate.
What are Electron Shells?
Electrons occupy quantized energy states and orbitals around the nucleus of an atom. A quantized energy state is basically a system where electrons can only occupy permitted energy values such as a staircase where an electron can only occupy one step or another but not the space in between. It moves up this staircase by absorbing energy and moves down the staircase by releasing energy. This system physically restricts the electrons to particular energy levels rather than being able to possess any possible energy level.
These permitted energy states are grouped into layers of electron shells. It is important to know that electron shells are not hard physical layers around a nucleus. The simplified representations of how electrons are organized do not accurately depict reality. Instead, think of electron shells as being organized more like a cloud around the nucleus than a set of rings and these regions are called orbitals. Therefore, electron shells are a collection of orbitals.
Electrons that are in lower energy states are closer to the nucleus while electrons in higher energy states are farther away from the nucleus.
The arrangement of electrons within an atom is referred to as the electron configuration which strongly influences how atoms react chemically to form bonds.
How Electrons Occupy Atomic Energy Levels
Electrons often occupy lower energy states before higher energy states. Within the electron energy field, there are regions referred to as sub-shells and orbitals. An orbital is the region in which an electron is likely to be found.
Electron shell diagrams are useful abstract teaching tools, but electrons do not travel around the nucleus in fixed circular paths like planets orbiting the Sun. Instead, electron behavior inside orbitals occurs primarily through probability distributions.
The key takeaway is that electrons are organized by energy, and electrons closer to the nucleus are less available for chemical interactions than electrons associated with the outer parts of an atom.
Principle of Minimum Energy
Before we transition to valence electrons, we explore the principle of minimum energy. The principle of minimum energy states that atoms tend to adopt the lowest-energy arrangement available. When atoms approach each other, they experience attractive and repulsive forces that interact until a stable bond forms, where the combined arrangement of the bond has less total energy than the atoms had when separated. Therefore, electron interactions only produce stable bonds when the resulting distribution lowers the total energy of the combined system.
What are Valence Electrons?
Valence electrons are the electrons located in the outermost shell of an atom and participate the most in chemical bonding and reactions. Valence electrons are accessible to neighboring atoms and are less shielded from chemical interactions with them. Through interactions between valence electrons, atoms can form covalent, ionic, metallic, and other types of chemical bonds.
The arrangement of valence electrons strongly influences whether an atom tends to gain, lose, or share electrons; how many chemical bonds it forms; its chemical reactivity; the polarity of those bonds; its oxidation behavior; and the structures it creates with other atoms.
Valence Electrons and Chemical Stability
Atoms form chemical bonds because the combined arrangement of electrons is more stable and at a lower energy state than when they are separated. In chemistry, this phenomenon is often stated using the octet rule, which proposes that atoms tend to form bonds that give them 8 electrons in their outer shell. This is not universally true, so the general guiding principle is that atoms form chemical bonds when changes in electron distribution create a more energetically stable structure.
How Valence Electrons Form Covalent Bonds
A covalent bond is a common type of chemical bond that forms when electron density is shared between atoms. The electrons are attracted to the positively charged nuclei of both atoms and help hold them together. Covalent bonding is commonly employed in adhesive chemistry. Carbon, a common component of organic adhesives, helps connect to other elements such as hydrogen, oxygen, chlorine, nitrogen, sulfur, silicon, and others. This enables carbon-containing adhesives to form long molecular chains, rings, branched structures, rigid thermosets, flexible elastomers, and crosslinked polymer networks.
Adhesives that contain carbon and depend on covalent bonding include epoxies, polyurethanes, acrylics, cyanoacrylates, polyvinyl acetate, and many rubber-based adhesives.
How Valence Electrons Form Ionic Bonds
Ionic bonds develop when the electron transfer between atoms creates oppositely charged ions. Remember, ions are atoms that have gained or lost electrons. When an atom loses one or more electrons, it becomes positively charged and is called a cation. When an atom gains one or more electrons, it becomes negatively charged and is called an anion. This creates electrostatic attraction between the ions, which pulls the ions together to form highly stable, three-dimensional structures known as crystal lattices. These ionic bonds form to create substances referred to as ionic compounds. Ionic compounds are relevant to adhesive bonding because they are found in mineral substrates, ceramic materials, concrete and masonry, surface salts, corrosion products, cleaning residues, and fillers and pigments.
How Valence Electrons Contribute to Metallic Bonding
Metallic bonding occurs when valence electrons are distributed among many metal atoms rather than bonding merely between two atoms. These de-localized electron distributions lead to metallic properties such as thermal conductivity, electrical conductivity, ductility, and metallic luster.
Metal surfaces are the interface on which the adhesives interact, but metal surfaces are exposed to air, water, and environmental substances before application. These interactions between metals and their environment lead to many hydroxides, oxides, and coatings forming on the metal surface. Therefore, the adhesive applied on the surface of the metal mainly interacts with the chemistry of the exposed surface rather than the clean bulk metal.
Valence Electrons and Electronegativity
Electronegativity is the tendency of an atom to attract shared electron density within a chemical bond. When two atoms have similar electronegativities, their electron density is distributed relatively evenly. These bonds are nonpolar or weakly polar. However, if the electronegativities are unbalanced, the atoms will attract shared electrons more strongly, leading to polar covalent bonds with partial positive and negative regions.
Less electronegative elements such as carbon and hydrogen interact develop uneven electron density with more electronegative elements such as oxygen, nitrogen, and chlorine. The partial positive and negative charges in a chemical bond can impact molecular polarity, dipole interactions, hydrogen bonding, chemical reactivity, water sensitivity, and interactions with substrate surfaces.
While electronegativity is one component of adhesive strength, other factors such as wetting, curing, stress accommodation, and resistance to environmental exposure all play a role.
Valence Electrons in Common Adhesive Elements
Several elements are central to adhesive and sealant chemistry because of the bonding structures their valence electrons enable them to form.
Carbon
Carbon is a central component of most organic adhesive polymers. It has a unique electron distribution of four electrons that enables it to create up to four stable covalent bonds in the form of chains, rings, branches, and crosslinked structures. These networks can be used to engineer bonding agents from pressure-sensitive adhesives to rigid structural epoxies.
Hydrogen
Hydrogen can form one covalent bond under typical conditions and is present in many organic adhesive polymers and additives. When hydrogen is bonded to an electronegative atom such as oxygen or nitrogen, it may further bond to another suitable chemical group. Hydrogen bonding contributes to cohesion within an adhesive as well as adhesion to a substrate.
Oxygen
Oxygen primarily participates in two covalent bonds and is found in many adhesives such as epoxides, carbonyl groups, hydroxyl groups, ethers, esters, and urethane linkages.
Oxygen is electronegative and attracts shared electron density strongly, which is why it is often polar and chemically reactive.
Nitrogen
Nitrogen is found in many adhesive groups such as amines, amides, polyurethanes, polyamides, and cyanoacrylates.
Groups that contain nitrogen can participate in curing reactions, acid-base interactions, hydrogen bonding, and other types of chemical reactions.
Silicon
Silicon is important in silicone polymers, silane adhesion promoters, glass, silica fillers, and mineral surfaces. Silicon polymers contain repeating silicon-oxygen bonds, which contribute to their thermal stability, flexibility, and resistance to weather.
Sulfur
Sulfur is found in polysulfide sealants, rubber-curing systems, and specialized adhesive formulations.
It participates in bonding through polymer backbones and crosslinks.
Fluorine
Fluorine is effective at attracting electron density and works with carbon to form strong bonds. However, fluoropolymers are highly chemically resistant and have low surface energy, which makes them durable but challenging to bond without specialized treatment.
In summary, the element itself does not determine the adhesive’s performance; the arrangement of atoms, polymer architecture, functional groups, and surface conditions all contribute to effective bonding.
Valence Electrons and Adhesive Curing
Chemical curing involves electron redistribution as chemical bonds are formed, broken, or reorganized. During typical adhesive curing reactions, the elements retain their identity, meaning that carbon remains carbon and oxygen remains oxygen, but what changes is how their atoms are connected through their electrons. For instance, when an epoxy resin reacts with a suitable curing agent, molecules react and form covalent bonds. These connections result in a crosslinking of molecules into a three-dimensional polymer network, solidifying the adhesive bond.
Other adhesive types cure using different electron-driven reactions. For example, acrylic adhesives use a process called polymerization, which occurs through an anionic process initiated by small amounts of surface moisture.
However, some adhesives do not cure through chemical reactions but instead through temperature changes or solidification.
Valence Electrons at the Adhesive-Substrate Interface
The bonding interface is where the adhesive contacts the substrate. For adhesion to initiate, the adhesive needs to spread across the substrate surface area and be in close contact with it. The distribution of valence electrons affects the chemical interaction at the interface through surface polarity, hydrogen bonding, acid-base behavior, oxidation, chemical reactivity, interactions between partial charges, and reactions involving primers and adhesion promoters.
Not all adhesives create new covalent bonds, but some rely on intermolecular attraction, mechanical interlocking, diffusion, or a combination of mechanisms.
For example, epoxy adhesives made from carbon interact with a substrate such as wood by forming polar chemical bonds with oxygen-containing hydroxyl groups in the wood.
Avoiding Common Misconceptions
When learning about electron shells and valence electrons, there are some key takeaways to avoid misconceptions.
Electron Shells Are Not Solid Physical Layer
Diagrams intended to depict electron shells are simplified abstract models used to represent the electron-energy organization. Electrons are best identified through quantum mechanical orbitals and probability distributions.
All Outer Electrons Do Not Behave Identically
An electron’s redistribution in bonding depends on its energy, element, orbital, and chemical environment. Variability can exist between electrons due to these reasons.
Polar Bonds Do Not Guarantee Strong Adhesion
While polarity can enhance intermolecular attraction, adhesive performance also relies on wetting processes, curing, surface cleanliness, joint design, environmental exposure, and mechanical properties of adhesive and substrate.
Adhesives Do Not Always Form Covalent Bonds with Substrates
Some adhesives form covalent bonds across the interface, but others rely on intermolecular attraction, diffusion, mechanical interlocking, or a combination of these mechanisms
Conclusion
Electron shells depict the structure of how electrons organize themselves inside an atom. Valence electrons are the electrons in the outermost shell and directly participate in most chemical bonding. The arrangement of electrons influences whether the atom gains, loses, shares, or redistributes the electron density. When atoms undergo these processes, they form covalent, ionic, or metallic bonds, which help determine molecular polarity, oxidation, chemical reactivity, and material structure.
Valence electrons also contribute to the formation of polymer chains, curing reactions, crosslinked networks, polar functional groups, surface oxides, and intermolecular attractions. While valence electrons explain much of the adhesion process, other factors such as wetting, surface preparation, and polymer mechanics are also important.
