Every adhesive, substrate, sealant, coating, primer, and surface contaminant is composed of atoms. Atoms are made from three primary particles known as protons, neutrons, and electrons. These components of an atom are all essential but they don’t all contribute the same to adhesive bonding. The number of protons is responsible for creating the identity of the atom and creating a positive charge in its nucleus. Neutrons play a somewhat passive role in the sense they help maintain the nuclear mass and stability of the nucleus but don’t play an active role in the chemical reactions that take place during adhesive bonding. However, electrons may play an especially pivotal role in initiating and sustaining bonds and chemical reactions. Protons and neutrons sit in the nucleus of an atom while the electrons are outside the nucleus.
Learning the roles that protons, neutrons, and electrons play helps us understand how adhesive molecular interactions form, why metals oxidize, how curing occurs, and why different substrates have different chemical reactivity at the bonding interface.
What is Inside an Atom?
An atom is the smallest functional unit that retains the identity of a chemical element. It has a positively charged nucleus and is surrounded by negatively charged electrons. The nucleus consists of 99.9% of the atom’s mass, while the electrons determine the size and chemical behavior of the atom. A proton has a relative charge of +1, while a neutron has a relative charge of 0, and an electron has a relative charge of -1. It’s the difference between the number of protons and electrons that determines whether the atom has a positive or negative electrical charge. An atom that has gained or lost electrons is called an ion.
What is a Proton?
A proton is a positively charged particle located in the atom’s nucleus. The number of protons is referred to as the atomic number. The atomic number determines the element the atom belongs to. For instance, every carbon atom has six protons, and every oxygen atom has eight protons. Likewise, every silicon atom has fourteen protons, and every aluminum atom has thirteen protons. You’ll often see the number of protons written in the top left-hand corner of each element on the periodic table. Once the number of protons is changed, the element itself changes. The chemical element is therefore defined as a species of atoms that have the same number of protons in their nuclei.
How Protons Influence Chemical Behavior
The protons usually do not leave the nucleus during chemical bonding. Their influence is mainly electrostatic. The positively charged nucleus attracts the negatively charged electrons and therefore the number of protons influences how strongly the nucleus can attract and retain electrons. The attraction between protons and electrons is not determined by proton count alone but can be affected by the electron’s distance from the nucleus, shielding from other electrons, the atom’s electronic arrangement, and interactions with neighboring atoms.
Why the Proton Number is Important in Bonding Science
The proton number is primarily responsible for defining the identity of the element. For example, carbon which has 6 protons is central to most organic adhesive polymers. Likewise, oxygen appears in epoxies, acrylics, and polyurethanes. The proton does not directly affect the ability to glue two surfaces together but the number of protons influences how the electrons of the atom and neighboring atoms behave,
What is a Neutron?
A neutron is a particle inside the nucleus of an atom that is electrically neutral. Since neutrons do not have any charge, they do not attract or repel electrons in the same way that protons do. A neutron is mainly responsible for the atomic mass, identity of an isotope, and nuclear structure and stability. An atom of the same element will share the same number of protons with other atoms of the element but may have a different number of neutrons. If the number of neutrons is different from other atoms, it is called an isotope. Isotopes are nuclides with the same atomic number but different mass numbers. Deuterium is an isotope of hydrogen because protium has one proton and no neutrons, but deuterium has one proton and one neutron. Both deuterium and protium are classified as hydrogen.
Do Neutrons Affect Chemical Bonding?
Neutrons usually do not participate in chemical bonding. Chemical bonding is mainly about the distribution and interaction of electrons. Isotopes behave relatively similarly in the sense that they influence molecular vibration, diffusion, bond-breaking rates, reaction rates, and equilibrium behaviors. These influences are regarded as isotope effects. Essentially, the neutrons contribute to atomic mass, isotope identity, and nuclear stability, but they have a relatively small effect on ordinary adhesive chemistry.
What is an Electron?
An electron is a negatively charged particle located around the nucleus of an atom. In the technical sense, electrons are described using quantum mechanical regions of probability called orbitals. For bonding-science purposes, it is best to understand electrons as occupying discrete energy levels around the nucleus. Electrons are responsible for much of an atom’s typical chemical interactions. Electrons contribute to chemical bond formation, molecular polarity, electrical charge, surface reactivity, oxidation and reduction, intermolecular attraction, adhesive curing reactions, and interactions between an adhesive and substrate.
Why Electrons Matter Most in Chemical Bonding
Chemical bonds occur through the interactions between atoms or groups of atoms that result in a stable combined structure. Chemical bonding changes the distribution of electron density between or among atoms. Atoms can form chemical bonds through the sharing of electron density, gaining or losing electrons, distribution of electrons across multiple atoms, and attracting or repelling full or partial electrical charges. For instance, a covalent bond shares electrons between atomic nuclei, which increases electron density. While the actual behavior of electrons is highly complex, the basic guiding principle is that chemical bonding is governed through the distribution of electrons and how that changes when atoms interact with each other.
Electrons and Electrical Neutrality
A proton has a positive charge, and an electron has a negative charge. A neutral atom has an equal number of protons and electrons, while an atom with an uneven number of protons and electrons carries a net electrical charge. A net electrical charge is the number of electrons subtracted from the number of protons.
An atom becomes positively charged when it loses one or more electrons and is then called a cation. An atom becomes negatively charged when it gains one or more electrons and is then called an anion.
An atom with a net electrical charge is called an ion. The attraction between oppositely charged ions leads to ionic bonding, which is responsible for the structures of many minerals, ceramics, and compounds.
Electrons and Adhesive Polymers
Most adhesives and sealants rely on polymers, which are large structures created from repeating or interconnected chemical units.
Polymers used in adhesives include epoxy, polyurethane, acrylic, rubber, silicone, cyanoacrylate, and polyvinyl acetate.
The atoms in these polymers are mainly held together through covalent bonds, which share electron density. Different combinations of atoms can create different electrical environments which influence properties such as polarity, chemical resistance, chain flexibility, water sensitivity, thermal stability, participation in curing reactions, and attraction to certain substrates. For example, oxygen- and nitrogen-containing polymers can create polar regions where electrons are unevenly distributed, which then interact with different substrates such as glass, wood, and concrete.
Likewise, carbon- and hydrogen-containing polymers such as polypropylene can create fewer polar surfaces, so the substrate may require surface treatment before application.
Electrons and Adhesive Curing
Curing is the process of the adhesive bond solidifying, which makes it capable of carrying loads or supporting movement. Curing can involve reactions between resin and hardener, polymer cross-linking, chain growth, reactions with atmospheric moisture, and reactions caused by heat, light, or catalysts.
Chemical bonding during curing causes atoms to reorganize and form new structures. The nuclei of the same elements remain the same, but the redistribution of electrons causes connections between atoms to change. For example, as an epoxy resin cures, it forms new covalent bonds that cross-link polymer networks. The protons stay in the nuclei, and the neutrons usually do not participate. The important chemical reactions occur primarily through interactions among the elements’ electrons.
Example: Aluminum at the Bonding Interface
A neutral aluminum atom contains thirteen protons and thirteen electrons. The number of protons determine the element’s identity as aluminum while the number of neutrons determine which isotope of aluminum is present. When aluminum is exposed to air, the atoms interact with oxygen through a redistribution of electrons to form an oxide layer. When an adhesive is applied to the surface of aluminum it interacts with aluminum oxide, hydroxyl groups, moisture, oils, and any other contaminant present. This example sufficiently demonstrates that protons determine the chemical identity, neutrons play mostly a passive role except in determining the isotope, and electrons interact with the atoms of other elements to form chemical bonds.
Avoiding Common Misconceptions
There are some general facts that must be known to avoid misconceptions about the adhesive bonding process.
Misconception 1: All three subatomic particles form chemical bonds directly
Chemical bonds are mainly formed through the activity of electrons while protons are mainly responsible for determining the identity of an element and its nuclear charge. Neutrons maintain a passive role except for indirect isotope effects.
Misconception 2: Electrons follow fixed circular paths
Electron shell representations are basic and abstract teaching models. In reality, electrons conduct themselves through orbitals and probability distributions.
Misconception 3: Atoms become different elements when they gain or lose electrons
Gaining or losing electrons does convert the element into an ion, but it does not fundamentally change the element. To change the element, the number of protons must change first.
Misconception 4: Neutrons determine the element
The number of protons is responsible for determining the element; neutrons determine the isotopes.
Misconception 5: Adhesion always creates new chemical bonds with the substrate surface
Some adhesives do form chemical bonds with the surface of the substrate, but many adhesives rely on intermolecular attraction, diffusion, or mechanical interlocking.
Conclusion
Understanding subatomic particles such as protons, neutrons, and electrons provides us with the knowledge of practical bonding concepts. We can know that material identity is determined by the number of protons, while electrons govern oxidation, polarity, and chemical reactivity. Electrons of one element interact with the electrons of another element and form polymer chains and networks. Likewise, surface preparation can greatly enhance the adhesive bonding outcomes. In the next article, we dive deep into electron behavior and how electron shells and valence electrons influence adhesive bonding.
