Quantum chemistry

Tiny parts make up everything.

These parts are very small. We use math to see them. This helps us learn how things work. It is like a secret map.

Atoms join together to make things. They use tiny bits to hold on. This hold is called a bond. The bits pull atoms together. But they also push back if too close. This balance makes things stay still. We can use computers to study this. It helps us see how things change.

Quantum chemistry is a special branch of science. It uses math to study tiny atoms and molecules.

Scientists use this to see how electrons move. Electrons are tiny parts that live around atoms. Knowing where they are helps us understand how things work.

One way to study this is valence bond theory. This way looks at how atoms join together. When two atoms get close, their parts overlap. This overlap makes a chemical bond. A bond is like a bridge between atoms.

The bond stays strong when the pull and push are balanced.

Another way is molecular orbital theory. This method looks at the whole molecule at once. It describes electrons as being spread out over everything. This helps scientists predict how molecules react to light.

Scientists also use density functional theory. This is a way to study large groups of atoms. It is very helpful because it does not take too much computer time. Using computers helps us see how atoms move and change. This makes it easier to study big and complex things.

Quantum chemistry is a fascinating branch of physical chemistry. It uses the rules of quantum mechanics to study tiny systems. Scientists look at molecules, materials, and solutions at the atomic level. They want to know how electrons affect physical and chemical properties. This field helps us understand the structure of atoms and ions. It even helps us see how chemical reactions happen through different pathways.

To understand these tiny systems, scientists use math to solve the Schrödinger equation. This equation helps find the electronic structure of a molecule. The electronic structure is the quantum state of its electrons. Most scientists use the Born–Oppenheimer approximation to help with these hard jobs. This method assumes that the electron wave function is shaped by the positions of the nuclei. Because most systems have many particles, scientists must use approximations. These help make the math possible for computers to solve.

Many smart people helped build this field over many years. Gilbert N. Lewis wrote a paper in 1916 about valence electrons. In 1927, Walter Heitler and Fritz London studied the hydrogen molecule. This was a huge milestone for the history of quantum chemistry. Later, Linus Pauling wrote many articles during the 1930s. He created a new framework called valence bond theory. His 1939 book became a standard text at many universities. Other important names include Robert S. Mulliken and Maria Goeppert Mayer.

There are different ways to look at how atoms bond. Valence bond theory focuses on how atoms pair up. A covalent bond forms when the orbitals of two atoms overlap. This overlap creates an electron pair. The strength of the bond depends on how much they overlap. Another way is molecular orbital theory, developed by Friedrich Hund and Robert S. Mulliken in 1929. This method describes electrons as being spread over the whole molecule. Another popular tool is density functional theory, or DFT. This method is very helpful for studying large molecules because it uses less computer time.

Quantum chemistry connects to how we see the world around us. It helps scientists predict spectroscopic properties, which is how light interacts with matter. It also explains chemical kinetics, which is how fast reactions happen. By studying molecular dynamics, we can see how molecules move. This is like watching a tiny dance at the atomic level. Even though the math is hard, it helps us understand the very building blocks of life.

Quantum chemistry is a specialized branch of physical chemistry. It applies the rules of quantum mechanics to chemical systems. Scientists use it to study molecules, materials, and solutions at the atomic level. The field focuses on calculating how electrons contribute to physical and chemical properties. These calculations help us understand the structure of atoms, molecules, and ions. They also allow us to describe chemical reaction pathways, intermediates, and transition states.

To understand these systems, researchers must determine the electronic structure. This refers to the quantum state of a molecule's electrons. The primary goal is to find solutions to the Schrödinger equation. This equation describes how particles behave at a tiny scale. However, an exact solution only exists for the hydrogen atom. Most systems involve three or more particles moving at once. Because of this complexity, scientists must use computational chemistry. They use systematic approximations to make these math problems solvable by computers. One common method is the Born–Oppenheimer approximation. This assumes the electronic wave function is shaped by the positions of the nuclei.

The history of this field involves many important discoveries. Gilbert N. Lewis provided a critical framework in 1916. He developed the first working model of valence electrons. In 1927, Walter Heitler and Fritz London reached a major milestone. They applied quantum mechanics to the diatomic hydrogen molecule. This explained the phenomenon of the chemical bond. Later, Linus Pauling integrated many ideas into a new framework. He wrote a series of articles during the 1930s. His 1939 text became a standard university textbook. It introduced many chemists to valence bond theory. Other key contributors include Robert S. Mulliken and Maria Goeppert Mayer.

There are two main ways to describe how atoms bond. The first is valence bond theory. This method focuses on the pairwise interactions between atoms. It looks at how atomic orbitals combine to form individual bonds. A covalent bond forms when half-filled orbitals overlap to create an electron pair. The strength of this bond depends on the amount of overlap. Atoms also have an ideal bond distance. This is the stable length where attractive and repulsive forces balance. Orientation matters for the type of bond created. A sigma (σ) bond forms from direct orbital overlap. A pi (π) bond forms from a side-to-side overlap of p-orbitals.

The second approach is molecular orbital theory. Friedrich Hund and Robert S. Mulliken developed this in 1929. This method is less intuitive than valence bond theory. Instead of focusing on local overlaps, it describes the whole molecule as one system. Electrons are described by mathematical functions that are delocalized over the entire molecule. This approach is the basis for the Hartree–Fock method. It is very useful for predicting spectroscopic properties. This is how light interacts with different types of matter.

Another essential tool is density functional theory, or DFT. The Thomas-Fermi model began this work in 1927. Modern DFT uses the Kohn-Sham method to describe many-electron systems. Instead of using wave functions, it uses electronic density. This makes the math much easier for computers to handle. The computational requirements for DFT scale much more slowly than other methods. This allows scientists to study large polyatomic molecules and even macromolecules. It is one of the most popular methods in computational chemistry today.

Quantum chemistry also explores chemical dynamics. This is the study of how molecules move and interact over time. Scientists can use quantum dynamics to solve the Schrödinger equation directly. They can also use semiclassical dynamics for a simpler approach. Some researchers use molecular dynamics to simulate motion using classical physics. Other methods, like path integral molecular dynamics, add quantum corrections to these simulations. These studies help us understand chemical kinetics, which is the study of reaction rates. By mastering these tools, scientists can predict how the building blocks of our world behave.