String theory

String theory was first proposed by Jöel Scherk and John Henry Schwarz in 1974 with a paper explaining one-dimensional pillars instead of point particles explaining the gravitational force.

String theory is a model of physics that clarifies the nature of matter and its interactions. It also states that space-time has several dimensions in addition to the ones we can perceive. So the whole universe is created by filaments. To understand this theory we have to understand an electron as a vibration that has the possibility of giving rise to other particles. Thanks to the theory we could later come to another quantum theory of gravity.

The 4 forces of string theory

String theory is composed of 4 forces:

• Electromagnetic force: through the particle known as the photon.
• "Strong" or nuclear force
• "Weak" force
• Gravity

The entire universe is made up of tiny strands of energy, known as strings. Each subatomic particle is born from one of the vibration nodes of a single type of string.

String theory together:

1. Einstein's theory of relativity → tells us what gravity is.
2. quantum mechanics → the world of stars.

Drawbacks:

According to string theory we live in a world of 10 dimensions (11 if we count time). These are divided into 9 spatial and 1 temporal. However, we can only perceive 4 of them: length, height, width, and time; so the other 7 are related to themselves.

On the other hand, the theory produces a superabundance of possible universes compatible with our own, at least of the order of 10 >500.

In 1982 Michael Green and John Schwarz created 3 models of string theory

• Model I: It has closed and open strings. Closed strings are block-shaped. The open strings do not touch each other.

• Model IIA: This type of string has symmetrical vibration patterns. This type of string is connected to structures that have an odd number of dimensions.

• Model IIB: unlike Type IIA, this type of string has asymmetrical vibratory partons. This type of string is connected to structures that have an even number of dimensions.

Repulsion of valence layer electron pairs, RPECV

Repulsión de pares electrónicos de capa de valencia (RPECV) o también conocido como VSEPR es la geometría que forma las moléculas o las redes covalentes.

The chemical physicist Gilbert Newton Lewis proposed the covalent bond between atoms in 1916. This bond is produced by the sharing of electron pairs. This means that each atom gets 8 electrons in the valence shell.

A clear example is the fluorine atom which has 7 electrons in the electronic valence shell.

In a structure, it is very difficult to distinguish the geometry, both the molecular geometry and the degree to which the bonds form each other. This is why the theory of repulsion of the electron pairs in the valence layer is used to determine the geometry of molecules. It was not until 1970 Gillespie that this theory made it easier to distinguish between geometry and angles.

The valence shell electron pair repulsion model

The RPECV model explains that since electrons repel each other because they are negatively charged, geometry can reduce the repulsions between the different electron pairs around the central atom.

Valencia's electron pair repulsion theory is the exemplar in chemistry for the shape of molecules, which is based on the degree of electrostatic repulsion of the electron pairs.

As we can see below, there are different types of molecular geometry:

• Trigonal Bipyramid

• See-saw

• T

• Linear

Types of repulsion

There are 3 types of repulsion between the electrons of a molecule:

• Non-bonding pair - non-bonding pair repulsion
• Non-bonding pair - bonded pair repulsion
• Bonded pair - bonded pair repulsion

The non-bonding pair repulsion is considered free, whereas the bonding pair repulsion must have a chemical bond.

Depending on the number of electrons in the valence shell we can see one arrangement of electrons or another.

A clear example of the valence shell electron pair repulsion theory is ammonia (NH₃). Ammonia has three bound electrons and one lone pair. The lone electrons affect the geometry by repulsion. The NH₃ molecule is part of the AB₃E group and has an E due to the unpaired electron pair.