Atomic theory is a physics and chemistry related substances scientific theory of nature. Contrary to the concept of infinitely separable matter, according to atomic theory, matter consists of discrete units of atoms. Modern Atomic Theory


Atomic theory is a physics and chemistry related substances scientific theory of nature. Contrary to the concept of infinitely separable matter, according to atomic theory, matter consists of discrete units of atoms.

Atomic theory

The atom was originally a concept in natural philosophy. For the West called atoms from the ancient Greek’s ατομος(meaning “indivisible”).

In Chinese, the atom’s earlier translation, “Mo Dang”, also originated from this.

The concept of the atom was once abandoned due to conflict with Christian doctrine and was not regained until modern times.

At the end of the 18th century, in the field of chemistry, a series of laws that could be described exactly in the course of chemical changes were discovered.

This provides an experimental basis for atomic theory to become a scientific theory. At the beginning of the 19th century, Dalton proposed his atomic theory to explain phenomena in chemistry.

As for the debate about whether the atom actually exists, it was not until the early 20th century that Einstein explained the Brownian motion from the perspective of molecular motion theory, and after experimental verification, it really got a positive answer.

From the end of the 19th century to the beginning of the 20th century, physicists discovered through experiments through a series of experiments related to electromagnetics and radioactivity that atoms that were originally considered “indivisible” were actually a series of subatomic particles (mainly electrons, protons, and neutrons).

Constituted, and these particles can exist independently. Since the atom was found to be separable, the physicist introduced a new term “basic particle” to describe the various components of the atom.

In the first half of the 20th century, with the deep understanding of the atomic structure and the quantum revolution in physics, modern atomic theoretical models were gradually established.

Modern atomic theory model

Discovery of subatomic particles

Main article: Electronic and raisin pudding models

In Joseph John Thomson in 1897 through to the cathode ray findings of the study electronics front, the atom was considered to be a substance essential components.

A Crook tube is a sealed glass container with two electrodes separated by a vacuum. When a voltage is applied between the electrodes, a cathode ray is generated.

Atomic theory

This ray will form a spot when it hits the other end of the glass tube. Thomson discovered through experiments that this kind of ray would be deflected in the magnetic field (which was known at the time), and the deflection also occurred in the electric field.

Through this, he concluded that the ray is not a kind of light, but a stream of particles made up of negatively charged, very light particles.

He called this particle “corpuscles”. This particle was later often referred to as “electronics” by scientists.

Thomson suggests that atoms are separable and that particles are their constituents. To explain the electrical neutrality of the atom as a whole, he proposed a raisin pudding model in which “particles” are embedded in atoms like raisins in puddings (although they are not static in Tom’s model), and positive charges are in atoms. Evenly distributed.

Nuclear discovery

Main article: Rutherford model

In 1909, Joseph Thomson’s student, Ernest Rutherford, objected to the raisin pudding model. He found that the positive charge and the vast majority of the mass of an atom are concentrated in a tiny part of its overall volume, and he suspects that the concentrated position is the exact center of the atom. Modern Atomic Theory

In the gold foil experiment, Hans Geiger and Ernest Marsden use under the guidance of Rutherford α particle bombardment of gold, and with a fluorescence observation deflect the trajectory of cases they screen.

If the electron mass is very small, the alpha particle momentum is very large, and the positive charge is evenly distributed in the atom as assumed in the raisin pudding model.

In the experiment, all the alpha particles will not show obvious trajectories when passing through the gold foil. Deflection. To their surprise, a small number of alpha particles have a large angular deflection.

Atomic theory

Thus, it can be confirmed that most of the mass of the atom is concentrated in one of the micro-elements (ie, “nucleus”). But it is not yet possible to make a conclusion about the electrical properties of this micro-element, which can be electrically neutral or not.

According to Coulomb’s law, when the alpha particle passes near the electrically neutral particle, the motion trajectory does not deflect; but if the particle is positively charged, the deflection will occur. Rutherford analyzed that the results of the experiment proved that the latter is correct.

Rutherford proposed a planetary model of the atom based on the results of the experiment. In this model, electrons surround a positively small, all-positive nucleus of a concentrated atom like a “planet”, because only the positive charge is concentrated enough to produce an electric field that causes a large angular deflection of the alpha particle’s trajectory.

Isotope discovery

Main article: Isotope

In 1913, radiochemical home Frederick Soddy Out of radioactive decay when products were experimental study, found in the periodic table within a single cell may contain more than one element. Margaret Todd then coined the term ” isotope ” to refer to elements with such a relationship. Modern Atomic Theory

In the same year, Joseph Thomson conducted an experiment. In the experiment, he let the helium ions flow through the electromagnetic field and finally hit a photosensitive plate.

There are two spots on the photographic plate, and this means that there are two different deflection trajectories. Thomson attributed it to the cesium ion containing different masses of strontium ions.

The phenomenon of mass differences between different cesium ions was explained after the discovery of neutrons in 1932.

Nuclear discovery

Main article: nucleus

1917, Rutherford with α particle bombardment of nitrogen gas, and emitted gas was observed hydrogen nuclei (Rutherford will notice this since he had to bombard particle α in the product hydrogen gas is also found hydrogen nuclear).

Rutherford proposed that the released hydrogen nuclei come from the nitrogen core (in fact, he splits the nitrogen core). Modern Atomic Theory

From his own work with his students Bohr and Henry Moselle, he learned that the total positive charge inside an atom is always an integer multiple of the charge of the hydrogen nucleus.

At the same time, the atomic weight of many of the elements measured at that time was approximately equal to an integer multiple of the atomic weight of hydrogen according to Prout’s assumption. It can be seen that the hydrogen atom is the lightest atom.

Thus, he concludes that the hydrogen nuclei are a single particle and an essential component of all nuclei. He named the particle proton.

Atomic theory

Through further experiments, Rutherford discovered that the mass of most atoms is much larger than the total mass of the protons contained in it.

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He speculates that the extra mass comes from an electrically neutral particle that was not discovered at the time. And temporarily referred to as ” neutrons .”

In 1928, Walter Bot in α particle bombardment beryllium when found to produce a high permeability, and electrically neutral rays.

It was then found that when the radiation was irradiated with paraffin, protons could be emitted. Initially, it was considered a high-energy gamma-ray because gamma rays can similarly bombard electrons in the metal. Modern Atomic Theory

But James Chadwick found that if it is electromagnetic radiation, its ionization is too strong under at least the conservation of energy and momentum.

In 1932, Chadwick used this type of radiation to illuminate the elements of several elements, such as hydrogen and nitrogen, by measuring the energy of the recoiled charged particles.

He concluded that the ray is actually an electrically neutral particle. Composition; such particles are not as zero in mass as gamma rays, but have a similar mass to protons.

Thus, he proposed that this article is the “neutron” that Rutherford predicted to exist. In 1935, he won the Nobel Prize in Physics for discovering the neutron.

The initial introduction of quantum concepts

Main article: Bohr model

The atomic planetary model has two distinct flaws:

  1. The electrons are charged, unlike the planets that surround the stars. And based on the electric classical dynamics in the Larmor equation, the speed of changing the charge emits electromagnetic waves, in this process will gradually charge the energy dissipation, and the planet electronic model will run this process occurs on the track, so that spiral The ground is close to the nucleus and eventually hits the nucleus in a very short time.
  2. It does not explain why the atomic emission spectrum of the experimental observation and the spectrum of the absorption spectrum appear as several discrete peaks. In classical physics, energy is continuously variable and the spectrum should be continuous.

At the beginning of the 20th century, quantum theory caused a revolution in physics. The concept of ” quantum ” was first introduced by Max Planck to explain blackbody radiation; he proposed that energy does not change continuously as in classical mechanics, but instead emits in a discrete form of each unit, the quantum form of energy.

Atomic theory

Or absorb. In 1905, Albert Einstein introduced this concept to optics and successfully explained the photoelectric effect. In 1913, Niels Bohr coined the concept to establish the Bohr model of the atom.

In the Bohr model, electrons can only move on a specific set of orbits, and their energy and angular momentum also have a series of specific, discrete values, and their orbital radii are proportional to the energy they have.

This model prohibits the electron from spiraling close to the nucleus because it cannot continuously lose or gain energy and can only make transient transitions between different energy levels. Modern Atomic Theory

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When a transition occurs, photons with corresponding energy are emitted or absorbed. At the same time, this model also provides a theoretical explanation for the hydrogen atom spectrum.

The Bohr model has certain theoretical flaws: it can only explain the spectrum of hydrogen atoms, and the interpretation of the spectrum of multi-electron atoms is not successful.

As the spectroscopic imaging technique is further refined, the hydrogen spectrum also exhibits spectral lines that cannot be explained by the Bohr model.

In 1916, Arnold Sommerfeld extended the circular orbit of electrons in the Bohr model to an elliptical orbit to explain the newly emerging spectral lines of the hydrogen spectrum, but this made the model complex, difficult to apply, and still not used in multiple electrons. In the case of an atom.

Quantized atomic theory model

Main article: Atomic domain

In 1924, Louis De Broglie proposed that all moving particles (especially subatomic particles like electrons) have wave characteristics to some extent. Inspired by this idea, Erwin Schrödinger began to explore the motion behavior of electrons: whether it is expressed in the form of waves, is it more appropriate than in the form of particles.

Atomic theory

In the Schrödinger equation published in 1926, he described electrons as wave functions instead of expressing them as point particles. This representation method explains many phenomena that cannot be explained by the Bohr model.

Although the concept of the wave function is very simple in mathematics, its physical image is unimaginable, and some objections were encountered at the time.

Max Born proposes that the wave function describes not the state of the electron itself, but all its possible states, and thus can be used to calculate the probability of electrons appearing somewhere around the core. Modern Atomic Theory

This reconciles two opposing electronic ways of describing it, describing it as a wave or describing it as a particle, and thus introducing a wave-particle duality theory.

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This theory suggests that electrons have both wave properties, such as diffraction, and particle properties, such as mass.

One consequence of describing electrons as wave functions is that it is mathematically impossible to simultaneously give the position and momentum of an electron, the principle of uncertainty published by Werner Heisenberg in 1927.

According to the Bohr model, electrons have a simple and fully determinable orbit. This argument is not compatible with the principle of uncertainty. So far, the Bohr model has been quickly eliminated by the new theory.

The modern model of an atom describes the location of electrons within an atom that may occur based on the probability that an electron will appear at a certain location.

An electron can be found at any distance from the core, but depending on its energy level, it will appear particularly frequently in a particular area, which is called the orbital area in which it is located.

Different orbital domains may have different shapes, such as spheres, dumbbells, rings, etc., but all centered on the nucleus.