Structure of Atom: Atom (Atom) refers to a chemical reaction substantially not be divided particles, an indivisible atom in a chemical reaction. But it can be divided into the physical state. Atoms consist of atomic nuclei and electrons that move around the nucleus. Atoms constitute the smallest unit of general matter and are called elements.

Structure of Atom

Therefore, it has a nuclear structure.

Structure of Atom

The smallest particles in a chemical change.

Note: Atoms are the smallest particles that makeup matter. It is wrong. Atoms can be divided into nuclei and extranuclear electrons. The nuclei are composed of protons and neutrons, and the number of protons is the basis for distinguishing different elements. Protons and neutrons can continue to be divided. So atoms are not the smallest particles that makeup matter, but atoms are the smallest particles in chemical reactions.


The power of the atom is great, and the energy of the nucleus will be relatively harmful after it is released. But it also has the advantage that if we are good at using it, we can help us. Among them, the radiation of atomic nuclei can be absorbed by plants to reduce our damage. But what we can usually do is to minimize the use of atomic energy, which can reduce the damage. Only by observing and understanding new methods in time can we better prevent them?

Dalton’s atomic model

The British natural scientist John Dalton transformed the speculative atomism of ancient Greece into a quantitative chemical theory and proposed the world’s first atomic theoretical model. His theory mainly has the following four points:

① All matter is composed of very tiny, indivisible matter particles, or atoms.

② Various properties and masses of atoms of the same element are the same, and atoms of different elements are mainly manifested by different masses.

③ The atom is a tiny, indivisible solid sphere.

④ Atom is the smallest unit to participate in chemical changes. In a chemical reaction, atoms are just rearranged, and will not be created or disappeared.

Although, after later confirmed, that the theoretical model is a failure, but the first time the atom from Dalton philosophy into chemistry studies, a clear direction for future efforts of chemists, chemical really from the ancient alchemy of after getting rid of it, Dalton was also hailed as “the father of modern chemistry”.

Raisin Pudding Model

The Raisin Pudding Model (Jujube Cake Model) was proposed by Thomson and was the first atomic model with a subatomic structure.

Thomson proposed an atomic raisin pudding model (jujube core model) based on the discovery of electrons. Thomson believed that:

① Positive charges are evenly distributed in atoms like fluids, electrons are scattered in positive charges like raisins, and their negative charges cancel out those positive charges.

② When excited, the electrons will leave the atom and produce cathode rays.

The alpha particle bombardment of the gold foil experiment (scattering experiment) by Thomson student Rutherford denied the correctness of the raisin pudding model (date cake model).

Saturn Model

In the same year that Thomson proposed the raisin pudding model, Japanese scientists proposed the Saturn model, which believed that the electrons were not evenly distributed, but concentrated on a fixed orbit around the nucleus.

Planet model

The planet model was proposed by Rutherford, based on the theory of classic electromagnetism, the main content is:

① Most of the volume of atoms is empty.

② There is a small nucleus with a very large density in the center of the atom.

③ All the positive charge of the atom is in the nucleus, and almost all the mass is concentrated in the nucleus. The negatively charged electrons move around the core at high speed in the nuclear space.

With the advancement of science, the fact that the linear spectrum of hydrogen atoms indicates that the planetary model is incorrect.

Bohr’s atom model

In order to explain the fact that the linear spectrum of hydrogen atoms, Rutherford’s student Bohr accepted Planck’s quantum theory and Einstein’s concept of photons. Based on the planetary model, he proposed a layered arrangement of extranuclear atoms. Structural model. The basic view of Bohr’s atomic structure model is:

① Atomic electrons in a circumferential track (having a radius determined orbit around the nucleus movement), does not radiate energy.

② The electrons moving in different orbits have different energies (E), and the energy is quantized. The energy value of the orbit increases with the increase of n (1,2,3, …), n is called quantum Number . The different orbits are named K (n = 1), L (n = 2), M (n = 3), N (n = 4), O (n = 5), P (n = 6), Q (n = 7).

③ If and only when the electron transitions from one orbit to another, it will radiate or absorb energy. If the radiated or absorbed energy is represented in the form of light and recorded, a spectrum is formed.

Bohr’s atomic model explains the linear spectrum of hydrogen atoms well, but it is powerless for more complex spectral phenomena.

Modern quantum mechanical model

The physicist’s De Broglie, Schrödinger, and Heisenberg, etc., after 13 years of hard demonstration, have explained many complex spectral phenomena based on the Bohr atomic model in the modern quantum mechanical model. The core is Wave dynamics. In the Bohr atomic model, the orbit has only one quantum number (primary quantum number), and modern quantum mechanics models introduce more quantum numbers (quantum number).

① Principal quantum number ( principal quantum number ), which determines different electronic sublayers, named K, L, M, N, O, P, Q.

② Azimuthal quantum number ( Angular Quantum Number ), the quantum number determined angle different energy levels, the symbol “l” total value of n (1,2,3, … n-1), with the symbol s, p, d, f, g, said that for a multi-electron atom, the motion state of the electron is related to l.

③ Magnetic quantum number ( magnetic quantum number ) The magnetic quantum number determines the orbit of different energy levels, the symbol “m” (see “magnetic moment” below). Useful only when a magnetic field is applied. The three quantities “n”, “l” and “m” determine the movement state of an atom.

④ There are two kinds of spins for spin mqn electrons in the same orbit. The essence of the “↑ ↓” spin phenomenon is still under discussion.

Subatomic Particles

Although the English name of atom is intended to be the smallest particle that cannot be further divided, with the development of science, atoms are considered to be composed of electrons, protons, and neutrons (hydrogen atoms are composed of protons and electrons).

Collectively referred to as subatomic particles. Almost all atoms contain the above three subatomic particles, but protium (hydrogen isotope) has no neutrons, and its ion (after losing electrons) is only a proton.

The proton carries a positive charge, and its mass is 1836 times that of the electron, which is 1.6726 × 10⁻²⁷ kg. However, part of the mass can be converted into atomic binding energy. Neutrons have no charge, and the mass of free neutrons is 1839 times the mass of electrons, which is 1.6929 × 10⁻²⁷ kg. The dimensions of neutrons and protons are similar, both in the order of 2.5 × 10⁻¹⁵ m, but their surfaces have not been precisely defined.

Although atoms are small, they cannot be divided by chemical methods, but they can still be divided by other methods because atoms also have a certain composition. The atom is composed of a positively charged nucleus in the center and a negatively charged electron outside the nucleus (the opposite of antimatter). The nucleus is composed of two particles, protons and neutrons.

In the standard model theory of physics, protons and neutrons are composed of elementary particles called quarks. Quark is a kind of fermions and one of the two basic components of matter. Another basic component is called lepton, and electrons are a kind of lepton.

There are six types of quarks, each with a fractional charge, either +2/3 or -1/3. The proton is composed of two upper quarks and one lower quark, and the neutron is composed of one upper quark and two lower quarks.

This difference explains why neutron and proton charge and mass are different. Quarks are joined together by strong interactions and mediated by gluons. Gluon is a member of the standard boson, a basic particle used to transmit force.

Subatomic particles have quantization characteristics and wave-particle duality. The formula is expressed as: λ = h / p = h / mv, where λ is the wavelength, p is the momentum, and h is the Planck constant (6.626 × 10⁻³⁴ J · S).


When a high-voltage direct current is applied to a glass tube with a vacuum inside and metal electrodes sealed at both ends, cathode rays are emitted from one end of the cathode. The fluorescent screen can show the direction of this ray.

If a uniform electric field is applied, the cathode ray will deflect to the anode and if a wheel is installed in the glass tube, the ray can rotate the wheel. It was later confirmed that the cathode ray is a group of negatively charged high-speed particles, that is, the flow of electrons. The electron was discovered.

The electron is the earliest discovered subatomic particle. So far, the electron is the lightest of all particles, only 9.11 × 10⁻³¹kg, which is a hydrogen atom [1 / 1836.152701 (37)]. Made by the famous “oil drop experiment”. The electron has a unit of a negative charge, namely 4.8 × 10⁻¹⁹ electrostatic unit or 1.6 × 10⁻¹⁹ Coulomb because its volume is too small, the existing technology cannot measure.

Modern physics believes that one of the electrons belonging to lepton is one of the basic units constituting matter (the other is quark).

Electronic Cloud

The electron has wave-particle duality. It cannot be sure that he is at a certain point in space at a certain moment as it describes the motion of an ordinary object, but can only indicate the probability (ie, probability) that it appears somewhere outside the atomic nucleus. The probability of electrons appearing in different parts of the nucleus is different.

The probability of electrons appearing in some places is large, and the probability of appearing in some places is very small. , The denser the black dots), then you get a slightly intuitive image. In these images, the nucleus seems to be covered by a negatively charged electron cloud, so it is called an electron cloud.

In an atom, electrons and protons attract each other because of the electromagnetic force. It is this force that binds electrons in an electrostatic potential well surrounding the nucleus. To escape from this well requires external energy. The closer the electron is to the nucleus, the greater the attraction. Therefore, compared with the outer electrons, electrons near the core require more energy to escape.

The atomic orbit is a mathematical equation describing the probability distribution of electrons in the nucleus. In practice, only a set of discrete (or quantized) orbits exist, and other possible forms will quickly collapse into a more stable form. These tracks can have one or more rings or nodes, and their sizes, shapes, and spatial directions are different.

Each atomic orbital corresponds to the energy level of an electron. Electrons can jump to a higher energy level by absorbing a photon with sufficient energy. Similarly, through spontaneous emission, electrons in the high energy state can also transition back to a low energy state, releasing photons. These typical energies, that is, the energy differences between different quantum states, can be used to explain the atomic spectrum.

Connect the places where the probability of extra-nuclear electrons are equal, as the interface of the electronic cloud, so that the total probability of the electronic cloud within the interface is very large (for example, 90% or 95%), and the probability outside the interface is very small. The included spatial range is called atomic orbital, where the atomic orbital and macroscopic orbit have different meanings.

The energy required to remove or add an electron in the nucleus is much smaller than the binding energy of the nucleon. These energies are called electron binding energy. For example, it takes only 13.6 eV to remove the ground state electrons in a hydrogen atom. When the number of electrons and the number of protons are equal, the atom is electrically neutral.

If the number of electrons is larger or smaller than the number of protons, the atom is called an ion. The outermost electron of an atom can move to an adjacent atom, or it can be shared by two atoms. Because of this mechanism, atoms can bond to form molecules or other kinds of compounds, such as ions or covalent network crystals.

An atomic orbital is Schrodinger equation engagement appreciated, Schrödinger equation is a second-order partial differential equation:

(δ²ψ / δx²) + (δ²ψ / δy²) + (δ²ψ / δz²) =-(8π²) / (h²) · (EV) ψ,

The solution ψ of this equation is a function of x, y, z, written as ψ (x, y, z). In order to describe the meaning of the wave function more vividly, spherical coordinates are usually used to describe the wave function, that is ψ (r, θ, φ) = R (r) · Y (θ, φ), where the R (r) function is The function related to the directional distribution is called the radial distribution function; Y (θ, φ) is related to the angle distribution and is called the angle distribution wave function.


All protons and neutrons in an atom combine to form a very small nucleus, which together can also be called a nucleus. The radius of the nucleus is approximately 1.07 × A ^ 1/3 fm, where A is the total number of nucleons.

The atomic radius is on the order of 105fm, so the radius of the nucleus is much smaller than the radius of the atom. The nucleons are bound together by residual forces that can act over short distances. When the distance is less than 2.5fm, the strong force is much greater than the electrostatic force, so it can overcome the mutual repulsion between positively charged protons.

Atoms of the same element carry the same number of protons. This number is also called the atomic number. For a particular element, the number of neutrons can be changed, which also determines which isotope of this element is the atom.

The number of protons and the number of neutrons determine which kind of nuclide this atom is. The number of neutrons determines the stability of the atom, and some isotopes are capable of spontaneous radioactive decay. Both neutrons and protons are a type of fermion.

According to the Pauli incompatibility principle in quantum mechanics, it is impossible for two fermions to have the same quantum physical state at the same time. Therefore, each proton in the nucleus occupies a different energy level, and the situation of neutrons is the same. However, the Pauli incompatibility principle does not prohibit a proton and a neutron from having the same quantum state.

If the number of protons and neutrons of an atomic nucleus is different, then the nucleus is prone to radioactive decay to a lower energy level and makes the number of protons and neutrons more similar. Therefore, atoms with the same or very close proton and neutron numbers are less likely to decay.

However, when the atomic number gradually increases, because the repulsive force between protons increases, more neutrons are needed to stabilize the entire nucleus, so it has an impact on the above trend. Therefore, when the atomic number is greater than 20, you cannot find a stable nucleus with the same number of protons and neutrons. As Z increases, the ratio of neutrons to protons gradually approaches 1.5.

Schematic diagram of nuclear fusion, where two protons are fused to form a deuterium nucleus containing a proton and a neutron, and a positron (electron antimatter) and an electron neutrino are released. The opposite process is nuclear fission.

If the nuclear mass-produced after nuclear fusion is less than the total atomic mass before fusion, then according to Einstein’s mass-energy equation, these differences in mass are released as energy. This difference is actually the binding energy between the nuclei, for two nuclei with atomic numbers before iron or nickel.

In the alpha particle scattering experiment, it was found that the mass of atoms is concentrated in a very small and positively charged substance, which is the nucleus.

The nucleus is also called a nucleus, and is composed of all protons and neutrons in the atom. The radius of the nucleus is approximately 1.07 × A ^ 1/3 fm, where A is the total number of nucleons. The atomic radius is on the order of 105fm, so the radius of the nucleus is much smaller than the radius of the atom.

Atom Nucleus

The nucleus consists of protons and neutrons (the hydrogen nucleus has only one proton), a quantum state.

Proton (Proton)

The proton is composed of two upper quarks and one lower quark, with a unit positive charge, and the mass is 183.6152701 (37) times the mass of the electron, which is 1.6726231 (10) × 10⁻²⁷kg, but part of the mass can be converted into atomic binding energy. Atoms with the same number of protons are the same element, atomic number = number of protons = number of nuclear charges = number of extranuclear electrons.

Neutrons (Neutron)

Neutrons are the subatomic particles with the largest mass in atoms. The mass of free neutrons is 1838.683662 (40) times the mass of electrons, which is 1.6749286 (10) × 10⁻²⁷kg. The dimensions of neutrons and protons are similar, both in the order of 2.5 × 10⁻¹⁵m, but their surfaces have not been precisely defined.

The neutron is composed of one upper quark and two lower quarks, and the charges of the two quarks cancel each other, so the neutrons are not significant, but the view that “neutrons are not charged” is wrong.

For a certain element, the number of neutrons can be changed, and the same kind of elements with different numbers of neutrons are called isotopes. The number of neutrons determines the stability of an atom, and the isotopes of some elements can spontaneously undergo radioactive decay.

Nuclear (Nuclear Force)

The nucleus is bound by a strong force in the area of ​​10⁻¹⁵m. Since protons are positively charged, according to Coulomb’s law, the repulsion between protons would cause the nucleus to burst, but there is a force in the nucleus that tightly binds the proton and neutron together. This force is the nuclear force. Within a certain distance, the nuclear force is much greater than the electrostatic force, overcoming the mutual repulsion between positively charged protons.

The range of action of the nuclear force is called the force range, and the range of action is around 2.5fm, at most no more than 3fm, That is, it cannot extend from one nucleus to another, so the nuclear force is a short-range force.

Nuclides (Nuclide)

The nuclei with the same number of protons and neutrons are called nuclides, and the x-axis represents the number of protons; the image obtained with the y-axis representing the number of neutrons is called a nuclide graph. At {0, 1, 2, 3,…, 20}, the function on the nuclide graph is approximately y = x, but as the number of protons increases, the Coulomb repulsion between protons increases significantly, and the nucleus needs more than usual The number of neutrons remains undecided. At x ∈ {21, 22, 23,…, 112}, the function is approximately y = 1.5x, and the number of neutrons is greater than the number of protons.

Binding Energy

In the atomic nucleus, the energy consumed by separating the nucleon from the atomic nucleus for work is called binding energy. Experiments have found that the mass of any nucleus is always less than the mass sum of its constituent nucleons (this difference is called the mass loss), therefore, the binding energy can be calculated from the Einstein mass-energy equation:

Binding energy = (rest mass of all protons and neutrons in the nucleus and-rest mass of the nucleus) × speed of light^2

Average binding energy

The average binding energy of each nucleus in a nucleus is called the average binding energy, and the calculation formula is:

The average binding energy of each nucleus = total binding energy ÷ number of nucleons

The larger the average binding energy, the harder it is for the nucleus to be broken down into single nuclei. It can be seen from the picture on the right:

① The average binding energy of heavy nuclei is smaller than that of middle nuclei, therefore, they are prone to fission and emit energy.

② The average binding energy of the light core is smaller than the average binding energy of the heavier core, therefore, when the light core is fused, it will release energy.

The van der Waals radius of an atom means that in the molecular crystal, the molecules are bound by van der Waals forces, such as half of the distance between two adjacent nuclear nuclei of rare gas.

The number of protons and neutrons in the nucleus can also be changed, but because of the strong force between them, high energy is required. When multiple particles gather to form a heavier nucleus, nuclear fusion occurs.

The opposite process of high-energy collisions is nuclear fission. In nuclear fission, a nucleus usually undergoes radioactive decay and splits into two smaller nuclei. Bombing with high-energy subatomic particles or photons can also change the nucleus. If in a process, the number of protons in the nucleus changes, then this atom becomes an atom of another element.

For two nuclei with atomic numbers before iron or nickel, the nuclear fusion between them is an exothermic process, that is, the energy released by the process is greater than the energy linking them together. Because of this, the hydrostatic balance.


Each element has one or more isotopes with unstable nuclei, which can undergo radioactive decay. During this process, the nuclei can release particles or electromagnetic radiation. When the radius of the atomic nucleus is greater than the radius of the strong force, radioactive decay may occur, while the radius of the strong force is only a few femtometres.

The most common radioactive decay is as follows:

  • Alpha decay: The nucleus releases an alpha particle, a helium nucleus containing two protons and two neutrons. The result of decay is to produce a new element with a lower atomic number.
  • Beta-decay: A phenomenon of weak interaction, in which a neutron is transformed into a proton or a proton is transformed into a neutron. The former is accompanied by the release of an electron and an anti-neutrino, while the latter releases a positron and a neutrino. The released electrons or positrons are called beta particles. Therefore, β decay can increase or decrease the atomic number of the atom by one.
  • γ decay: The energy level of the atomic nucleus is reduced, and electromagnetic radiation is released, usually after the alpha particles or beta particles are released.

Important parameters

Mass number

(Mass number) Since the masses of protons and neutrons are similar and much larger than electrons, the relative atomic mass is defined by the sum of the number of protons and neutrons of an atom, which is called the mass number.

Relative atomic mass

The rest mass of an atom is usually expressed in uniform atomic mass units (u), also known as Dalton (Da). This unit is defined as one-twelfth of the mass of electrically neutral carbon 12, approximately 1.66 × 10⁻²⁷kg. Lightest hydrogen isotopes protium atom is the lightest weight of about 1.007825u. The mass of an atom is approximately the product of the mass number and the atomic mass unit. The heaviest stable atom is lead-208, with a mass of 207.9766521u.


Even the heaviest atoms are difficult for chemists to manipulate directly, so they usually use another unit mole. The definition of the mole is that for any element, one mole always contains the same number of atoms, which is about 6.022 × 10²³[2010 CODATA data]:

NA = (6.02214129 +/- 0.000000027) x 1023mal-1

Therefore, if the atomic mass of an element is 1u, the mass of one atom of this atom is (1.66 × 10⁻²⁷x6.022 × 10²³ = 9.99652×10⁻⁴≈10×10⁻⁴ = 0.001kg) 0.001kg, which is 1 gram. For example, the atomic mass of carbon-12 is 12u, and the mass of one mole of carbon is 0.012kg.