Atomic Physics

ray, particle

ray is a kind of radiation. It consists of particles, i.e. the nuclei of helium. A helium nucleus has two protons and two neutrons; therefore it carries a positive charge. Since particle is positively charged and rather heavy, its penetrating power is very weak. Its range in air is only a few centimetres. ray is usually produced by decay of atomic nuclei.


Atoms are the basic units of matter. Their length scale is about . An atom consists of a nucleus and electrons surrounding the nucleus. The nucleus is very small in size, but it carries most of the mass of the atom. It is a collection of positively charged protons and electrically neutral neutrons. The electrons surrounding the nucleus carry a negative charge and move around the nucleus at some specific energy levels. The number of electrons is the same as the number of protons in an atom. Since the charges carried by a proton and an electron are equal in magnitude but opposite in sign, atoms are neutral. The number of protons (atomic number) determines the element which the atom belongs to.

atomic number

Atomic number is the number of protons in a nucleus. Atomic number determines the element which the atom belongs to. Atoms with the same atomic number have the same chemical properties.

ray, particle

ray is a kind of radiation. It consists of particles, i.e. electrons. particle carries a negative charge and is much lighter than particle. It has weak penetrating power (a little bit stronger than particle). A thin piece of aluminum plate is good enough to block the ray. ray is usually produced by decay of atomic nuclei.

chain reaction

The neutrons released in fission will collide with other nuclei and stimulate another round of fission. Once we start this process, the reaction will go on repeatedly. Therefore it is called chain reaction. When an atomic bomb explodes, chain reactions lead to repeated fission of uranium nuclei and release a huge amount of energy.


Electrons are negatively charged particles with a mass about 1800 times less than nucleons. They move around the nucleus at specific energy levels.


A basic classification of matter. Each element consists of one kind of atoms only. The atoms of different elements have different number of protons. In a neutral atom, the number of electrons is equal to that of protons. The number and arrangement of electrons determine the chemical properties of an element.

energy level

The electrons in an atom move around the nucleus at some fixed orbital called energy levels. An electron at a certain energy level processes a specific amount of energy.


ray is a kind of radiation. It consists of electromagnetic waves with very short wavelength and very high energy. ray does not carry a charge, and has high penetrating power. It can penetrate a few centimetres of lead. ray is usually produced by decay of nuclei, or presents in cosmic ray.


The time required for the radioactivity of an object to reduce to half of the original. Radioactive decay is a random process. In one half-life, about half of the radioactive substance in the object have decayed. Therefore the radioactivity reduces to half of the original.


When an atom loses one or more electrons, the number of protons will be greater than the number of electrons. Therefore it has an overall positive charge and it is called an ion.


Ionization is the process that an atom loses some of its electrons and becomes an ion.


Two atoms with the same atomic number but different mass number are called isotopes. They have the same number of protons but different number of neutrons. They have the same chemical properties.

mass number

The total number of protons and neutrons in a nucleus.


A group of atoms bonded together by sharing some of their outermost electrons. Molecules are one of the basic constitutions of matter.


A neutron is a neutral (with zero charge) particle. It is a main component of an atomic nucleus.

nuclear fission

Nuclear fission is the process that one nucleus splits into more than one nuclei, leading to formation of other elements. Large amount of energy is released during the process. There are two types of nuclear fission: spontaneous and stimulated fission. Some heavy nuclei are not stable so they will undergo spontaneous fission and give lighter nuclei. Sometimes fission is stimulated by collisions with other particles. For example, the fission of uranium 235 in nuclear reactors is started by the collisions with neutrons.

nuclear reactor

A nuclear reactor contains the materials for nucleus fission. It provides an environment for radioactive material to perform chain reaction under control. Energy is given out in the process. The nuclear reactor is the central part of a nuclear power plant.


Nucleons are the particles in atomic nuclei. It is a general name for protons or neutrons.


A nucleus is the central part of an atom. It is very small in size. Its radius is only one part in a hundred thousand of the radius of an atom. A nucleus consists of protons and neutrons. Since the mass of each proton or neutron is about 1800 times the mass of an electron, the nucleus carries almost all the mass of the atom. The nucleus is positively charged because protons carry a positive charge and neutrons have no charge.


A proton is a positively charged particle. It is a main component of an atomic nucleus. The mass of proton is almost the same as that of neutron. It carries a positive charge with the same magnitude, but of a sign opposite to that of an electron.


Radiation is a mode of energy transport, usually in the form of electromagnetic waves (e.g. ray) or beams of high energy particles (e.g. particle).

Radioactive decay, nuclear decay

A natural process in which an atomic nucleus emits radiation and reaches a more stable state, or transforms into another nucleus. There are three possible ways of nuclear decay that emit radiation, namely, decay, decay and decay. Substances that undergo nuclear decay and emit radiation are said to possess radioactivity.


Radiation loses energy during propagation in an object. Eventually it will be stopped or absorbed by the object. The longest distance it can travel is called range. The range of radiation is determined by many factors, such as the energy of the radiation, its properties (like charge and mass) and the properties of the object. For example, the range of radiation in air is usually much longer than that in metal.