The atom is the building block of all the matter we observe. It is generally an electrically neutral body consisting of a nucleus and a cloud of electrons equal in number to the number of protons in the nucleus. It is the smallest possible particle of a chemical element that retains its chemical properties. The number of protons in an atom is called its atomic number, and uniquely identifies a chemical element. The simplest and lightest atom is the hydrogen atom with a nucleus consisting of a single proton. There are roughly 1078 atoms in our universe.
|Charge||Postive charge||No charge||Negative charge|
|Position||In the nucleus||In the nucleus||Moving around the nucleus|
|Structure|| Up quark |
| Up quark |
Simplified atom model
This view of the atom was developed by Ernest Rutherford in 1911 and improved by Niels Bohr in 1913/1914. The model explains some of the properties of the atom with high accuracy. Electrons circle around the nucleus in shells and can jump from shell to shell while absorbing or emitting a well defined amount of energy.
Moving to a higher shell requires the electron to absorb a photon the outside. If there is free room in a lower shell the electron will at a random moment fall back while emitting a photon of an exact frequency. These absorbtions and emissions of well defined frequencies makes it possible to identify different atoms by the light they emit or absorb.
While this schematic view of the atom is very familiar and is easy to understand it fails to explain many of the properties of atoms and the fully quantum mechanical model was developed. Even if this model is obsolete it is still used because it is easy to label the different parts and show the basic behaviour of the atom.
Quantum mechanical atom model
In 1927, Werner Heisenberg formulated the uncertainty principle that proved to be fundamental to the further understanding of atoms: "The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa".
The consequence is that atoms can't be viewed as well defined particles but as a diffuse cloud (a superposition of probability density function) where the electrons have no well defined position but has a certain probability to be observed in a given position.
Quantum mechanical properties in the description of the atom:
- The Principal Quantum Number - Electron energy
- The Orbital Quantum Number - Orbital angular momentum magnitude
- The Magnetic Quantum Number - Orbital angular momentum direction
- The Spin Magnetic Quantum Number - Electron intrinsic spin direction
The chemical behavior of atoms is due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. These configurations are determined by the quantum mechanics of electrons in the electric potential of the atom; the principal quantum number determines particular electron shells with distinct energy levels. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.
Beginning in the fourth shell, there are subshells that have lower energy states than those in the adjacent inner shell. Since the electrons fill the levels in order of energy, electrons can start filling subshells in an outer shell before an inner shell is completely full. This explains why, for example, the electron configuration for calcium can be 2,8,8,2 when the third shell can hold up to 18 electrons.
An electron shell can hold up to 2n2 electrons, where n is the principal quantum number of the shell. The occupied shell of greatest n is the valence shell, even if it only has one electron. In the most stable ground state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell. An excited atom's electrons will spontaneously fall into lower levels, emitting excess energy as a photons, until it returns to the ground state.
The constituent protons and neutrons of the atomic nucleus are collectively called nucleons. The nucleons are held together in the nucleus by the strong nuclear force.
Nuclei can undergo transformations that affect the number of protons and neutrons they contain, a process called radioactive decay. When nuclei transformations take place spontaneously, this process is called radioactivity. Radioactive transformations proceed by a wide variety of modes, but the most common are alpha decay (emission of a helium nucleus) and beta decay (emission of an electron). Decays involving electrons or positrons are due to the weak nuclear interaction.
In addition, like the electrons of the atom, the nucleons of nuclei may be pushed into excited states of higher energy. However, these transitions typically require thousands of times more energy than electron excitations. When an excited nucleus emits a photon to return to the ground state, the photon has very high energy and is called a gamma ray.
Nuclear transformations also take place in nuclear reactions. In nuclear fusion, two light nuclei come together and merge into a single heavier nucleus. In nuclear fission, a single large nucleus is divided into two or more smaller nuclei.