Theory Overview of an Atom

An atom is a nucleus plus electrons. The atom's nucleus is composed of neutrons and of protons. Neutrons are electrically neutral particles. Protons are positively charged ones. Electrons are orbiting the nucleus. They are negatively charged. The electric charge of a particle determines how it's behaving relative to another particle. Particles with opposite charges attract each other. Particles with like charges repel. The number of protons in an atom is matching the number of the electrons, as the neutrons are neutral. This yields an electrically neutral atom. An atom is mostly made of vacuum. The distance is enormous between the nucleus and the electrons. The strong nuclear interaction that binds nucleons together in an atomic nucleus is essentially the same between protons and neutrons. For protons, however, the electric repulsion between identically charged particles adds together. A breaking of mirror symmetry exists. Neutrons mainly act as moderators in the nucleus, avoiding that the protons with a same charge repel each other. The electrons are insecable particles as neutrons and protons are made of quarks

The atomic number (or 'Z') designates the number of protons in the atom's nucleus, hence the correlative number of electrons. 'A' is for the mass number of the atom which is the total number of protons and neutrons. The mass of an atom (the unit is the 'amu') is approximatively equivalent to the mass number. 'Isotopes' are atoms of a same element but with different mass number. Considering that any atom of a same compound always has a same number of protons, the variation may only occur about the number of neutrons

Atoms are mainly stable. Some however, may they be isotopes or not, are decaying, i.e. they are turning into other elements. This is done by a change in their number of protons. Such a decay is the famed 'radioactivity'. When atoms decay, they expel protons, or neutrons. When an atom expels at the same time 2 protons and 2 neutrons (which is a nucleus of the isotope helium-4 of the helium), this is called alpha decay (such helium-4 nuclei are sometimes called alpha particles). Another way of decay is to convert a proton into a neutron or reciprocally. This is the beta decay and the process must keep the same amount of electric charge, hence a neutron decaying into a proton produces a proton (+1), an electron (-1) and a neutral antineutrino, or a proton decaying into a neutron produces a neutron, an anti-electron or positron (+1), and a neutral electron neutrino; products of such a decay are called beta particles. Such decays are responsible for the heat of planets' core, or they allow dating rocks and elements in the Universe

Nuclei and electrons are bound together by the 'electromagnetic force' due to that particles of opposite electric charge are attracting each other. The nearer the nucleus and the electrons, the stronger the attraction. Protons and neutrons in the nucleus are linked by the 'strong force'. The strong force is overwhelming the electromagnetic force which would make the protons with the same electric charge blow apart. It needs too the presence of neutrons. The strong force operates only at short distance. Only neighbouring protons and neutrons feel it. As distance diminishes the strong force turns into its opposite, as it becomes a repulsive force, hence particles in a nucleus are evenly spaced. It's the strong force which determines the adequate number of protons and neutrons to be found in a nucleus. The strong force is mediated by tiny and fleeting particles called pions. The decay in the atoms is governed by the 'weak force'. The weak force acts at a short distance only, as it affects every particles of the atom. The weak force may change one particle into another, e.g. a proton into a neutron. The gravity, this other force of the natural world, is not apparent at the level of the atomic particles

Atoms are described by this specific field of physics, the quantum physics. The quantum physics states that matter both acts like a particle and like a wave. This actually mostly applies to electrons. This quantum explanation of the atom came after the more classical views of Rutherford (1911 -the atom was seen like a solar system) and of Bohr (the electrons were found at discrete radii). Quantum physics mainly determines the structure of the electrons, hence it mainly determines the ability of atoms to bond to each other, and the energy level of the atoms too. The motion of an electron relatively to the atom's nucleus is now seen like an ensemble of orbits, or a cloud of possible positions. Such a cloud is called an 'orbital' as it may be circular or two-lobe shaped. High densities in the clouds hint to the most likely positions of the electrons. To keep with this new description of the electrons, different shells, with sub-shells, are seen around the atom's nucleus. Each shell is constituted by the electrons which have a same orbital. The farthest the shell from the nucleus, usually the higher the energy of the electrons. The stability of an atom, as far as this relationship between the nucleus and the electrons is concerned, is due to its electrons orbiting at such distances that their wavelength builds a positive interference which reinforces the wave. The quantum description of the electrons further determines how the atoms are interacting between them. It is this structure of the electrons clouds which determines that interaction. If the structure of the last shell of electrons is not complete, that is if the atom is unstable, the atom will interact easily. It will be exchanging or sharing electrons with one other or more atoms (this is due to the fact that the atoms always seeks the lowest energy state possible; in this case, it's requiring less energy to fill their outer shell by linking to other atoms than otherwise). The last shell of electrons is called the 'valence shell'. Interaction between atoms may take several forms according to what happens. A 'covalent bond' is when one or more atoms are sharing electrons through their valence shells. Molecules, for example, are based on covalent bonds only. A 'ionic bond' is when atoms having gained or lost electrons in their valence shell, and having become electrically charged (that is a 'ion' -a ion is a 'cation' if positive, 'anion' if negative) are linked together on the basis of their opposite charges. Such ionic bonds usually yields crystals or salts. 'Metallic bonds' occur when atoms are completely sharing their outer shell of electrons. The resulting structure is a crystal, but electrons move freely in it. That's why the metals are good conductors of electricity! It is this shell structure, at last, which is pertinent too to the changes of energy of an atom. An atom, at rest, is tending to the lowest state of energy possible, that is all of its electrons are at their places in the orbitals and in the shells. When an atom is hit -by another atom or by a particle for example, it's gaining energy. This actually means that its electrons are moving to a higher orbital. Then the electrons are falling back to their original state as they release the energy they gained under the form of a photon. Such a photon is ranging from the X-rays to the visible light, according to the energy which was absorbed

A quantum microscope allowed by June 2013 to the first direct observation of the orbital structure of an excited hydrogen atom, zapping the atom with laser pulses, forcing the electron of it to be ejected along direct and indirect trajectories. The phase difference between these trajectories produce a interference pattern which can be photographed. Atoms can take on different shapes depending on their excited states