Atom (science)

An atom (from the Greek atomos, indivisible) is the smallest physical unit that can retain its chemical properties. While atoms were once thought to be the smallest instances of mass in the universe, we now know that they are made up of smaller subatomic-particles.

Structure of the Atom

Atoms are made of a dense nucleus formed by combinations of the two nucleons (positively charged protons and zero charge neutrons) and surrounded by a much larger "cloud" of electrons. The number of protons contained in the nucleus determines the atomic number and in turn which element it is classified as. The number of neutrons further specifies the isotope number of that element. The number of electrons surrounding the nucleus is typically assumed to be equal to the number of protons in order to keep the entire atom electrically neutral. Atoms that are not neutral are called ions, they are designated by their charge in units of elementary charge, which is equal to the negative of the number of surplus electrons present around the atom. Thus an atom with one extra electron is charged -1, and one missing is charged +1.

Forces in the atom

The nucleus of the atom contains a high concentration of positively charged particles with no counter balancing negatively charged particles to keep it stable. In order to explain the existence of the nucleus scientists introduced the two nuclear forces, the strong force and the weak force. We now understand that the nucleus is held together by the residual strong force despite its significant positive charge. The electrons which surround the atom are electro-statically attracted to the nucleus due to their negative charge.

Electron Quantum States

Electrons surround the nucleus of an atom, where each takes on a unique quantum mechanical position in the atom's structure.

Nuclei Quantum States

The nucleus of an atom itself also has a shell like behavior not dissimilar to electron shells. This leads to some extra stable and semi-stable states when the number of protons or neutrons in the nucleus is equal to the "magic numbers" 20, 28, 50, 82, and 126 which is believed to correspond to complete shells.

Decay

Most combinations of nucleons are inherently unstable and undergo a number of radioactive decays in order to form more stable nuclei. In all of the common decays a particle is emitted from the nucleus in order to compensate for some instability in the atom.

• Alpha decay most frequently occurs in atoms which are simply too big, atoms are limited in size because the residual strong force which holds them together only acts over very small distances so that the rate of electrostatic repulsion grows faster than the rate of strong attraction as the nucleus grows. Alpha decay emits an Alpha particle which is denoted with the Greek letter α.
• Beta+ decay occurs in atoms which are proton heavy. In this decay a neutron decays into a proton and emits both a positron and a neutrino. The only natural Beta+ emitter is ⁴⁰K.
• Beta- decay occurs in atoms which are neutron heavy. In this decay a proton decays into a neutron emits both an electron and a neutrino. A lone proton can beta- decay.
• Gamma decay occurs in atoms where the nucleus is in an excited state. The nucleus will turn this excess energy into a high energy photon and return to a lower state. Gamma decays typically follow either an alpha or beta decay that leaves the nucleus with excess energy.
• Electron capture occurs when a proton inside the nucleus captures an electron and becomes a neutron.

History of the Atom

J.J. Thomson's "Plum Pudding" Model

The first widely accepted model of the atom was J.J Thomson's Model of a positively charged cloud with negatively charged particles electrons, interspersed throughout. Thomson proposed and searched for configurations for which these particles had normal modes of vibration and were stable. He built his model using only electrostatic forces, which required that the electrons be in constant motion, this led to further difficulties and he was never able to create a model that matched observed data.

Rutherford Model

Ernest Rutherford, one of Thomson's students, disproved his theory in his now famous scattering experiment by showing that there was a dense positively charged core in each atom. Rutherford posited that the structure of an atom more closely resembled a solar system with the nucleus at the center and electrons orbiting around it.

Bohr's model of the Hydrogen Atom

1913

Today's view

For most practical purposes Rutherford's model has proven an acceptable approximation, however the quantum mechanical interpretation is that the

See Also

• Element
• Molecule

References

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