Chemistry Definitions


Diamagnetism:
This is when a substance is repelled from a magnetic field because it contains paired electrons. These paired electrons are in a state of field cancellation with one another. The presence of an external magnetic field will disrupt the coherence of this di-electron state, raising its energy. This causes it to repel away from the field in search of a lower energy state. Copper (Cu) and zinc (Zn) are diamagnetic.

Di-electron:
A single, coherent, quantum state made up of two electrons with antiparallel spin that are completely superimposed upon one another. This is a very stable state since it allows for maximum cancellation of magnetic field and a reduction in quantum angular momentum, and thus, a significant lowering of energy. Examples include the 1s2 shell in helium (He) and the covalent bond in dihydrogen (H2). For more on electrons, see Electrons & The Hierarchy of Forces.

Effective Nuclear Charge (Zeff):
The protons in the nucleus attract the electrons in the electron cloud around it. The more protons in the nucleus and the more electrons in the orbitals, the more strongly the two are attracted towards each other. This shrinks the size of the atom. In general, effective nuclear charge increases as we move to the right in a row on the periodic table, since we are adding both protons and electrons when we move to the next element. This means atoms get smaller as we move to the right in a row on the periodic table. In general, effective nuclear charge decreases as we move down a column (Group) on the periodic table, since we are adding electron shells. The outer electrons are further from the nucleus, which both decreases their attraction to it, and the new shell adds diameter to the atom.

Electron:
A negatively subatomic particle with a spin of S=½ and a charge of 10-19 Coulombs. It has a toroidal sub-structure in its momentum space, comprising of a photon making two revolutions per wavelength. For more on electrons, see Electrons & The Hierarchy of Forces.

Electronegativity:
When two atoms bond, electronegativity is a measure of how strongly an atom’s nucleus pulls the two bonded electrons toward itself. When there is an electronegativity difference between the two bonding atoms, the more electronegative atom will pull electron density more strongly, resulting in an imbalance in the electron sharing and a polar bond. In general, the smaller the atom, the higher its electronegativity. The reason is because of effective nuclear charge. (See above.)

Ferromagnetism:
This is when a substance can retain its internal magnetic field alignment and therefore act as a permanent magnet. When exposed to an external magnetic field, the unpaired electrons throughout the substance align with the field, and when the field is removed, the electrons remain in alignment with one another. Iron (Fe), cobalt (Co), and nickel (Ni) are ferromagnetic.

Hybridization:
When different orbitals in the same shell, like an s-orbital and a p-orbital, resonate together since they are occupying the same volume of space. This combination of their electron densities changes their shape in order to achieve greater symmetry and stability.

Ion:
An atom that has either gained or lost one or more electrons. This means there is no longer a balance between the positive protons in the nucleus and the negative electrons enveloping it. The atom now has an overall charge. If it lost electrons it will be a positive ion (also known as a cation). If it gained electrons it will be a negative ion (also known as an anion).

Ionization Energy:
This is the amount of energy needed to remove an electron from an atom, to overcome the attraction from its nucleus. (An analogy might be giving a rocket enough thrust to escape a planet’s gravity.) The higher the ionization energy, the harder it is to remove the electron. The lower the ionization energy, the easier it is to remove the electron. The most reactive elements will be those with either low or high ionization energies. This is because they will either be eager to donate an electron or steal an electron (respectively) in a chemical reaction. In general, ionization energy increases as we move to the right in a row on the periodic table, since effective nuclear charge is increasing (see above). In general, ionization energy decreases as we move down a column (Group) on the periodic table, since the valence electrons are further from the nucleus and therefore less strongly attracted to it.

Magnetic Susceptibility (χm):
This is a measure of how strongly an element will be attracted (or repelled) by an external magnetic field. A negative value means the element is diamagnetic — it has all of its electrons paired and it repels from an external magnetic field. A positive value means the element is paramagnetic — it has unpaired electrons and it is attracted into an external magnetic field.

Orbital:
A region of space occupied by electron density. There are several different types of orbitals that vary in size and shape. The simplest is a sphere that envelops the nucleus, known as an s-orbital.

Paramagnetism:
This is when a substance is attracted to a magnetic field because it contains one or more unpaired electrons. In the presence of an external magnetic field, the magnetic field of an unpaired electron will cause it to align with the magnetic field. This causes cancellation of magnetic field to occur between the electron and the direction of the field, and the substance will therefore experience an attraction into the magnetic field. The dioxygen (O2) molecule and most metals are paramagnetic.

Reactivity:
This is a measure of how readily an atom will give or take electrons in order to achieve greater stability and symmetry. If it has a low ionization energy and wants to lose electrons, a high ionization energy and wants to gain electrons, or if it has a high electron affinity and thus a strong desire to gain electrons, it will react vigorously with other elements in order to make those electron exchanges.

Valence Electrons:
These are electrons in the outermost shell of the atom. Sodium has 1 valence electron. Oxygen has 6. Electrons in the inner shells are known as core electrons.

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