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## Standard potential

The standard potential means the potential of a redox reaction, when it is at equilibrium, in relation to zero. When the standard potential exceeds zero, we are dealing with an oxidation reaction. When the standard potential is below zero, we are dealing with a reduction reaction. The standard potential of electrons is expressed in volt (V), by the symbol V0.

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## Energy of second ionisation

Besides the energy of the first ionisation, which indicates how difficult it is to remove the first electron from an atom, there is also an energy measure for second ionisation. This energy of second ionisation indicates the degree of difficulty to remove the second atom.

As such, there is also the energy of a third ionisation, and sometimes even the energy of a fourth or fifth ionisation.

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## Energy of first ionisation

The ionisation energy means the energy that is required to make a free atom or molecule lose an electron in a vacuum. In other words; the energy of ionisation is a measure for the strength of electron bonds to molecules. This concerns only the electrons in the outer circle.

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## Electronic Shell

The electronic configuration of an atom is a description of the arrangement of electrons in circles around the core. These circles are not exactly round; they contain a wave-like pattern. For each circle the probability of an electron to be present on a certain location is described by a mathematic formula. Each one of the circles has a certain level of energy, compared to the core. Commonly the energy levels of electrons are higher when they are further away from the core, but because of their charges, electrons can also influence each another’s energy levels. Usually the middle circles are filled up first, but there may be exceptions due to rejections.
The circles are divided up in shells and sub shells, which can be numbered by means of quantities.

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## Isotopes

The atomic number does not determine the number of neutrons in an atomic core. As a result, the number of neutrons within an atom can vary. Then atoms that have the same atomic number may differ in atomic mass. Atoms of the same element that differ in atomic mass are called isotopes.
Mainly with the heavier atoms that have a higher atomic number, the number of neutrons within the core may exceed the number of protons.
Isotopes of the same element are often found in nature alternately or in mixtures.

An example: chlorine has an atomic number of 17, which basically means that all chlorine atoms contain 17 protons within their core. There are two isotopes. Three-quarters of the chlorine atoms found in nature contain 18 neutrons and one quarter contains 20 neutrons. The mass numbers of these isotopes are 17 + 18 = 35 and 17 + 20 = 37. The isotopes are written as follows: 35Cl and 37Cl.
When isotopes are noted this way the number of protons and neutrons does not have to be mentioned separately, because the symbol of chlorine within the periodic chart (Cl) is set on the seventeenth place. This already indicates the number of protons, so that one can always calculate the number of neutrons easily by means of the mass number.

A great number of isotopes is not stable. They will fall apart during radioactive decay processes. Isotopes that are radioactive are called radioisotopes.

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Ionic radius is the radius that an ion has in an ionic crystal, where the ions are packed together to a point where their outermost electronic orbitals are in contact with each other. An orbital is the area around an atom where, according to orbital theory, the probability of finding an electron is the greatest.

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Even when two atoms that are near one another will not bind, they will still attract one another. This phenomenon is known as the Vanderwaals interaction.
The Vanderwaals forces cause a force between the two atoms. This force becomes stronger, as the atoms come closer together. However, when the two atoms draw too near each other a rejecting force will take action, as a consequence of the exceeding rejection between the negatively charged electrons of both atoms. As a result, a certain distance will develop between the two atoms, which is commonly known as the Vanderwaals radius.
Through comparison of Vanderwaals radiuses of several different pairs of atoms, we have developed a system of Vanderwaals radiuses, through which we can predict the Vanderwaals radius between two atoms, through addition.

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## Boiling point

The boiling point of an element or compound means the temperature at which the liquid form of an element or compound is at equilibrium with the gaseous form. We usually presume the air pressure to be 1 atmosphere.
For example: the boiling point of water is 100 oC, or 373 K.
At the boiling point the vapour pressure of an element or compound is 1 atmosphere.

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## Melting point

The melting point of an element or compound means the temperatures at which the solid form of the element or compound is at equilibrium with the liquid form. We usually presume the air pressure to be 1 atmosphere.
For example: the melting point of water is 0 oC, or 273 K.

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## Density

The density of an element indicates the number of units of mass of the element that are present in a certain volume of a medium. Traditionally, density is expressed through the Greek letter ro (written as r).Within the SI system of units density is expressed in kilograms per cubic meter (kg/m3). The density of an element is usually expressed graphically with temperatures and air pressures, because these two properties influence density.