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Chemical properties

Chemical properties of elements and compounds

Atomic number - Atomic mass - Electronegativity according to Pauling - Density - Melting point - Boiling point - Vanderwaals radius - Ionic radius - Isotopes - Electronic schell - Energy of first ionisation - Energy of second ionisation - Standard potential

Atomic number

The atomic number indicates the number of protons within the core of an atom. The atomic number is an important concept of chemistry and quantum mechanics. An element and its place within the periodic table are derived from this concept.
When an atom is generally electrically neutral, the atomic number will equal the number of electrons in the atom, which can be found around the core. These electrons mainly determine the chemical behaviour of an atom. Atoms that carry electric charges are called ions. Ions either have a number of electrons larger (negatively charged) or smaller (positively charged) than the atomic number.

Atomic mass

The name indicates the mass of an atom, expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass in always very close to the mass (or nucleon) number, which indicates the number of particles within the core of an atom; this means the protons and neutrons. Each isotope of a chemical element can vary in mass. The atomic mass of an isotope indicates the number of neutrons that are present within the core of the atoms. The total atomic mass of an element is an equivalent of the mass units of its isotopes. The relative occurrence of the isotopes in nature is an important factor in the determination of the overall atomic mass of an element. In reference to a certain chemical element, the atomic mass as shown in the periodic table is the average atomic mass of all the chemical element's stable isotopes. The average is weighted by the relative natural abundances of the element's isotopes.

Electronegativity according to Pauling

Electro negativity measures the inclination of an atom to pull the electronic cloud in its direction during chemical bonding with another atom.
Pauling's scale is a widely used method to order chemical elements according to their electro negativity. Nobel prize winner Linus Pauling developed this scale in 1932.
The values of electro negativity are not calculated, based on mathematical formula or a measurement. It is more like a pragmatic range.
Pauling gave the element with the highest possible electro negativity, fluorine, a value of 4,0. Francium, the element with the lowest possible electro negativity, was given a value of 0,7. All of the remaining elements are given a value of somewhere between these two extremes.


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.

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.

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.

Vanderwaals radius

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.

Ionic radius

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.


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.

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.

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.

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.

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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