An amagat is a unit of number density. It is similar to Loschmidt constant which is the molecular density quoted at STP (T = 273.15 K, P = 1 atm). 1 amagat is defined as the number of molecules of an ideal gas per unit volume at STP. 1 amagat is equivalent to Loschmidt constant. The symbol used for amagat is amg or Am (rarely). It is a non-SI unit.

The number is named after Émile Hilaire Amagat. He was a French physicist and also known for Amagat’s law of additive volume.

Amagat's law of additive volumes is the law of partial volumes. The law relates the total volume of a mixture with the volumes of individual components. Amagat's law is very similar to Dalton's law of partial pressure. The law is only valid for ideal gases. The law is named after Emile Amagat who was a French Physicist. He published his law of partial volumes in 1880.

The Loschmidt constant is also called as the Loschmidt number. The symbol used for the Loschmidt constant is n₀. It is the number of molecules of an ideal gas per unit volume. The Loschmidt constant has the unit of the reciprocal cubic metre. At STP (P = 1 atm, T = 273.15 K), The value of the constant is 2.686 781 1 × 10²⁵ m⁻³. The constant is a measure of number density.

The Avogadro constant or (the Avogadro number earlier) is the number of elementary units in one mole of any substance. The Avogadro constant is denoted as N_A. It has the dimension of the reciprocal amount of substance (mol⁻¹). The approximate value of N_A is 6.022 × 10²³ mol⁻¹. This means one mole of any substance contains 6.022 × 10²³ elementary particles. The Avogadro constant is named after Italian scientist Amedeo Avogadro.

Accuracy refers to the closeness between the measured value and actual value of a quantity. While precision indicates closeness among the measured values of a quantity. Accuracy indicates how much the measured value agrees with actual value, and precision indicates how close the measured values agree among themselves. The results which are easily reproducible have good precision.

In chemistry, we may deal with different varieties of calculations. Sometimes, we may come across very large numbers like Avogadro’s constant (N_A = 6.022 × 10²³ mol⁻¹), or sometimes, with very small numbers like the diameter of hydrogen (120 pm). Many times, chemistry experiments involve the handling of large experimental data like vapour pressure data. So, there is a need for the right technique to manage numbers conveniently and maintain the accuracy of numbers. This is where the concept of significant figures arises.

To maintain consistency in SI units, the International System of Units has adopted specific rules and style conventions for writing SI units. General rules for writing SI units were first specified by the 9th CGPM in 1948. Later, various international bodies like ISO extended these rules. These rules are very important in scientific writing. Adherence to these rules is strictly recommended while submitting and reviewing a manuscript. The general guidelines are described below.

The SI stands for the International System of Units or Le Systeme International d’Unités (in French). It is the modern system of measurement. The article lists some of them.

The International System of Units or in short SI (in French Le Systeme International d’Unités) is the currently accepted system of measurement. It originated from France. The SI was established in 1960 by the 11 CGPM (General Conference on Weights and Measures or Conférence Générale des Poids et Mesures in French). The SI system is the updated version of the metric system. It is presently accepted worldwide; though, certain countries like the USA still follows traditional English units. The SI system includes the seven base units, names of 22 derived units, and a set of prefixes.

Everything around us, in the earth, and beyond earth is matter. Matter consists of atoms. Every type of atom is associated with an element. There are 118 elements known to us, and they are arranged in the modern periodic table. This article mentions some of the most abundant elements in different systems.