molecule 的定義

元素指自然界中一百多種基本的金屬和非金屬物質，它們只由一種原子組成，其原子中的每一核子具有同樣數量的質子，用一般的化學方法不能使之分解，並且能構成一切物質。

一個分子是由多個原子在共價鍵中透過共用電子連接一起而形成。它可以由相同化學元素的原子組成，如氧氣 O2；也可以是不同的元素，如水分子 H2O。抽象地，一個單一原子也可當作是一分子（單原子分子），但在實際使用時，「分子」指的通常是多個原子的化學化合物。

在氣體元素中，稀有氣體（如氬 Ar）是1。固體元素中，黃磷（P4）原子數是4，硫（S8）的是8。所以，氬（Ar）是單原子，氧氣（O2）是雙原子的，臭氧（O3）則是三原子的。

由分子組成的物質叫分子化合物。

化學式
要反映分子中各種原子的真實數量，就要利用化學式。例如乙烯和丙烯的化學式分別為C2H4和C3H6。

但化學式相同並不代表兩個分子是一樣的物質，因為分子中原子的排列和組合，亦即分子的結構，也是決定分子性質的要素。同樣的原子但排列不同的分子叫同分異構體。同分異構體有同一化學公式但因不同結構的關係有不同的特質。立體異構體是一種特別的異構體，它們可以有很相似的物理及化學性質，而同時有十分不同的生物化學性質。

由量子力學的定律的演算，分子有固定的平衡幾何狀態——鍵的長度和之間的角度。純物質都是由相同幾何結構的分子組合而成的。分子的化學式和結構是決定它的特質，尤其是它的化學活性的兩要素。

A mass formula is an equation or set of equations in physics which attempts to predict the mass or mass ratios of the subatomic particles.

An important step in high energy physics was the discovery of the Gell-Mann-Okubo mass formula predicting relationships between masses of the members of SU (3) multiplets.

The development of an accurate mass formula is one of several fundamental aspects to developing a working theory of everything, which can overcome the incompatibilities between current classical and quantum physics theories.

There are currently no universal mass formulae which are generally accepted as correct by the mainstream physics community, however several versions of potential mass formulae have been presented and are currently being explored by some (largely amateur) physics theorists.
The molecular mass (abbreviated MM) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). Due to this relativity, the molecular mass of a substance is commonly referred to as the relative molecular mass, and abbreviated to Mr.

CONVERSION FACTOR of molecular mass to molar mass

molar mass = molecular mass * (6.022*10^23)

The molecular mass can be calculated as the sum of the atomic masses of all the atoms of any one molecule. The molecular mass can also be measured directly using mass spectrometry. In mass spectrometry, the molecular mass of a small molecule is usually reported as the monoisotopic mass, that is, the mass of the most common isotope of each element. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not the same as found on a typical periodic table. The average molecular mass is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element. This average mass can be calculated using the elemental mass numbers on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule.

The molar mass of a substance is the mass of 1 mol (the SI unit for the basis SI quantity amount of substance, having the symbol n). This has numerical value of the molecular mass multiplied by avagadro's constant 6.022*10^23, and the SI unit kg/mol, usually also found as stated in g/mol .

Molecular mass or molar mass are used in stoichiometry calculations.

Since molecules are created by chemical reactions, not nuclear reactions, a molecule's molecular mass exactly equals the sum of the atomic masses of its constituent atoms.

The gram-molecular weight is the molecular weight of a chemical compound expressed in grams. Thus the molecular weight of calcium carbonate is 100 therefore the gram-molecular weight is 100 g. The Gram Molecular Weight is frequently used, particularly for calculating the concentration of solutions. Thus a molar solution will contain the gram molecular weight of a chemical dissolved to produce one litre of solution.

In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds [1] [2] [3] [4] [5]. Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure substance that still retains its composition and chemical properties.[6] Certain pure substances (e.g., metals, molten salts, crystals, etc.) are best understood as being composed of networks or aggregates of atoms or ions instead of molecular units.

In the molecular sciences, a molecule is a sufficiently stable, electrically neutral entity composed of two or more atoms.[7] The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition. [8]