An atom is the smallest unit of an element. However, it is not indivisible as its Greek name suggests. It consists of a tiny nucleus surrounded by an 'electron cloud' [1] several thousand times the diameter of the nucleus [2] . More than 99.9 % of the mass (or 'matter') is concentrated in the tiny nucleus, while less than 0.1 %, in the form of electrons, is dispersed over a surrounding space about 10 million times as voluminous as the nucleus, suggesting a picture of an almost 'empty' atom. The size (diameter) of atoms ranges from about 60 pm (helium) to 500 pm (cesium) [3] , and the mass varies between 2 × 10-27 and 5 × 10-25 kg.
More on how we know about atoms
The term 'electron cloud' was introduced by Feynman in his popular physics lectures. It illustrates the uncertainty of an electron's position and movement. Other physicists have invoked the picture of a fast spinning propeller: invisible, can be anywhere, but the outer boundary is clearly defined and normally cannot be penetrated.
This size comparison between atom and its nucleus refers to Rutherford's discovery of the tiny nucleus (see his famous experiment ). Modern atomic theory suggests that the 'electron cloud' has no outer boundary and, therefore, the size of an atom is undefined.
Due to the undefined boundary of the electron cloud, there is no absolute size of an element's atom. However, the distance between the centers of two equal atoms bonded in the crystal of an element can be measured, e.g. by X-ray crystallography. The atomic radius is defined as half this distance and varies with the type of bonding. Most atomic radii are in the order of 100 pm, or 0.1 nm. An atom's volume, and hence its radius, can also be estimated from the element's molar mass, its density, and the Avogadro constant.