The diameter of an atom is about 10 −8 cm. And atoms are very, very small -so much so that, until the early twentieth century, chemists and physicists had no accurate means of isolating them to determine their mass. Mass does not necessarily relate to size, though there is enough of a loose correlation that more often than not, we can say that an item of very small size will have very small mass. Putting an Atom's Size and Mass in Context Its invariability makes mass preferable to weight as a parameter of scientific measure. Therefore a person's weight varies according to gravity, and would be different if measured on the Moon, whereas mass is the same throughout the universe. Weight is a measure of force affected by Earth's gravitational pull. (Of course it would make no more sense to measure atoms in pounds or kilograms than to measure the width of a hair in light-years but pounds and kilograms are the most familiar units of weight and mass respectively.) Though the two are relatively convertible on Earth, they are actually quite different. However, the pound is a unit of weight in the English system, whereas a kilogram is a unit of mass in the metric and SI systems. Mass is, after all, basically the same as weight, is it not? In fact it is not, though people are accustomed to thinking in those terms since most weight scales provide measurements in both pounds and kilograms. One might ask why such pains have been taken to make the distinction. Though 'weigh' is used as a verb in this essay, this is only because it is less cumbersome than 'measure the mass of.' (In addition, 'atomic weight' may be mentioned when discussing studies by scientists of the nineteenth century, who applied that term rather than atomic mass.)

Indeed, the use of ' atomic weight' today merely reflects the fact that scientists in the past used that expression and spoke of 'weighing' atoms. The first of these is not as accurate as the second, which explains why atomic mass was chosen as the subject of this essay. Some textbooks and other sources use the term atomic weight instead of atomic mass. The measurement of atomic mass was thus a historic challenge that had to be overcome, and the story of the ways that scientists met this challenge is an intriguing one. More specifically, the work of a chemist requires the use of accurate atomic proportions in forming the molecules that make up a compound. One answer is that everything is made of atoms. But what about something so insignificant in mass that comparing it to a gram is like comparing a millimeter to the distance between Earth and the nearest galaxy? Obviously, special units are needed for such measurements then again, one might ask why it is necessary to weigh atoms at all. Every known item of matter in the universe has some amount of mass, even if it is very small.

Ca atomic mass