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Asking ‘how big is an atom’ is something we learn to wonder when we are in primary school. Can you remember when you were first introduced to the idea that everything is made up of atoms? If so, then you can remember the first time you wondered how big one was. In order to form an answer, we need to look at both mass and radius. Atomic dimensions are thousands of times smaller than light wavelengths, so they can not be viewed using an optical microscope, but a scanning tunneling microscope can be used to observe them.
Atomic mass comes mostly from the protons and neutrons. The total number of these in an atom is called the mass number. The mass of an atom at rest is expressed as a Dalton(Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12(1.66×10-27kg. Hydrogen-1is the atom with the lowest mass and it has an atomic weight of 1.007825 Da. An atom has a mass approximately equal to the mass number times the atomic mass unit.
Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of the atomic radius. This is a measure of the distance out to which the electron cloud extends from the nucleus. This assumes the atoms exhibit a spherical shape, but this is only obeyed by atoms in vacuum or free space. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a chemical bond. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms and the quantum mechanical property known as spin.
To give you an idea how small an atom really is try to visualize these examples. A typical human hair is about 1 million carbon-12 atoms in width. A single drop of water contains about 2×1021 atoms of oxygen, and twice that many hydrogen atoms. A one carat diamond contains about 1022 atoms of carbon. Even with examples to think of, it is hard to imagine ‘how big is an atom.’
If you’d like more info on the Atom, check out NASA’s Article on Analyzing Tiny Samples, and here’s a link to NASA’s Article about Atoms, Elements, and Isotopes.
We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.