# THE ATOM – Its Size (Atomic radius)

Atoms – Its Symbols

Atoms – Atomic Mass

What is an atom?

As we know, an atom is the smallest building block of everything that exists. It is composed of three sub-atomic particles: the proton, neutron, and electron. Protons and neutrons make up the nucleus of the atom, a dense and positively charged core, while the negatively charged electrons can be found around the nucleus in an electron cloud.

• Atoms can be different by having different numbers of protons, neutrons and electrons. All atoms with the same atomic number behave in the same way chemically and are classified as the same chemical element. Elements are the substances consisting of one type of atoms.
• Atoms are the units of chemical change.

### Size of an atom (Atomic radius)

We know that each atom has a nucleus inside and electrons zooming around outside the nucleus. It should seem reasonable that the size of an atom depends on how far away its outermost (valence) electrons are from the nucleus. If they are very close to the nucleus, the atom will be very small. If they are far away, the atom will be quite a bit larger. So the atomic size is determined by how much space the electrons take up.

Law of Conservation of Mass

Problems based on Law of Conservation of Mass

Law of Definite Proportions / Constant Composition

Dalton Atomic Theory

Problems based on law of Definite Proportions / Constant Compositions

Measuring the size of atoms is, in some ways, like measuring the size of cotton balls or automobile tires. The value we get depends on the conditions under which they are measured. A “free” cotton ball has a different size than when it is in the package. The radius of the tire is different when measured to the top of the tire than when measured to the bottom of the tire resting on the ground. Different values for the sizes of atoms are obtained depending on both the method used and the conditions in which the atoms find itself – free or bonded to other atoms.

Atomic radius is generally stated as ‘ the total distance from an atom’s nucleus to the outermost orbital of electron’. In simpler terms, it can be defined as something similar to the radius of a circle, where the center of the circle is the nucleus and the outer edge of the circle is the outermost orbital of electron. Atomic radius is measured in nanometer (nm).
1nm  =  1 /10m
or
1m   = 10nm
Hydrogen atom is the smallest atom (0.037 nm) of all the elements.