The Sizes of Atoms and Molecules

An atom is the basic unit of an element, consisting of a nucleus surrounded by electrons. When two or more atoms bond together, they form a molecule.

To expand on that, a molecule:

  • is made up of two or more atoms joined by covalent bonds, where they share pairs of electrons.
  • can be made of atoms from the same element or different elements.
  • is defined by its specific number and types of atoms. For example, a water molecule is always H2O, containing two hydrogen atoms and one oxygen atom.

Atoms and simple molecules, like water which has only a few atoms, are typically about 0.1 nanometres in diameter.

“Nano” means billionth or 10−9. Therefore, a nanometre (nm) is one billionth of a metre. To visualise this scale, consider that a human hair is approximately 90,000 nanometres wide. Lining up 900,000 atoms or simple molecules would stretch across the width of a human hair.

Factors Affecting the Size of Atoms

Although all atoms and simple molecules are tiny, some are relatively larger or smaller than others. You can get an idea by looking at the following table:

An illustrated periodic table emphasising the relative sizes of atoms. The table features coloured circles within each element's cell, with the size of each circle representing the relative atomic size. The circles decrease in size from left to right across a period, illustrating the decrease in atomic radius across a period due to increased nuclear charge. Vertically, the circles increase in size from top to bottom within a group, showing the increase in atomic size down a group as additional electron shells are added. A double-headed arrow across the top indicates an increase in atomic size from right to left across a period, and a double-headed arrow along the right side indicates an increase in size from top to bottom down a group.

There are two factors that affect an atom’s size:

  • Number of electron shells: Atoms with more electron shells are larger. This is why atoms get larger as you go down a group in the periodic table.
  • Number of protons in the nucleus (atomic number): A higher number of protons increases the attractive forces between the positively charged protons and negatively charged electrons, pulling the electrons closer to the nucleus. This effect reduces the atom’s size, explaining why atoms generally become smaller across a period in the periodic table.

Considering these principles, hydrogen is the smallest atom due to having just one electron shell and a single proton, while francium is the largest atom, situated at the bottom of its group in the periodic table.

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