An atom is the fundamental unit of matter. That means it’s the smallest building block of all things that we can see, touch or smell. But this answer is still quite cryptic. So let’s look at what people mean when they talk about atoms.
The word ‘atom’ stems from an ancient Greek term meaning ‘uncuttable’, or ‘indivisible’. The Greek philosopher Democritus thought that if you picked up a stick and broke it in half, each part would still be a stick, just a smaller one. You could break a stick again in half, and again, and again, and you would still have a stick. Now if you were to keep repeating this until you had the smallest stick possible, one that was ‘indivisible’, that teeny-weeny stick would be an atomic stick or a stick-atom.
Democritus’ idea was that all the large things in the world were built from atomic building blocks, and that the characteristics of these building blocks determined how the matter would behave in large quantities. While the Greek philosopher was wrong about exactly what things were elemental building blocks, the idea about the smallest indivisible particle was correct.
His idea was to futuristic and held no use in the ancient days. It was forgotten for over 2,000 years, until in 1808 John Dalton formulated the modern atomic theory.
He was a chemist and a schoolteacher and had identified that there were fundamental building blocks in chemistry and regardless of the quantity, these always possessed the same characteristics in how they looked, smelt, felt and behaved when mixed with other substances. Today, we have identified 118 different atoms. Chemists order them in what is called the periodic table.
Dalton also recognised that atoms could somehow connect to other atoms to form molecules. He knew that, for example, water was made from oxygen atoms (number 8 in the periodic table) and hydrogen atoms (number 1). What he did not know, however, was how many oxygen and hydrogen atoms he needed to make one water molecule. This was only understood once scientist learnt how to count and weigh individual atoms.
When you combine two or more atoms the new structure is called a molecule—everything you can touch and see is made from molecules. Each molecule has a particular blueprint and exactly where and how the individual atoms connect can drastically change how the molecule behaves—the smallest change, for example, can turn medicine into poison.
One thing that both Dalton and Democritus were wrong about is that the atom could not be broken down into smaller parts. Today we know that each atom consists of usually three building blocks: electrons, protons and neutrons. The hydrogen atom is the only one that only has a proton and an electron but no neutron.
Protons and neutrons stick together, forming what is called the nucleus of the atom; the electrons whizz around the nucleus in what is called electron orbitals. The numbering of the periodic table is the number of protons that each atom has and the row in which they are placed tells you how far out the electron orbits reach.
The electron is believed to be a fundamental particle meaning it’s not made of any smaller particles, the proton and neutron can be broken down even further into what physicists call quarks.
A further detail that Dalton and Democritus had wrong was that one atom could not be converted into a different atom. While this is generally true, scientists have found two processes called nuclear fission and nuclear fusion that allow them to transform one atom into a different one. In nuclear fusion they take two small atoms and smash them together until they stick together making one larger and heavier atom. In nuclear fission they start with a large atom and shoot very fast neutrons at it trying to split it into two smaller atoms.
Of the 118 atoms that we have discovered, only the first 98 can be found in nature. The atoms with number 43 (technetium) and number 61 (promethium), and all elements with a number higher than 92 occur so rarely in nature that scientists found them first in the lab through a basic form of nuclear fusion. —By Dr. Till J. Weinhold