The periodic table is the map of the chemical world. Columns collect atoms which share properties - all of the elements on the far right - He, Ne, Ar… - are all gases and all nearly chemically inert. The region at the bottom harbors elements more likely to be radioactive. Metals pool in the middle.
Each atom of an element has a characteristic number of protons - positively charged particles - in their nucleus. An atom with five protons is boron. One with 82? Lead.
Most atoms also have a number of uncharged particles - neutrons - in their nuclei as well. The sum of the number of protons and neutrons in a given nucleus is called its mass number. A boron atom with six neutrons has a mass number of 11: five protons and six neutrons. Take away a neutron and it’s still boron, but the mass number is now 10.
Atoms with different mass numbers but the same number of protons are termed isotopes. Most elements have several naturally occuring isotopes. The most abundant form of the element carbon has a mass number of 12. One percent of carbon atoms, however, have an extra neutron and a mass number of 13.
Scottish novelist and physician Margaret Todd coined the term for her distant relative Frederick Soddy at a dinner party in 1913. He had described his research to her and she responded that any good discovery need a Greek term to describe it. She suggested combining the Greek “iso” for same and “topos” for place - to emphasize that the mass number of an element doesn’t affect it’s place in the periodic table: argon-36 and argon-40 are both inert gases. Soddy went on to win the Nobel Prize in 1921 for his discovery - perhaps because his distant relation had coined him a such good term?