# Why are ideas on the nature of matter different from those held 200 years ago, and different from those held 2500 years ago?

Greek philosophers held that matter was composed of small indivisible species, each called an $\alpha \tau o \mu o s$ (which is Greek for $\text{indivisible}$ or rather $\text{uncuttable}$). In the 19th and 20th centuries, physicists and chemists examined this definition experimentally, and proposed that an atom was a fundamental particle, that could not be $\text{chemically}$ altered. Later discoveries in the 20th century proposed that atoms were themselves composed of more fundamental particles, protons, neutrons, and electrons, whose number determined atomic identity and chemistry. The number of protons in the atomic nucleus ($Z$) determined the atomic identity; the number of smaller fundamental particles, called electrons, and opposite in charge to the proton, determined the chemistry of that element. The atomic nucleus also contains neutrons, neutral particles which add to the mass of the element.
As a point of interest, historically, electrons were conceived to have a negative charge, and the protons in the atomic nucleus were assigned a positive charge. If we were doing this over again, electrons would be assigned a positive charge - it would save generations of students trying to solve quantum equations who got the answer wrong because they wrongly counted an odd number of electrons and got a $\text{plus}$ result instead of a $\text{minus}$ or vice versa.