Defining the Atom

An atom is the smallest part of an element that maintains the same characteristics as that element. This concept dates back to 530 B.C. when Greek philosopher Democritus named them "atoms," meaning uncuttable or indivisible. To grasp how incredibly tiny atoms are, consider that a single copper penny contains approximately 24,000,000,000,000,000,000,000 atoms!

John Dalton's Atomic Theory established four fundamental principles: all elements consist of indivisible atoms; atoms of the same element are identical; atoms combine in simple ratios to form compounds; and chemical reactions rearrange atoms but never transform one element into another.

The anatomy of an atom includes a dense nucleus (containing protons and neutrons) with electrons orbiting around it. Protons carry positive charges, neutrons have no charge, and electrons carry negative charges. The number of these subatomic particles defines what makes each atom unique, while the total number of protons and neutrons gives us the mass number.

💡 Although the nucleus contains almost all of an atom's mass, it takes up virtually none of its volume—if an atom were the size of a football stadium, the nucleus would be smaller than a marble at the center!

The three subatomic particles differ dramatically in size and mass. Protons and neutrons have approximately the same mass $about 1.67 × 10^-24 grams$, while electrons are roughly 1/1840 the mass of a proton $9.11 × 10^-28 grams$. Despite these differences, atoms remain electrically neutral because the number of positive protons equals the number of negative electrons.