Who first proposed the idea that matter is made of indivisible particles, and in what era?

Isaac Newton, in the 17th century, based on his experiments with light John Dalton, in 1803, based on chemical reactions Democritus, an ancient Greek philosopher around 400 BCE — but it was philosophical, not experimental J.J. Thomson, in 1897, based on cathode ray experiments

What was the key evidence that led John Dalton to develop his atomic theory in 1803?

He observed atoms directly through a powerful optical microscope Chemical reactions always combine elements in fixed, simple mass ratios (law of definite proportions) — best explained by discrete atoms combining in fixed numbers He measured the weight of individual atoms using a balance He discovered that atoms emit light of specific colours when heated

Which of the following was NOT part of Dalton's original atomic theory?

Elements consist of tiny atoms that are the smallest unit of the element All atoms of the same element are identical and have the same mass Atoms contain smaller particles (electrons and a nucleus) Compounds are formed when atoms combine in whole-number ratios

What is a cathode ray tube, and what did experiments with it reveal?

A glass tube filled with mercury vapour that glows when electricity passes through it — like a fluorescent lamp A sealed glass tube with metal electrodes from which air has been removed — experiments showed it produced a beam of identical, negatively charged particles (electrons) from any metal electrode A tube used to produce X-rays for medical imaging A vacuum chamber used to isolate individual atoms for study

J.J. Thomson is credited with discovering the electron in 1897. What did this prove about atoms?

Atoms are made of multiple smaller atoms called electrons Atoms are not indivisible — they contain smaller, negatively charged particles (electrons) All atoms carry a negative charge equal to the charge of one electron Electrons are a type of atom specific to metals

How did Thomson determine that cathode ray particles are negatively charged?

The particles produced a spark when they hit a grounded metal surface He applied electric and magnetic fields near the beam — the particles deflected toward the positive plate and away from the negative, indicating negative charge He dissolved the particles in acid and measured the pH change He observed that the particles always moved at the speed of light

Thomson proposed the 'plum pudding' model of the atom after discovering the electron. What did this model look like?

A tiny central nucleus with electrons orbiting it like planets around the Sun Negatively charged electrons ('plums') embedded in a diffuse ball of positive charge ('pudding') — overall neutral A solid sphere of atoms packed together with electrons in the gaps Pairs of positive and negative particles orbiting a central neutron

Why was the plum pudding model eventually abandoned?

The plum pudding model was disproved when X-rays were discovered to pass through atoms Thomson himself discovered it was wrong when he observed electrons orbiting the atom Rutherford's gold foil experiment showed that positive charge was concentrated in a tiny central nucleus, not spread throughout the atom The model could not explain why atoms emit specific colours of light

Dalton assumed all atoms of the same element are identical and have the same mass. Later, isotopes were discovered. How did this update Dalton's theory?

Isotopes have different numbers of electrons — they are charged versions of atoms Isotopes are completely different elements that happen to look similar chemically Isotopes are atoms of the same element with different numbers of neutrons — same atomic number (same element), different mass number (different mass) Isotopes showed that Dalton's atomic theory was completely wrong

The discovery of the electron proved atoms are divisible. Why was this a revolutionary development in science?

It proved that chemistry is impossible — atoms cannot bond if they have internal parts It showed that atoms have internal structure — opening the door to understanding chemistry, electricity, and eventually nuclear physics and quantum mechanics It showed that atoms are made of light, not matter It had little practical impact — the electron was too small to matter