What were alpha particles used in Rutherford's experiment?

Negatively charged particles — electrons — fired from a heated wire Positively charged particles — helium nuclei (2 protons + 2 neutrons) — emitted by radioactive radium Neutral particles — neutrons — produced in a nuclear reactor High-energy photons (gamma rays) from a radioactive source

What result did Rutherford's team expect to see, based on the plum pudding model?

All alpha particles should bounce straight back — the atom is a solid sphere Alpha particles should pass straight through or deflect only slightly, because the positive charge is spread diffusely throughout the atom The alpha particles should be absorbed by the gold and disappear The alpha particles should split the gold atoms in two

What did Rutherford actually observe?

All alpha particles deflected at 90° — consistent with a uniform distribution of charge All alpha particles passed straight through — confirming the plum pudding model Most particles passed straight through; some deflected at large angles; a very small number bounced almost directly backward Most particles bounced back — confirming gold atoms are solid spheres

Why do most alpha particles pass straight through the gold foil?

Gold is a special metal that allows all charged particles to pass through The alpha particles are negatively charged and are not affected by the positive nucleus Because atoms are mostly empty space — the nucleus is tiny (10⁻¹⁵ m) relative to the atom's overall size (10⁻¹⁰ m) The electrons surrounding the atom repel the alpha particles enough to push them through

Why do a small fraction of alpha particles deflect at large angles or bounce back?

They collide with electrons in the outer shells, which deflect them back They are absorbed by the nucleus and re-emitted in a random direction They pass very close to, or directly at, the tiny positively charged nucleus — the intense electric repulsion at such close range deflects or repels them They are deflected by magnetic fields created by electron spin in the gold atoms

What did Rutherford's experiment tell us about where the mass of an atom is concentrated?

Mass is evenly distributed throughout the atom, including the electrons The electrons carry most of the mass; the nucleus is very light Almost all the mass is in the tiny central nucleus — the rest of the atom (the electrons) contributes almost nothing to the total mass Mass is concentrated in the electron shells, not the nucleus

Rutherford proposed the existence of the proton. What property does a proton have?

A proton is a neutral particle in the nucleus that helps stabilise it A proton is a positively charged particle in the nucleus with a charge equal in magnitude to the electron's charge and a mass about 1,836 times greater than the electron A proton is an electron that has gained positive charge by gaining energy A proton is a positively charged particle that orbits the nucleus

If the nucleus of an atom has 79 protons (like gold), how many electrons does a neutral gold atom have?

118 electrons — gold atoms have more electrons than protons 197 electrons — equal to gold's mass number 79 electrons — the number of electrons equals the number of protons in a neutral atom It varies — the number of electrons changes as atoms interact

Rutherford's model of the atom (electrons orbiting a nucleus) has a major problem: classical physics predicts it shouldn't work. What is this problem?

The electrons would collide with each other and fall off the nucleus Accelerating electrons should emit electromagnetic radiation and spiral into the nucleus within a fraction of a second — but atoms are stable The nucleus would repel the orbiting electrons because they are both positively charged The gravitational attraction between nucleus and electrons is too weak to maintain an orbit

James Chadwick discovered the neutron in 1932. Why had the neutron been predicted before it was found?

Rutherford's equations only balanced if there was a charged particle of opposite sign to the proton Scientists knew neutrons existed from theory — they just hadn't looked for them yet Nuclei were too heavy for their number of protons alone — an equal number of neutral particles was needed to account for the extra mass Electrons were too light to be inside the nucleus, so another particle must exist