Structure of Atom

Atom

Smallest particle of an element

Atomic Structure – arrangement of smaller particles within an atom

Two Main Regions In Atom

Nucleus – very small dense central region

protons – positively charged particles

neutrons – neutral particles

● Electron Cloud – large region around nucleus

electrons – negatively charged particles

Discovery of Electrons

JJ Thomson – used Crooke’s Tube (cathode ray tube)Gas under low pressure in a sealed glass tubeHas a positive end (Anode) and negative end (Cathode)When anode and cathode are connected to the + and – terminals of a battery, the tube produces a beam of particles called electrons

Thomson’s Atom

Plum-pudding model

Since total atom is neutral, there must be an equal number of + and - charges

Atom is a positively charged area with negative electrons randomly spread throughout

Nucleus

Ernest Rutherford – Gold foil experiment

Shot positive charged particles at a very thin piece of gold foil

Lead block

Uranium

Gold Foil

Florescent Screen

What he expected

What he got

+

How he explained it

+

Atom is mostly empty spaceSmall dense, positive piece at centerAlpha particles are

deflected by it if they get close enough

Rutherford’s Model Of Atom

Positively charged nucleus containing protons and neutrons with negatively charged particles in orbit around it

Electrons stay in orbit due to attraction between + and – charges

Problems With Rutherford Model

Charged particle when moving in a curved path give off light so they lose energy

If they lose energy, they would gradually get closer and closer to the nucleus and crash into it

Bohr Model of Atom

Similar to Rutherford model, but in this model electrons orbit in specific, well defined energy levelA gain or loss of energy moves the electron from one energy level to anotherPositively charged protons and neutral neutrons held together tightly by the Strong Nuclear Force