Atomic Theory

The Bohr-Rutherford Model of the Atom

Atomic Theory

• Modern atomic theory has evolved a lot in the last 200 years.

• Dalton first came up with the idea that matter is made up of small particles called atoms.

• Other scientists soon discovered negatively charged electrons that could be transferred between atoms.

• Later, others discovered positively charged protons, and neutral neutrons.

JJ Thompson

• JJ Thompson theorized that the positive charge in an atom was spread out, and the negative charges were dotted throughout it.

• This is called the “raisin bun” model, with electrons being like raisins in the bun.

Ernest Rutherford

• Ernest Rutherford discovered that atoms had a dense, positive nucleus. This is how he did it.

• Rutherford fired positive particles through a thin piece of foil only a few hundred atoms thick. He expected them to pass straight through.

• Instead, some were deflected, indicating a dense positive point in the atom. The nucleus.

Niels Bohr

• Bohr further adaped the model of the atom by discovering that electrons exist in specific energy levels.

Excited Electrons

• Bohr was able to discover this because when atoms are excited they release light.

• When an element is excited by adding energy (light, heat or electricity) the atom releases light. A neon sign is one example of this.

Excited Electrons

• Electrons in the neon atoms get when electricity runs through them.

• When they do this, they move faster. Once they move faster, they have to “jump” to a higher energy level.

• As they fall they need to release the energy, in this case as red coloured light.

Representing Atoms

• There are several different ways to represent atoms. The Bohr-Rutherford model is one such way.

• Ex: Al1327

An Easier Way

• This method is an extension of the Bohr-Rutherford model. Instead of representing the electrons as dots, we just count how many are in each shell.

• Ex: Al1327

An even EASIER way

• The Lewis dot diagram is an even easier way to show atoms. It shows only the outer shell of electrons.

• Since only the outer shell is involved in bonding, this is ideal for us.

• Note: You need to know the pattern in the periodic table.

• Ex: Na Ex: S