Atomic Models & Properties

Q: How have scientists’ views of the atom changed over time?

A: Scientists have theorized what an atom looks like, evolving from a simple particle to revolutionizing quantum mechanical models.

Q: Who was Democritus and what did he believe?

A: Greek philosopher in 400 BCE. He believed all matter can be divided into smaller pieces until a single indivisible particle is reached. A single particle cannot be further “cut.”

Q: According to Democritus, what are the properties of atoms?

A:

• Different sizes

• In constant motion

• Separated by empty spaces

Q: Who reintroduced the atom theory in 1807 and what was his model called?

A: John Dalton reintroduced the theory. His model is called the Billiard Ball Model.

Q: What were John Dalton’s main proposals about atoms?

• All matter is made up of tiny indivisible particles called atoms.

• All atoms of an element are identical.

• Atoms of different elements are different.

• Atoms are rearranged to form new substances in chemical reactions, but are never created or destroyed.

Q: How did Dalton describe atoms in his Billiard Ball Model?

A: Atoms are solid, hard spheres that are movable, like billiard balls.

Q: What problem existed with Dalton’s Billiard Ball Model?

A: It did not explain why some objects attract or repel each other.

Q: Who discovered the electron and when?

A: J.J. Thomson in 1897 discovered that all atoms contain very small negatively charged particles called electrons.

Q: What did Thomson observe about positive and negative charges?

A: Positive and negative charges are attracted to each other.

Q: How did Thomson discover electrons using a cathode ray tube?

A:

• He noticed that when rays were emitted from a cathode ray tube (e.g. found in neon signs), all the rays moved to the positive end of the circuit.

• Therefore, he concluded that the particles must be negatively charged

Cathode Ray

• Prior to Thomson’s experiment, it was widely known that when a power outlet was used, rays from the cathode end moved towards the anode end

• However, it was thought that this phenomenon occurred due to the material of the cathode

• When a magnet was placed on the cathode, some of the rays deflected, while some moved towards the anode

  • Therefore, the movement was dependent on the particles- not material 

Q: What is the Plum Pudding Model?

A: Thomson’s model:

• Atoms contain negatively charged electrons.

• Since atoms are neutral, the rest must be positively charged.

• Electrons are evenly distributed throughout the atom.

• Called “Plum Pudding” because electrons are embedded in a positive “pudding.”

Q: What was Rutherford’s Gold Foil Experiment?

A: In 1909, Rutherford tested Thomson’s model by shooting positive particles at thin gold foil.

Q: What did Rutherford predict if Thomson’s model was correct?

A: That particles would pass through the foil or be deflected at a small angle.

Q: What were the actual results of the Gold Foil Experiment?

A: Most particles passed through undetected, but some were deflected by something very large and positive.

Q: What did Rutherford conclude from the Gold Foil Experiment?

A:

• The atom has a central nucleus with positive charge.

• Electrons surround the nucleus.

• Most of the atom is empty space.

• Discovered the proton (positively charged particle).

Q: What were the limitations of Rutherford’s Atomic Model?

A:

• Contradictions with electron and energy emission.

• If the nucleus is entirely positive, why doesn’t it burst apart?

• Did not explain why like atoms repulse each other.

• Could not fully explain the total mass of the atom.