Atomic Structure: Electrically Charged Particles
Elective Nature of Matter
When two bodies with electric charges of the same sign approach, they repel each other.
When two bodies with electric charges of opposite signs approach, they attract each other.
Electroscope
An electroscope is a device used to detect charged bodies. There are two ways to detect if bodies are charged:
- Contact: If a charged rod touches the metal ball, the charge will reach the sheets. Since the sheets have the same charge, they repel.
- Induction: If a charged rod is brought near, without touching the electroscope, the charges are reordered. Positive charges go to the top, and negative charges remain on the plates, causing them to repel.
The Electric Pendulum
The electric pendulum was used by Coulomb to study attraction and repulsion between charged bodies. The pendulum consists of a ball hanging by a thread. If we touch a negatively charged rod to the pendulum bob, it acquires a negative charge.
Particles That Form Atoms
At the beginning of the nineteenth century, Dalton determined that matter is made of atoms. Different experiments also showed that matter could gain or lose electrical charge. Therefore, these electric charges are part of the atoms. There are three types of particles in atoms:
- Proton
- Neutron
- Electron
Dalton’s Atomic Theory
Dalton considered atoms indivisible. This was surpassed by experiments showing that atoms were made up of electrically charged particles. One of these experiments was carried out by Thomson with cathode ray tubes.
Cathode Ray Tubes
Cathode ray tubes have a positive and a negative pole. Inside the tube is hydrogen gas. When an electrical discharge was applied between the plates, a lightning-like beam appeared. This beam was formed by negatively charged electrical particles. These particles were called electrons. After Thomson, Rutherford discovered the proton, a positively charged particle. Finally, in 1931, Chadwick discovered the third particle, called the neutron.
Plum Pudding Model
After demonstrating that the atom was not indivisible, as Dalton stated, Thomson proposed an atomic model. Thomson’s model was described as a mass of positive charge with electrons embedded within it. The atom was considered neutral.
Rutherford Model
Rutherford’s experiment, which aimed to check the validity of Thomson’s atomic model, involved bombarding a thin gold foil with a beam of alpha particles (positively charged). He observed that most alpha particles passed through unaffected or with slight deviations, some experienced significant deviations, and a few bounced back.
Rutherford’s Atomic Model
To explain the large deviations that some alpha particles experienced, Rutherford proposed a new atomic model. According to Rutherford, the atom consists of:
- Nucleus: The central part containing all the positive charge and almost the entire mass of the atom.
- Shell: Surrounding the nucleus, where the negatively charged electrons are located. These electrons revolve around the nucleus at great distances.
Rutherford suggested that the nuclei of atoms had to have other particles with a mass almost equal to that of the proton but with no electric charge. He called them neutrons. Chadwick discovered them in 1932.