Comprehensive Study Notes on Atomic Structure and Sub-atomic Particles

Characteristics and Properties of the Electron

Electrons are defined as negatively charged sub-atomic particles. According to the transcript, these particles possess a charge of $-1.6 \times 10^{-19}\,C$ and a mass of $9.1 \times 10^{-31}\,kg$. The standard symbol used to denote an electron is $e^-$. In terms of physical characteristics, electrons are extremely small and are located outside the nucleus of the atom.

Comparative Properties of Sub-atomic Particles

The fundamental sub-atomic particles—the electron ($e^-$), the proton ($p^+$), and the neutron ($n^0$)—differ significantly in their charges, masses, and historical discovery. The electron carries a negative charge of $-1$, possesses a negligible mass approximately equal to $\frac{1}{2000}$ of a proton, and was discovered by J.J. Thomson in the year 1897.

The proton carries a positive charge of $+1$ and has a mass designated as $1\,unit$. The discovery of the proton is attributed to E. Goldstein in 1886. The neutron is a neutral particle with a charge of $0$ and a mass of approximately $1\,unit$. It was discovered by J. Chadwick in 1932.

The Discovery and Characteristics of Protons

E. Goldstein detected the presence of positively charged particles, known as protons, through his famous experiment involving anode rays, also referred to as canal rays. These rays consist of positively charged radiations. The proton has specific physical characteristics: its charge is exactly $+ 1.6 \times 10^{-19}\,C$, and its mass is measured at $1.673 \times 10^{-24}\,gm$. Comparatively, the mass of a proton is approximately $2000$ times the mass of an electron.

Postulates of Thomson's Atomic Model

When asked what the two postulates proposed by Thomson are, the model provides the following description: First, an atom consists of a positively charged sphere, and the electrons are embedded within this sphere. Second, the negative and positive charges are equal in magnitude; consequently, the atom as a whole is electrically neutral.

Observations and Conclusions from Rutherford's Alpha Particle Experiment

Rutherford's alpha particle experiment led to several critical observations regarding atomic structure. First, it was observed that most alpha particles passed through the gold foil undeflected, which indicates that atoms are predominantly empty space. Second, the experiment revealed a positively charged nucleus because a few alpha particles deflected, and approximately $1$ in $12,000$ particles bounced back. This indicated the presence of a small, dense, positively charged center.

Third, regarding nucleus size, the nucleus was found to be extremely small, measuring about $\frac{1}{10^5}$ the size of the atom, meaning it occupies negligible space compared to the overall empty space within the atom. Fourth, the experiment concluded that the entire atomic mass is concentrated in the nucleus.

Features of Rutherford's Model of the Atom

Based on his observations, Rutherford proposed a model with specific features. He stated that there is a positively placed nucleus in an atom where nearly all the mass resides, consisting of protons and neutrons. He also proposed that electrons revolve around the nucleus in well-defined orbits. Finally, he emphasized that the size of the nucleus is very small compared to the total size of the atom.

Drawbacks of Rutherford's Model and the Proposal of Bohr's Model

There were significant drawbacks identified in Rutherford's model of the atom. According to the theory, electrons revolve around the nucleus in well-defined orbits; however, because electrons are charged particles, they would continuously lose their energy while moving. Eventually, this loss of energy would cause the electrons to fall into the nucleus, which would make the atom highly unstable.

This instability was the major drawback of the Rutherford model and remained unexplained by him. To overcome these limitations, Neil Bohr proposed a modified model for the structure of the atom in 1912.

Atomic Number and Elemental Identity

The atomic number is defined as the total number of protons lying in the nucleus of any atom. This number serves as the identity of an atom; changing the atomic number effectively means changing the identity of the atom itself. Atomic number is denoted by the symbol $Z$. Mathematically, the atomic number equals the number of protons. In a neutral atom, the number of protons and electrons are equal.

The transcript provides a partial list of the first twelve elements: 1 represents H (Hydrogen) 2 represents He (Helium) 5 represents B (Boron) 11 represents Na (Sodium) 16 represents SS (Sulfur) Additionally, the elements "N" and "Meg" are mentioned in the contextual list.