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Fundamental Structure of the Atom

The term "atom" originates from a Greek word which translates to "cannot be split," reflecting the historical belief that these particles were the smallest possible units of matter. In modern chemistry, we understand that all atoms of a particular element are identical to one another, while different elements are composed of different types of atoms. Atoms are not indivisible but are instead composed of sub-atomic particles. A key stabilizing force within the atom is the electrostatic attraction that exists between the positively charged protons and the negatively charged electrons.

An atom is composed of three primary subatomic particles: protons, neutrons, and electrons. Protons carry a relative charge of $+1$, have a relative mass of $1$, and are represented by the symbol $P$. They are found within the central nucleus of the atom. Neutrons carry no charge (a relative charge of $0$), also possess a relative mass of $1$, and are represented by the symbol $n$. Like protons, neutrons are located in the nucleus. Electrons are negatively charged with a relative charge of $-1$ and are found outside the nucleus. The transcript notes the electron as having a mass value associated with the expression $1545 \times 10$.

Evolution of the Atomic Model

The understanding of atomic structure has evolved significantly through various historical models. In 1903, the British physicist J.J. Thomson proposed the Plum Pudding model, also known as the Chocolate Chip Cookie model. Following his discovery of electrons, Thomson envisioned the atom as a positively charged sphere with negative electrons embedded within it, similar to chocolate chips in a cookie. In this model, there was no distinct nucleus.

By 1909, Ernest Rutherford discovered the proton, and in 1911, he identified the nucleus. Rutherford's model shifted the perspective to a nuclear atom where orbital electrons, which are negatively charged, move around a central nucleus containing positively charged protons. However, Rutherford's initial nuclear model in 1911 was different from the modern atomic model because it did not yet include neutrons within the nucleus.

Rutherford's Gold Foil Experiment

In the Gold Foil Experiment, a radioactive source emitted a beam of $\alpha$ particles toward a thin sheet of gold foil surrounded by a detector. Scientists observed three distinct behaviors: first, most $\alpha$ particles traveled through the foil undetected; second, some $\alpha$ particles were deflected by small angles; and third, occasionally, an $\alpha$ particle traveled back from the foil. These observations led to critical interpretations regarding atomic structure.

The fact that most particles passed through indicated that the atom is mostly empty space. The deflection of some particles suggested that the nucleus is positively charged, just as the $\alpha$ particles are, causing repulsion. Finally, the particles that bounced back demonstrated that the nucleus is a dense center that contains most of the atom's mass. The conclusion was that gold atoms must be mostly empty space with their particles packed into a dense nucleus at the center.

James Chadwick and the Discovery of the Neutron

In 1932, James Chadwick discovered the third type of subatomic particle, which he named the neutron. The discovery of the neutron completed the primary components of the atomic nucleus. Neutrons play a vital role in the stability of the atom because they help to reduce the electrostatic repulsion between the positively charged protons, thereby stabilizing the nucleus. For example, a Carbon atom consists of $6$ protons and $6$ neutrons surrounded by electrons.

Purity in Elements and Mixtures

A pure substance consists of only one type of atom. For instance, pure gold, designated as $24$ carat, is composed entirely of gold atoms ($24/24$ atoms are $Au$). In contrast, $18$ carat gold is a mixture that includes gold atoms along with copper or silver. The purity of $18$ carat gold can be calculated as follows:

$\text{\% of purity} = \frac{18}{24} \times 100 = 75\%$

Similarly, $14$ carat gold contains only $14/24$ gold atoms. Silver is often marked with