13. NUCLEI

In the previous chapter, we learned that in every atom, the positive charge and mass are densely concentrated at the centre of the atom forming its nucleus. The overall dimensions of a nucleus are much smaller than those of an atom. Experiments on scattering of a-particles demonstrated that the radius of a nucleus was smaller than the radius of an atom by a factor of about 10^4. This means the volume of a nucleus is about 10^{-12} times the volume of the atom. In other words, an atom is almost empty. If an atom is enlarged to the size of a classroom, the nucleus would be of the size of a pinhead. Nevertheless, the nucleus contains most (more than 99.9%) of the mass of an atom.

This chunk introduces the concept of the atomic nucleus as the central part of an atom. It explains that the nucleus contains almost all of the atom's mass, despite being significantly smaller in size. To visualize this, if an atom were enlarged to a classroom, the nucleus would only be the size of a pinhead, highlighting how much space is mostly empty in an atom. Understanding this concept sets the foundation for exploring what the nucleus is made of and how its components are held together.

Imagine a large balloon (the atom) filled with air. The air represents the empty space within the atom. Now think of a tiny marble (the nucleus) placed at the center of the balloon—though the marble is small, it holds the majority of the mass of the entire balloon, just as the nucleus does within an atom.

The mass of an atom is very small compared to a kilogram; for example, the mass of a carbon atom, 12C, is 1.992647 × 10–26 kg. Therefore, a different mass unit is used for expressing atomic masses. This unit is the atomic mass unit (u), defined as 1/12th of the mass of the carbon (12C) atom. The atomic masses of various elements expressed in atomic mass unit (u) are close to being integral multiples of the mass of a hydrogen atom. Accurate measurement of atomic masses is carried out with a mass spectrometer.

This chunk explains how atomic masses are measured and the importance of the atomic mass unit (u). The atomic mass unit is based on the mass of a carbon atom, providing a convenient way to express the extremely small masses of atoms in a more manageable form. The idea that atomic masses are close to integral multiples of the hydrogen atom indicates a consistency in atomic mass measurements, making it easier to compare different elements.

Think of measuring very small objects, like grains of salt or tiny beads. Using a regular scale for such small weights can be cumbersome. Instead, we often use specialized tools that measure in milligrams, making it easier to compare weights. Similarly, the atomic mass unit simplifies the comparison of atomic weights, allowing scientists to understand and communicate these very small figures effectively.

The positive charge in the nucleus is that of the protons. A proton carries one unit of fundamental charge and is stable. The other two isotopes of hydrogen are called deuterium and tritium. Tritium nuclei, being unstable, do not occur naturally and are produced artificially in laboratories.

In this section, the focus is on the role of protons and the discovery of neutrons in the nucleus. Protons are identified as positively charged particles that make up part of the atomic structure. The existence of neutrons was established when scientists noticed that certain isotopes had more mass than expected based on the number of protons alone, leading to the conclusion that there must be additional neutral particles (neutrons) present in the nucleus.

Imagine a classroom filled with students (protons) each holding a colored balloon (representing charge). If you suddenly find some balloons that don’t have any color but still add weight to the balloons' total mass, you realize there must be invisible heavier people (neutrons) contributing to the classroom’s overall weight without being counted in the colorful balloon count.