The nucleus of an atom is indeed heavier than its outer shell, despite having fewer protons and neutrons. To understand this phenomenon, let's delve into the composition and structure of an atom.
An atom consists of a central nucleus, which contains protons and neutrons, surrounded by orbiting electrons. Protons are positively charged particles, while neutrons have no charge (they are neutral). Electrons, on the other hand, are negatively charged particles.
The mass of a proton is approximately 1 atomic mass unit (amu), and the mass of a neutron is also around 1 amu. In comparison, the mass of an electron is extremely small, approximately 0.0005 amu. Therefore, the mass of the electrons can be neglected when considering the total mass of an atom.
Now, let's consider the number of protons and neutrons in the nucleus compared to the number of electrons in the outer shell. The number of protons in the nucleus determines the atomic number of the atom and defines its chemical properties. The number of neutrons, on the other hand, can vary, resulting in different isotopes of an element.
The electrons, which are negatively charged, occupy specific energy levels or shells surrounding the nucleus. These energy levels can accommodate only a certain number of electrons. The innermost shell can hold a maximum of 2 electrons, the second shell can hold up to 8 electrons, the third shell can hold up to 18 electrons, and so on. However, the outermost shell, known as the valence shell, has a specific capacity for electron occupancy, and it tends to be less than the inner shells.
The reason the nucleus is heavier than the outer shell is primarily due to the masses of protons and neutrons. Although the outer shell can contain more electrons, their individual masses are much smaller compared to protons and neutrons. Protons and neutrons, being roughly 2,000 times heavier than electrons, contribute significantly more to the overall mass of an atom.
Therefore, even though the nucleus contains fewer particles (protons and neutrons) compared to the number of electrons in the outer shell, the substantial mass of protons and neutrons causes the nucleus to be much heavier.