Atomic mass can increase or decrease depending on the changes in the number of protons, neutrons, or electrons in an atom. Here's how each of these factors affects the atomic mass:
Protons (atomic number): The atomic number represents the number of protons in the nucleus of an atom. Each element has a unique atomic number, and it determines the element's identity. When the number of protons changes, the atom becomes a different element altogether. For example, hydrogen has an atomic number of 1, while helium has an atomic number of 2. As you move up the periodic table, the atomic number and atomic mass generally increase.
Neutrons: Neutrons are electrically neutral particles found in the nucleus of an atom. The number of neutrons can vary within the same element, leading to different isotopes. Isotopes have the same number of protons but different numbers of neutrons. Since neutrons have a mass of approximately 1 atomic mass unit (AMU), adding or removing neutrons from the nucleus can change the atomic mass. Isotopes of an element have different atomic masses but retain the same atomic number.
Electrons: Electrons are negatively charged particles that orbit the nucleus of an atom. They have a negligible mass compared to protons and neutrons. The number of electrons does not significantly impact the atomic mass because their mass is much smaller compared to the protons and neutrons. In most stable atoms, the number of electrons is equal to the number of protons, maintaining electrical neutrality.
Therefore, changes in the number of protons and neutrons are primarily responsible for variations in atomic mass. By adding or subtracting protons or neutrons, or by having different isotopes, the atomic mass of an element can increase or decrease.