The position of an atom in the periodic table is primarily determined by its atomic number (Z), rather than its mass number (A). The periodic table arranges elements in order of increasing atomic number, which represents the number of protons in the nucleus of an atom.
The reason for using atomic number as the primary criterion for organizing the periodic table is that it reflects the fundamental properties and characteristics of an element. The number of protons in an atom's nucleus determines its chemical behavior, including its electron configuration, valence electrons, and the arrangement of electrons in its orbitals. These properties, in turn, determine an element's chemical reactivity, bonding tendencies, and its place in the periodic table.
While the mass number (A), which represents the total number of protons and neutrons in the nucleus, does provide information about an atom's mass, it does not correlate directly with an element's properties or its position in the periodic table. Elements with different mass numbers but the same atomic number belong to the same chemical element and exhibit similar chemical behavior. This is because the number of protons, rather than the number of neutrons, is the primary determinant of an element's properties.
In summary, the atomic number (Z) is the key factor in determining an atom's position in the periodic table because it reflects the element's fundamental properties and its behavior in chemical reactions. The mass number (A) is important for determining an atom's mass but does not influence its placement or properties in the periodic table.