Final answer:
The chemical properties of an atom are primarily determined by its valence electrons, which are indicated by the atomic number and organized in the periodic table to reflect similarities in properties.
Step-by-step explanation:
The chemical properties of an atom are primarily determined by its electrons, specifically the number of valence electrons. These are the electrons in the outermost shell of an atom. The atomic number, denoted by the letter Z, reveals the number of protons in an atom's nucleus, and since atoms are electrically neutral, it also indicates the number of electrons. In a neutral atom, the chemical properties are influenced by the arrangement of these electrons in various energy shells, with the outermost electrons playing the most significant role in chemical reactions and bonding. The periodic table arranges elements in such a way that those with similar valence electron configurations, and hence similar chemical properties, are in the same column.