Determining the number of electrons, protons, and neutrons in atoms in the past without the knowledge of the atomic number or atomic weight would have been challenging. However, scientists have developed various methods and techniques over the years to make such determinations.
Early discoveries and models: In the late 19th and early 20th centuries, scientists such as J.J. Thomson, Ernest Rutherford, and Niels Bohr made significant contributions to our understanding of atomic structure. Thomson's experiments with cathode rays led to the discovery of electrons, while Rutherford's gold foil experiment provided evidence for a dense, positively charged nucleus. Bohr's model of the atom proposed that electrons orbited the nucleus in discrete energy levels.
X-ray crystallography: In the early 20th century, the technique of X-ray crystallography emerged as a powerful tool for determining atomic structures. By analyzing the diffraction patterns produced by X-rays passing through crystalline materials, scientists could deduce the arrangement of atoms within the crystal lattice. This helped in identifying the periodicity of elements and provided insights into the atomic arrangements of various compounds.
Mass spectrometry: Mass spectrometry is another technique that has been instrumental in determining the composition of atoms. By subjecting a sample to ionization and then separating and detecting the resulting ions based on their mass-to-charge ratios, scientists can obtain information about the isotopic composition and relative abundance of elements present in the sample.
Chemical analysis: Chemical analysis has long been used to determine the composition of substances. By carefully studying the reactions and properties of various substances, scientists can infer the types and quantities of atoms present. For example, stoichiometry—the study of the quantitative relationships between reactants and products in chemical reactions—can provide information about the ratios of atoms involved.
Although the atomic number and atomic weight are now fundamental properties of elements, it's important to note that our understanding of atomic structure has evolved over time. The techniques mentioned above, along with advancements in spectroscopy, quantum mechanics, and other fields, have contributed to our current understanding of atomic composition.