Explain step by step: challenges faced when making a periodic

Creating the periodic table, particularly in its early stages, presented several significant challenges:

1. Incomplete Knowledge of Elements: Many elements were yet to be discovered. Early chemists had to work with a limited set of known elements, making it difficult to see the full pattern.
2. Inaccurate Atomic Mass Determinations: The primary method for ordering elements was by atomic mass. However, experimental techniques for determining atomic masses were not always precise, leading to errors and inconsistencies in early arrangements. For example, Mendeleev had to correct the atomic mass of beryllium.
3. Lack of a Fundamental Ordering Principle: While atomic mass was used, the underlying reason for periodicity (atomic number, related to protons and electron configuration) was unknown. This meant that sometimes elements had to be placed out of strict atomic mass order to fit their chemical properties (e.g., tellurium and iodine).
4. Identifying Recurring Chemical Properties: Systematically classifying and comparing the chemical and physical properties of all known elements was a monumental task. Recognizing the recurring patterns (periodicity) that allowed elements to be grouped into families was crucial but complex.
5. Predicting Undiscovered Elements: A major challenge and triumph was the ability to leave gaps for and predict the properties of elements that had not yet been found. This required immense confidence in the periodic law and careful extrapolation of trends.
6. Anomalies and Exceptions: Some elements did not perfectly fit the initial atomic mass ordering based on their chemical properties. These "anomalies" (like argon and potassium, or cobalt and nickel) were difficult to reconcile until the concept of atomic number was introduced.
7. Understanding Valency and Reactivity: Relating the combining power (valency) of elements to their position in the table was a key challenge. Early chemists had to deduce these relationships from experimental observations.
8. Lack of Knowledge of Subatomic Particles: The existence of protons, neutrons, and electrons, and their role in determining an element's identity and properties, was unknown. This made understanding the why behind periodicity impossible for early pioneers.