Question

1. how is the modern periodic table arranged? 2. when the periodic table was being built, how did we know about elements before their discovery?

Explanation:

1. The modern periodic table arranges elements in order of increasing atomic number. Elements with similar chemical properties are grouped in columns (groups), and elements with the same number of electron - shells are in rows (periods).
2. Scientists used the periodic trends in properties such as atomic mass, chemical reactivity, and physical properties of known elements. By observing these trends, they could predict the existence and properties of undiscovered elements. For example, Mendeleev left gaps in his periodic table for elements he predicted would be discovered later.

Answer:

1. Elements are arranged in order of increasing atomic number, with elements of similar chemical properties in groups and elements with the same number of electron - shells in periods.
2. Scientists used periodic trends of known elements to predict the existence and properties of undiscovered elements.