|Science in Action Notes: Matter & Chemical Change (2.0)||| Print ||
2.0. - An understanding of the nature of matter has developed through observations over time.
2.1. - Evolving Theories of Matter (pgs. 113-120) - Timeline
Today the 'quantum model' describes the atom as a cloud of electrons around a nucleus
2.2. - Organizing the Elements (p. 122-125)
Finding a pattern in an unknown helps scientists to organize ideas and information. It also helps scientists to interpret what the information means and explain these ideas, based on what they have learned.
Looking for Patterns
Early chemists used symbols of the sun and the planets to identify the metallic elements known to them.
This later became a problem, when more elements were discovered, because they ran out of planets. (History of Chemical Symbols). John Dalton developed a new set of symbols in the early 1800's to improve communication between chemists.
Berzelius later revised Dalton's symbols by replacing them with letters, instead of pictures. He represented the elements by their first letter (capitalized), or their first two letters (first one capitalized and the second letter was lower case).
An Order for the Elements
Elements were listed in order of their atomic mass. Atomic mass is the mass of one atom of an element. It is represented in atomic mass units (amu).
John Newland's "law of octaves" identified the pattern in which the properties of the elements seemed to repeat at regular intervals, similar to the octave scale in music. Demitri Mendeleev later revised the pattern in 1869.
Finding a Pattern
Mendeleev collected the 63 elements known at the time and arranged them according to their properties (which he wrote on a file card). He arranged the cards into a 'solitaire-like' table. By sorting and arranging the elements in this way, Mendeleev was able to identify gaps where elements, not yet discovered, would be able to fit.
1.3. - The Periodic Table Today ( Periodic Table Video Notes )
About 112 elements are known today.
They are organized into what is called 'The Periodic Table of Elements'
In 1875 gallium was discovered and proved that Mendeleev's organization of the elements worked, because it fit in where he had placed a (?). The next (?) was not replaced until 1939 when francium was discovered.
Read more about the Periodic Table of Elements
Check and Reflect p. 134