2.3.1 Creating the Periodic Table Flashcards
Creating the Periodic Table
- Mendeleev organized the known elements into a table using atomic mass and chemical reactivity and predicted the existence of undiscovered elements.
- The discovery of argon posed new issues that instigated further changes in the order of the table.
- Arranging the elements on the periodic table according to their atomic numbers resolved all remaining inconsistencies
Mendeleev
- Mendeleev organized the 63 elements known in his time in the order of their atomic masses and into groups based on their chemical reactivities and physical properties.
- Mendeleev found a regular repeating pattern in the table of elements he developed that contained a few inconsistencies. For example, iodine and tellurium did not group properly given their atomic masses.
- He also noticed gaps in the sequence and used this
information to make predictions of the existence of
undiscovered elements that he called eka-aluminum and eka-silicon. These elements were later discovered and called gallium and germanium
noble gases
- The discovery of argon posed new issues that instigated further changes in the order of the table.
- Ramsay discovered the element argon in 1894. It had an atomic mass that placed it between potassium and calcium on the periodic table. Potassium combines with oxygen in a 2:1 ratio, whereas calcium combines in a 1:1 ratio. It was unclear what an element placed between them should do, moreover argon showed little reactivity with any substances.
- The discovery of neon and krypton in 1898, two non-reactive gases like argon, motivated the creation of a whole new group of elements, called the noble gases, that included argon. Neon and krypton had atomic masses that suggested they belonged to the right of the last column in the periodic table. Argon was placed in this new grouping despite the issue of its atomic mass.
Mendeleev used an analogy to playing cards to help him organize the known elements into a periodic table. The suits and numeric values of the cards were analogous to what aspects of the known elements.
Chemical properties and atomic mass
How many elements were known at the time that Mendeleev began to develop his periodic table?
60 − 70
Which of the following statements is true?
The order of elements in the periodic table is determined by the number of protons an element has
The rows of the periodic table are referred to as periods. Which of the following is true (without exception) regarding elements in the same period on the periodic table?
They have atomic numbers that increase, as you move from left to right in the table
Suppose you are a scientist working on developing the periodic table, and there is a gap in your table. From the information in the illustration, what could you predict about the missing element?
The number of electrons in an atom of the neutral element.
- Since the periodic table is arranged by increasing atomic number, you know that the atomic number of the missing element will be between 50 and 52, or 51. The atomic number is equal to the number of protons in an atom of the element. In a neutral atom, the number of protons equals the number of electrons. Thus, the atomic number also tells you the number of electrons an atom of the element has.
Placing argon into the periodic table was problematic because:
Both because it did not combine with other elements also because it atomic mass did not reflect its actual position
Who was responsible for arranging the elements in a periodic table?
Mendeleev
Lithium and chlorine combine in a 1:1 ratio, and magnesium and chlorine combine in a 1:2 ratio. Suppose you want to chemically combine sodium (Na) and bromine (Br). In what proportions do you predict they will combine?
One Na to one Br.
- Sodium is in the same group as lithium, and bromine is in the same group as chlorine. Therefore these two elements will combine in the same ratio as lithium and chlorine, a 1:1 ratio.
The elements in any _________________ in the periodic table have similar chemical properties.
Group
