Periodic table is the representation of elements in an order
The history of periodic table can be summarized as
a) 1829 : Johann Dobereiner proposed the triad rule. According to him we can classify the elements in a triad where the atomic mass of middle element is average of atomic mass of the near by two atoms.
For example : Li, Na and K is a triad. Where atomic mass of Na is average of atomic mass of K
atomic mass of Na = 7+ 39 / 2 = 23
However, with the further discovery of elements the law was rejected.
b) 1864 : John Newland proposed the law of octave. According to him we can arrange the elements in a set of seven elements where the property of second set of seven elements will resemble the properties of first of seven elements or property will be repeated after seven elements. This was also rejected due to its limited applicability.
c) Meyer : He arranged some 28 elements into six different families based on their atomic masses. The members of each familiy shared some common properties.
d) 1869 : Mendleev : He actually developed a periodic table based on atomic mass of elements. He arranged the elements into groups and periods. He even left space for some undiscovered elements, which were later on discovered.
e) 1916 : Henry Moseley : He finally arranged elements based on their atomic number based on X-ray studies. He proposed the modern periodic law that the periodic properties of elements are due to atomic number of elements.
One of the common indicators that a chemical reaction has occurred is if the react ants reacting produce a precipitate that is insoluble within the solution and forms.
<span>TRUE
The atoms of a chemical element can exist in different types. These are called isotopes. They have the same number of protons (and electrons), but different numbers of neutrons. Different isotopes of the same element have different masses.</span>
Answer:
Reaction happens in both directions..
Explanation:
This is dynamic equilibrium, where the reactants and products are converted into each other at the same rate. If the system is closed and the temperature is held constant, the chemical reaction will achieve dynamic equilibrium.