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All of them tend not to react with each other until you get lower down the group
Atom.
when a substance can not be chemically reduced
No. An element, by definition, does not contain any other element. The only element that contains carbon is carbon; however, carbon does make compounds with other elements - for example, carbon dioxide and carbon monoxide with oxygen.
An element.
On the periodic table, the elements fall into groups (the vertical columns) which are often called families. The reason is that elements in a group are chemically similar to each other.
All of them tend not to react with each other until you get lower down the group
Atom.
when a substance can not be chemically reduced
Paper is a mixture. An element is one single piece of the Periodic Table of Elements such as carbon, or gold. A compound is 2 or more elements chemically bonded. A mixture is 2 or more elements just mixed together, without being bonded chemically, though it may contain other mixtures and compounds in it.
H2 is an element as it does not include any other elements from the Periodic Table.
No. An element, by definition, does not contain any other element. The only element that contains carbon is carbon; however, carbon does make compounds with other elements - for example, carbon dioxide and carbon monoxide with oxygen.
An element.
No. helium is chemically inert and does not combine with other elements.
There are more metals than any other type of element in the periodic table.
The Periodic Table was invented by Dmitry Mendeleyev. He grouped every known element and he used this table to predict the existence of other several elements. Mendelevium name pays homage to Dmitry Mendeleyev.
Dobereiner is remembered for what were called "Dobereiner's Triads" which are groups of three elements having similar chemical properties. This is a forerunner of the periodic table, but it is not actually a table, and it does not offer any explanation for the chemical similarities observed within the triads. Mendeleev's Periodic Table of the Elements shows the relationship of every element to every other element, and explains why they all have the chemical properties that they have.