# The Periodic Table

##### 1.13 Describe how Mendeleev arranged the elements, known at that time, in a periodic table by using properties of these elements and their compounds
• Ordered his table in order of atomic mass, but not always strictly – i.e. in some places he changed the order based on atomic weights.
• Left gaps for elements that he thought had not been discovered yet.
##### 1.14 Describe how Mendeleev used his table to predict the existence and properties of some elements not then discovered
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
##### 1.15 Explain that Mendeleev thought he had arranged elements in order of increasing relative atomic mass but this was not always true because of the relative abundance of isotopes of some pairs of elements in the periodic table
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
##### 1.16 Explain the meaning of atomic number of an element in terms of position in the periodic table and number of protons in the nucleus
• Elements are arranged in order of atomic (proton) number (bottom number) and so that elements with similar properties are in columns, known as groups.
• Elements in the same periodic group have the same amount of electrons in their outer shell, which gives them similar chemical properties.
##### 1.18 Identify elements as metals or non-metals according to their position in the periodic table, explaining this division in terms of the atomic structures of the elements
• Metals = elements that react to form positive ions.
• Majority of elements are metals.
• Found to the left and towards the bottom of the periodic table., because they lose electron(s) in order to form these positive ions, forming an electronic structure that is stable, like that of a noble gas
• Non-metals = elements that do not form positive ions.
• Found towards the right and top of the periodic table, because they gain electron(s) in order to form these negative ions, forming an electronic structure that is stable, like that of a noble gas

(see 1.18)