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1.- describe the trend in ionization energy as the atomic number increases across a period.

2.- describe the trend in ionization energy as the atomic number increases down a group.

1. Across a period , the ionization energy increases  because the formation of a cation becomes more difficult owing to the increase in nuclear charge.

2. Ionization energy decreases down the group because the outer electrons become progressively separated from the nucleus and so are less tightly held .

Explanation:

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.