Mendeleev: A Russian scientist credited with creating the foundation of the modern periodic table.
Periodic Law: When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties.
This is often a section that students either find confusing, or they simply memorize the trend but have no idea why it happens. 1) the shielding effect - as more and more energy levels are added around a nucleus, the nucleus is less and less able to 'grab' the outside (valence) electrons tightly.
2) nuclear charge - as more and more protons are added across the periods of the table, the nucleus becomes more and more positively charged.

Down a group: shielding increases, nuclear charge also increases, but the shielding effect is stronger than the nuclear charge effect. Across a period: shielding is held constant (across any period all atoms have the same number of energy levels) but nuclear charge increases.
Tip: To turn text into a link, highlight the text, then click on a page or file from the list above. Scott has always been a a€?hands-ona€? scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. We say that the inner 'rings' or energy levels of electrons shield the outside ring from the pull of the nucleus.

As nuclear charge increases, the atom is able to pull it's electrons more tightly toward the nucleus, creating a smaller atom.
He has never shied away from questioning a€?conventional wisdoma€? and his original approach to problems has often produced significant breakthroughs.

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