The Nature Index 2025 Research Leaders — previously known as Annual Tables — reveal the leading institutions and countries/territories in the natural and health sciences, according to their output in ...
Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest ...
Elements in the same horizontal row are in the same period. The periods are numbered from top to bottom. The period number is the same as the number of occupied electron shells.
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