Most atoms have three different subatomic particles inside them: protons, neutrons, and electrons. The protons and neutrons are packed together into the center of the atom(which is called the nucleus) and the electrons, which are very much smaller, whizz around the outside. Most of an atom is empty space.
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This will be classified as light on the API scale due to the large percentage of lighter fractions such as paraffins and naphthenes.
Answer: The electron arrangement for gallium is:
Ar 3d10 4s2 4p1