About 300 b. c., aristarchus suggested that the sun was at the center of the universe. why?

the brightness of the planets would change.
the planets appeared to move among the constellations.
the planets always moved opposite to the stars.
the planets disappeared periodically.

The correct answer is option A and B

About 300 B.C., Aristarchus suggested that the sun was at the center of the universe because a) the brightness of the planets would change and b) the planets appeared to move among the constellations.

18 centuries before  Copernicus, Aristarchus predicted that  all planets orbit the sun. In other words, he suggested the Solar System is heliocentric.

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.