Many chemistry instructors teach beginning chemistry students the fundamentals of atomic structure by having them draw atoms based on the Bohr model of the atom. The Bohr model essentially treats atoms as miniature solar systems in which the small electrons orbit a much more massive nucleus, similar to the way planets orbit the sun. The nucleus contains uncharged neutrons and positively charged protons, whereas the orbiting electrons possess negative charges. Most helium atoms contain two protons, two neutrons and two electrons.
Draw a circle about 2 inches in diameter on a piece of paper. The circle represents the nucleus of a helium atom.
Add two “+” symbols inside the circle to represent the two positively charged protons in a helium atom’s nucleus.
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Draw two small zeros inside the circle to represent the two neutrons in the nucleus.
Add a larger circle around the existing circle so that the distance between the circles is about one-half inch. This circle represents the orbit of the electrons. Draw two small letter e’s on the outer circle to represent the helium atoms’ two electrons in orbit around the nucleus. You have now drawn a representative model of a helium atom.
In a neutral atom such as helium, the number of electrons will always equal the number of protons.