Physics · Atomic structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
An atom has a tiny, positively charged nucleus made of protons and neutrons, with electrons around it. The atom is mostly empty space, yet almost all of its mass is in the nucleus.
Electrons sit in energy levels. An electron that absorbs electromagnetic radiation moves to a higher level, further from the nucleus. When it drops back, it emits radiation. An atom that loses an outer electron becomes a positive ion.
Isotopes are atoms of the same element with different numbers of neutrons. The model of the atom changed as new evidence arrived: plum pudding, then the nuclear model after the alpha scattering experiment, then Bohr's energy levels and Chadwick's neutron.
Students say isotopes have different numbers of protons or electrons. Only the number of neutrons differs; a different proton number would be a different element.
Answer: 17 protons, 20 neutrons and 17 electrons.
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