Subatomic Particles

Objective: Locate particle charges and identify an element.

An atom contains a compact nucleus of positively charged protons and electrically neutral neutrons, surrounded by electrons occupying quantum states rather than little fixed planetary tracks. Atomic number Z is the number of protons and defines the element. In a neutral atom, electron count equals proton count; ions have different electron counts without changing element identity. Most atomic mass is in the nucleus, while the atom’s extent is associated with electron distribution. A simplified shell diagram is a useful introductory model but should not be mistaken for exact electron paths. Charges of proton and electron have equal magnitude and opposite sign.

Objective: Apply subatomic particles to calculations, evidence, and specified situations.

Atomic number Z is the proton count. Mass number A counts protons plus neutrons, so N=A−Z. For an atom or ion, signed charge in elementary-charge units equals proton count minus electron count. Electrons therefore number Z−charge. Ordinary ion formation changes electrons rather than the nucleus. An isotope label supplies a particular integer mass number, whereas a periodic-table atomic mass is usually a weighted average. Always identify which quantity the question gives before subtracting.