Definition
Proton: positive charge, in the nucleus. Neutron: no charge, in the nucleus. Electron: negative charge, orbits the nucleus.
Almost all of an atom's mass is concentrated in the nucleus because protons and neutrons are roughly times heavier than electrons.
Remember
Electrons stay in orbit around the oppositely charged (attracting) nucleus because their motion provides the centripetal force balancing that attraction — similar to how a planet stays in orbit around the Sun instead of falling in.
Maximum electrons in shell number : .
Example 1 — capacity of the third shell
How many electrons can the third shell hold at most?
- : .
Answer: electrons
Remember
An electron's energy increases in each shell further from the nucleus, because it takes more energy to overcome the nucleus's pull as an electron moves farther away.
Definition
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons — so they differ in mass number but have identical chemical behaviour, since chemistry is governed by the number and arrangement of electrons, not neutrons.
Example 2 — neutrons in an isotope
Find the number of neutrons in O (oxygen has atomic number ).
- Neutrons mass number atomic number .
Answer: neutrons
Definition
A radioactive isotope has an unstable nucleus that spontaneously emits radiation (alpha, beta, or gamma) as it decays toward a more stable form.
Remember
Radiocarbon dating uses the known, steady decay rate of a radioactive isotope of carbon to estimate the age of once-living material — the same principle lets certain radioisotopes date rocks and minerals over far longer timescales, and lets others be used in medicine (radiographic inspection) or industry.
Quick Recap
- ●Proton (+), neutron (neutral): in nucleus, carry almost all the mass. Electron (): orbits, negligible mass
- ●Maximum electrons per shell
- ●Isotopes: same protons, different neutrons — same chemistry, different mass
- ●Neutrons mass number atomic number
- ●Radioactivity: an unstable nucleus emitting radiation as it decays; used in dating, medicine and industry
