All matter is made of atoms. An atom is an extremely small particle consisting of a central nucleus and electrons moving around it.
Basic Knowledge
An atom is approximately 10-10 m (0.1 nm) in size, while the nucleus is even smaller, approximately 10-14〜10-15 m. The nucleus consists of protons (+) and neutrons (charge 0), and electrons (-) move around it. Protons and electrons have equal magnitudes of charge, so the atom as a whole is electrically neutral.
Notation for atoms:
・Atomic number Z = number of protons (= number of electrons in a neutral atom)
・Mass number A = number of protons + number of neutrons
The notation is AZX (example: 126C has 6 protons and 6 neutrons).
Isotopes are atoms with the same number of protons but different numbers of neutrons. Their chemical properties are almost the same.
Atom (the smallest unit making up matter; on the order of 10-10 m)
Nucleus (the center of an atom; consists of protons and neutrons)
Proton, neutron, and electron (charges of +1, 0, and -1, respectively)
Atomic number Z (the number of protons; determines the kind of element)
Mass number A (the number of protons + neutrons)
Isotope (same number of protons, different number of neutrons; 12C and 14C)
Further Study (Background and Significance)
The history of the discovery of the atom is a drama in chemistry. John Dalton (1803) proposed atomic theory, J. J. Thomson (1897) discovered the electron, and Ernest Rutherford (1911) demonstrated the existence of the nucleus through alpha-particle scattering experiments. Later, Niels Bohr (1913) proposed a model of electron orbits based on the spectrum of the hydrogen atom, leading to the modern quantum-mechanical view of the atom.
Isotopes include stable isotopes (which do not decay) and radioactive isotopes (radioisotopes), the latter changing into other elements while emitting radiation. 14C has a half-life of approximately 5730 years and is used for archaeological dating (radiocarbon dating). The standard for atomic mass is defined as 12C=12, and atomic weight is an average value that takes the natural abundance of isotopes into account (the atomic weight of carbon = 12.01).
- Atom = proton (+), neutron (0), and electron (-)
- Nucleus = protons + neutrons; electrons move around it
- Atomic number Z = number of protons (= number of electrons in a neutral atom)
- Mass number A = number of protons + number of neutrons
- Isotopes = atoms differing only in the number of neutrons (12C, 13C, and 14C)
- Radioactive isotopes decay into other elements
- The standard for atomic weight is 12C=12
Points to Note (Easily Confused)
① The atomic number (Z) is the number of protons, while the mass number (A) is the number of protons + neutrons. The number of neutrons is A-Z. ② Isotopes (different numbers of neutrons) and allotropes (different structures of simple substances) are completely different. ③ Distinguish between atomic mass (of an individual atom) and atomic weight (an average based on isotope abundance). ④ The mass of an electron is approximately 1/1840 that of a proton or neutron and can be considered negligible.
Practice
- State the numbers of protons, neutrons, and electrons in 146C.
- Explain the difference between isotopes and allotropes.
- What atom is used as the standard for atomic weight?