Fix the mistake: The Constituent Particles of Matter

Structure of the Atom

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.

Important Terms
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).

Key Points
  • 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

  1. State the numbers of protons, neutrons, and electrons in 146C.
  2. Explain the difference between isotopes and allotropes.
  3. What atom is used as the standard for atomic weight?

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