Atoms

What are the three main building blocks of atoms?

1. Protons
   • Positively-charged subatomic particles found in the nucleus that contribute to the weight of the atom.
   • The number of protons for any given element never changes and gives atoms their ``uniqueness'' - any changes in the number of protons will by default change the atom into a different element.
2. Neutrons
   • Neutrally-charged subatomic particles that also contribute to the weight of the atom, i.e. neutrons have roughly the same mass as protons.
   • The number of neutrons is variable. When atoms of the same element possess different amounts of neutrons, they are called isotopes.
3. Electrons
   • Negatively-charged subatomic particles that move around the nucleus
   • Electrons do not contribute to the mass of an atom (because they have 1,837 times less mass than a proton or a neutron).
   • The number of electrons can change. When atoms of the same element possess different amounts of electrons, they are called ions.

Figure 28.1: Convention for depicting atomic mass $(A)$ and atomic number $(Z)$.

What is the difference between nuclear charge ($Z$) and effective nuclear charge ($Z_{eff}$)?

Nuclear charge, $Z$, is simply the amount of charge the nucleus has (and not to be confused with the atomic number designation mentioned above).

Effective nuclear charge, $Z_{eff}$, is the amount of nuclear charge the electrons actually ``feel'':

That is, if an atom has many shells of electrons, the positive charge exerted by the nucleus on any given electron will be ``blocked'' by the layers of intervening electrons. This makes the ``felt'' positive charge ($Z_{eff}$) significantly less than the positive charge of the actual nucleus ($Z$) as the number of layers between the nucleus and the electron increases.

Conversely, if an atom has no shells between its lone electron and the nucleus (as in the case of hydrogen) then the $Z$ will be very close to the $Z_{eff}$.

Figure 28.2: Comparison of $Z_{eff}$ for a small atom and a large atom.

What is the Bohr model?

The Bohr model is one of the earliest models used to describe how an electron orbits around the nucleus - similar to a planet orbiting the sun.

What are limitations of the Bohr model?

The Bohr model treats electrons like the rings of Saturn - circular and predictable. Further, the Bohr model is a limited model because it does not apply to atoms possessing more than one electron.

Figure 28.3: The Bohr model: This model suggests that electrons move in circular orbits around nucleus and possess a fixed number of orbits. This makes the Bohr model only empiric and therefore it does not accurately describe the structure of atoms.

What field of study opened up the door to understanding how more than one electron interact with each other around the nucleus?

Quantum mechanics.

What ``space'' do electrons occupy according to quantum mechanics?

Orbitals.

Note: Orbitals, as we will see, are areas where an electron can be expected to reside and are the accepted description of electron position around a nucleus. Orbits, in contrast, are elliptical, fixed paths that an electron was purported to use to travel around a nucleus.