Notes on Atomic Structure, Electron Shells, and Orbital Theory
Size and Structure of Atoms
The simplified models (figures two and four) greatly exaggerate the size of the nucleus relative to the whole atom. Example: if a helium atom were the size of a football stadium, the nucleus would be the size of a pencil eraser at center, and the electrons would be like two tiny gnats buzzing around the stadium. Conclusion: Atoms are mostly empty space.
When two atoms approach during a chemical reaction, their nuclei do not come close enough to interact.
Subatomic Particles and Chemical Reactions
Of the three subatomic particles discussed, only electrons are directly involved in chemical reactions.
An atom’s electrons vary in the amount of energy they possess. Energy is defined as the capacity to cause change (e.g., doing work).
Potential energy is the energy matter possesses because of its location or structure. Example: water in a reservoir on a hill has potential energy due to its altitude.
When dam gates open and water flows downhill, the energy can be used to do work (e.g., turn turbines to generate electricity). The water has less energy at the bottom; work is needed to elevate water against gravity. This illustrates why energy changes are directional and involve energy transfer.
Energy, Potential Energy, and Electron Energy Levels
Electrons in an atom have potential energy due to their distance from the nucleus. Negatively charged electrons are attracted to the positively charged nucleus.
Electrons move rapidly around the nucleus at discrete average distances, defined as electron shells. In diagrams, shells are shown as flat circles around the nucleus, but in reality electrons move in three dimensions.
Each shell represents a different amount of potential energy that an electron can have.
Electrons with the lowest potential energy spend time in the first shell (closest to the nucleus). They are attracted to the nucleus but do not fall into it due to their momentum.
The farther an electron shell is from the nucleus, the higher the potential energy of the electrons in that shell.
An electron in a given shell must absorb a discrete amount of energy to move to a shell farther away from the nucleus. Analogy: a ball on a staircase can rest only on discrete steps, not between steps.
Electrons can move from one shell to another only by gaining or losing the discrete amount of energy equal to the difference in potential energy between the old and new shell.
Absorbing energy to move outward: energy might come from a photon (e.g., light) or another energy source delivering the exact amount needed to boost the electron to a higher shell.
An electron boosted to a higher energy level is called an excited electron.
Electrons do not stay in excited states for long; they drop back to lower shells, releasing energy in the process.
When excited electrons drop, they release the same amount of energy they absorbed to move out to the higher shell; i.e., energy released on return matches the absorbed energy (ΔE). This energy can appear as heat or as light.
In photosynthesis, electrons boosted to higher potential energy by sunlight drive the process that powers life on Earth (to be explored in a later chapter).
It takes work to move a given electron farther from the nucleus; thus, more distant electrons have greater potential energy.
Changes in potential energy occur in fixed steps, not continuously, reflecting quantized energy levels.
A ball-on-a-staircase analogy helps visualize that an electron’s energy is quantized and that transitions occur between defined levels.
Quantization and 3D Space of Electrons
An electron’s potential energy is determined by its energy level.
An electron can exist only at certain energy levels, not in between them.
An electron’s energy level is correlated with its average distance from the nucleus.
Electrons are found in different electron shells, each with characteristic average distance and energy level.
Shells can be represented as concentric circles in diagrams, but this is a simplification; orbitals occupy three-dimensional space.
Orbitals: Shapes, Occupancy, and Electron Distribution
The first shell has only one spherical s orbital: 1s. The second shell has one larger spherical s orbital (2s) and three dumbbell-shaped p orbitals (2p). The third shell and higher shells also have s and p orbitals, plus more complex shapes.
An electron distribution diagram (like for neon) shows shells and orbitals; separate orbitals exist within each shell.
The three-dimensional shapes represent electron orbitals; these are volumes of space where electrons are most likely to be found.
Each orbital holds a maximum of two electrons. Thus, the first shell’s 1s orbital holds up to 2 electrons.
Hydrogen’s single electron resides in the 1s orbital; Helium’s two electrons fill the 1s orbital.
The four orbitals in the second shell can hold up to 8 electrons (2 in each orbital): specifically, one 2s orbital and three 2p orbitals.
Neon (Ne) has 10 electrons: distribution includes 1s, 2s, and 2p orbitals, with the second shell's four orbitals accommodating up to 8 electrons in total.
Valence Electrons, Reactivity, and the Periodic Table
The chemical behavior of an atom is determined mostly by the number of electrons in its outermost shell (the valence shell). These outer electrons are called valence electrons.
In lithium (Li), there are two electrons in the first shell and one electron in the second shell; therefore, the second shell contains the valence electron.
Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior (e.g., fluorine and chlorine both have 7 valence electrons and tend to form compounds with sodium; NaCl is table salt; sodium fluoride is used in toothpaste to prevent tooth decay).
An atom with a completed valence shell is unreactive (inert).
The far right of the periodic table includes helium, neon, and argon, which have full valence shells and are chemically inert.
All other atoms have incomplete valence shells and are chemically reactive.
Periodic Table Structure and Electron Distribution
A modified version of the periodic table shows electron distribution for the first 18 elements, from hydrogen to argon.
Elements are arranged in three rows (periods), corresponding to the number of electrons and shells in their atoms.
The left-to-right sequence within each period corresponds to the sequential addition of electrons and protons.
In an uncharged atom, the number of electrons equals the number of protons.
Electron shells are depicted as concentric circles, with the innermost shell closest to the nucleus having the lowest energy.
Orbital Diagrams: Visualization and Reality
The concentric-circle representations are simplifications; electrons’ locations cannot be known precisely due to quantum behavior.
Orbitals describe the three-dimensional space where electrons spend most of their time; the phrase that electrons occupy 90% of the time refers to orbital probability regions.
Neon’s orbitals are shown as a combination of the 1s, 2s, and 2p orbitals to illustrate how electrons are distributed among shells and orbitals.
Claims about electrons: they are constantly moving in three dimensions, unlike the static ball-on-a-staircase analogy.
Reactivity and the Role of Unpaired Electrons
The reactivity of an atom arises from the presence of unpaired electrons in one or more orbitals of the atom’s valence shell.
When atoms interact, it is the unpaired electrons that participate to complete valence shells.
Key Takeaways and Connections to Broader Concepts
Atoms are mostly empty space; the nucleus is tiny relative to the whole atom.
Only electrons are directly involved in chemical reactions; energy levels are quantized into shells and orbitals.
Energy changes occur in discrete steps; electrons absorb energy to move to higher shells and emit energy when returning to lower shells.
The valence shell dictates chemical behavior; elements with full valence shells are inert, while those with incomplete shells are reactive.
The arrangement of electrons in shells and orbitals underpins the periodic table’s structure and the chemical properties of elements, with real-world relevance to biology (e.g., photosynthesis), medicine, and materials science.
ext{For transitions between shells: } \Delta E{abs} = Ej - Ei \quad (j>i) ext{Electron emission on return: } \Delta E{emit} = Ej - Ei \quad (j>i)