Models of the Atom Foothill Chemistry

Let’s Talk About Models When things are too large, too complex, too expensive, or unable to be studied at actual size. Help us to understand what we want to know.

Rutherford’s Model Rutherford’s Model could not explain the photoelectric effect or chemical properties of atoms

Bohr Model Electrons are not found just anywhere surrounding the nucleus, but in specific circular paths, or orbits around the nucleus.

Bohr Model Each possible electron orbit in Bohr’s model has a fixed energy. The fixed energies an electron can have are called energy levels. A quantum of energy is the amount of energy required to move an electron from one energy level to another energy level. Like the rungs of the strange ladder, the energy levels in an atom are not equally spaced. The higher the energy level occupied by an electron, the less energy it takes to move from that energy level to the next higher energy level.

Electron Cloud Model Erwin Schröedinger used mathematical models to develop it.

Quantum Mechanical Model The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations around the nucleus. Heisenberg Uncertainty Principle – It is impossible to determine simultaneously the position and velocity of an electron or any other particle. The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

Atomic Orbitals An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron. Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron is likely to be found. Orbital shapes have different letters – s, p, d, and f

Atomic Orbitals An atomic orbital is often thought of as a region of space in which there is a high probability of finding an electron. Each energy sublevel corresponds to an orbital of a different shape, which describes where the electron is likely to be found. Energy sublevels are represented by letters - s, p, d, and f Each sublevel has a different shape. The different shapes are represented by corresponding numbers 0-3

s and p Orbitals Different atomic orbitals are denoted by letters. The s orbitals are spherical, and p orbitals are dumbbell-shaped.

d Orbitals Four of the five d orbitals have the same shape but different orientations in space

Energy Sublevel Configurations The number and kind of atomic orbitals depend on the energy sublevel Principal Energy Level Number of Sublevels Type of Sublevel n=1 1 1s (1 orbital) n=2 2 2s (1 orbital), 2p (3 orbitals) n=3 3 3s (1 orbital), 3p (3 orbitals), 3d (5 orbitals) n=4 4 4s (1 orbital), 4p (3 orbitals), 4d (5 orbitals), 4f (7 orbitals)

Electrons and Orbitals Each orbital can only hold a maximum of two electrons Energy Level n Maximum Electrons 1 2 8 3 18 4 32

Quantum Numbers Principal Quantum Number Represented by n Energy level occupied by the electron Number of orbitals in the energy level = n2; number of electrons = 2n2 Angular Momentum Quantum Number Represented by l Shape of the orbital Numbers 0 to 3 corresponding to s, p, d and f respectively. Magnetic Quantum Number Represented by m Orientation of the orbital around the nucleus -l to +l Spin Number Only 2 values +1/2, -1/2

Pauli Exclusion Principle No two electrons can have the same set of four quantum numbers. Therefore, no more than two electrons can occupy an orbital, and these two must have opposite spins

Aufbau Principle An electron occupies the lowest energy orbital that can receive it.

Hund’s Rule Orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in a singly occupied orbital must have the same spin state. The arrangement of electrons with the maximum number of unpaired electrons is the most stable arrangement.

Electron Configuration The arrangement of electrons in an atom Ground-State Electron Configuration – The lowest energy arrangement for each element Hydrogen - 1s1 Helium – 1s2 Noble Gas Configuration Lists the noble gas immediately preceding the element to represent the electron configuration of that noble gas The electron configuration continues with the next orbital following the noble gas configuration Be [He]2s1

Electron Configurations

Orbital Notation Can use boxes or underlines