Electrons in Atoms

GPS Standards SC3a – Discriminate between the relative size,

charge, and position of protons, neutrons, and electrons in the atom. Identify the inadequacies in the Rutherford atomic

model. Identify the new proposal in the Bohr model of the

atom. Describe the energies and positions of electrons

according to the quantum mechanical model. Describe how the shapes of orbitals related to different

sublevels differ.

Essential Question How are Rutherford’s, Bohr’s, and the

quantum mechanical models related to each other?

Notes Inadequacies in Rutherford’s Model

Could not explain why metals and metal compounds give off characteristic colors when heated in a flame

Could not explain why heated metals glow red, then yellow, then white

Could not explain the chemical properties of elements

Treated the electron as a particle

The Bohr Model Revised Rutherford’s model to include information about

how the energy of an atom changes when it absorbs or emits light

Proposed that an electron is found only in specific circular paths, or orbits, around the nucleus

Each proposed orbit has a fixed energy called an energy level Higher the energy of an electron, the farther it is from the

nucleus Quantum – the amount of energy required to move an electron

from one energy level to another energy level Gave results in agreement with experiments for the

hydrogen atom Failed to explain the energies absorbed and emitted by

atoms with more than one electron Treated the electron as a particle

Quantum Mechanical Model Schrodinger

Devised a mathematical equation describing electron as a wave

Quantum mechanical model modern description of the electrons around an atom based on mathematical solutions to Shrödinger’s

equation Based on the probability of finding an electron within a

particular volume of space around the nucleus By treating the electron as an electron wave instead of a

particle, most of the problems associated with Bohr’s model were alleviated. There are still some problems that we will look at later. The model is still a work in progress.

September 7, 2011 Essential Question

How are quantum numbers used to describe electrons?

Quantum Numbers Each electron around an atom has a set of 4 quantum

numbers which describe the “energy address” of the electron.

Principal quantum number (n) First quantum number Represents the energy level in which the electron is found

(larger value of n = higher energy) Determines the size of an orbital (larger value of n = larger

orbital size) The values of n are successive integers beginning with 1 (n =

1, 2, 3, 4, …., ) Each energy level represents 1 period on the periodic table. Maximum number of orbitals in an energy level = n2

Maximum number of electrons in an energy level = 2n2

Angular momentum quantum number (l) Designates the shape of the orbital in which the

electron is found Indicates the sublevel of the electron Values of l = successive integers from zero to n-1 (l = 0,

1, 2, …., n-1) Each energy level has a number of sublevels equal to

the value of n. Energy level n=1 has 1 sublevel (l=0) Energy level n=2 has 2 sublevels (l=0 and l=1) Energy level n=3 has 3 sublevels (l=0, l=1, and l=2) Energy level n=4 has 4 sublevels (l=0, l=1, l=2, l=3)

Commonly used labels of the sublevels l=0 is the s-sublevel l=1 is the p-sublevel l=2 is the d-sublevel l=3 is the f-sublevel

Magnetic quantum number (ml) Determines the orientation of the orbital within the

sublevel Each energy level has an s-sublevel that contains 1 s-

orbital Beginning with the 2nd energy level, each energy level has

a p-sublevel containing 3 p-orbitals. Beginning with the 3rd energy level, each energy level has

a d-sublevel containing 5 d-orbitals. Beginning with the 4th energy level, each energy level has

an f-sublevel, containing 7 f-orbitals Values of ml are integers from –l to +l

Orbital – a region in the space surrounding the nucleus where the probability of finding an electron is above 90%

Spin quantum number (ms) Each orbital can hold a maximum of 2 electrons. Spin makes the electron act like a tiny magnet Values of ms are +1/2 or -1/2