Ch. 13 – Electrons in Atoms

13.1 = Models of the atom

Down the wrong path…

•400 BC – Democritus proposed that atoms make up substances

•Aristotle disagreed with him and thought matter was uniform throughout

•This was accepted for the next 2,000 years!

Evolution of Atomic Models

John Dalton (1808)- atom was a solid mass (did not know about subatomic particles)

Billiard ball model

J.J. Thomson (1904)- discovered the existence of electrons as part of an atom (however, he did not identify their location).

Plum pudding model

Ernest Rutherford (1911)- discovered the nucleus. Decided that the space surrounding the nucleus contained electrons.

These models do not provide enough information to explain all of the properties of an element…

Many properties of atoms, ions, and molecules are related to specific arrangements of their electrons

Examples: colors given off when heated in flame, magnetic properties, covalent and ionic bonding, etc.

Niels Bohr (1913) – he proposed that electrons travel in fixed orbits

Planetary model

If an excited electron drops from E3 to E2, what is the wavelength that it gives off? Planck’s constant = 6.63 x 10-34 Js

Erwin Schrodinger (1926)- used mathematical equation to describe probable location and energy of an electron (the quantum mechanical model)

Electron cloud – region outside the nucleus where the electron is most likely to be found

Evolution of Atomic Models cont…

These models provide more information based on an electron’s…

Energy level- region where electron is most likely moving

The higher the energy level the further it is from the nucleusLow energy electrons = near the nucleus!

•Quantum- amount of energy required to move an electron from it’s present energy level

-Energy is lost and gained-Not always the same (depends on

energy level-higher energy level = less distance

between energy levels thus less energy is required to move from one level to the next

-lower energy levels have greater distance between them = more energy to transfer between levels)

The Quantum Mechanical Model

Does not define an exact path of an electron but a probable location.

The region where electrons are likely to be found are called, atomic orbitals.

Atomic Orbitals

The Principal Quantum Number (n)- represents the energy level (n=1,2,3,4,etc)

• The principal energy levels are assigned values in order of increasing energy levels (ladder example)

Within each principal energy level the electrons are found in sublevels (L).

(number of sublevels = principal energy level) Table 13.1

Atomic OrbitalsThe sublevels are represented by the letters s, p, d, and f.

Electrons in each of these sublevels travel in a pattern that have a distinctive shape to that sublevel.• s- spherical shaped cloud• p- dumbbell shaped cloud (contains node)

Node- region close to nucleus where electron is not likely to be found.

• d- cloverleaf shape (contains node)• f- complex and harder to visualize

Letters denote atomic orbitals…

Electron probability clouds

Principal energy level

# of sublevels

Distance of electrons from nucleus increases with n

13.2 - Electron Arrangement in Atoms

Electrons in an Energy level

The maximum # of electrons that can occupy a principle energy levels is represented by the formula, 2n²

-Note: n = principle quantum #

Electrons in a Sublevels: 2 electrons p: 6 electrons

d: 10 electrons f: 14 electrons

Electron Arrangement in Atoms-Electron Configurations-

Electron Configurations- how the electrons are arranged around the nucleus

Three rules tell you how to find the electron configurations of atoms:

• 1. Aufbau Principle• 2. Pauli Exclusion Principle• 3. Hund’s Rule

Aufbau Principle- Electrons enter orbitals of lowest energy first.

s sublevel is the lowest, then p, d, f, etc…

Hund’s Rule- When electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with parallel spins.

Electron Arrangement in Atoms-Pauli Exclusion Principle-

Pauli Exclusion Principle- An atomic orbital may describe at most two electrons

An s or p orbital may contain 1 or 2 electrons.If two electrons are in the same orbital they have opposite spins (clockwise or counter-clockwise)• The position of the electrons represent the

directions of the spin ( )

Periodic Table Arrangement

Writing Electron Configurations

There is a shorthandWrite the energy level and the symbol for every sublevel occupied by an electron. Attach a superscript to indicate the number of electrons in that sublevel.

• Examples:H = 1s¹ (1=energy level, s=symbol,

¹=superscript)He = 1s²