The Atom

Objectives

Previous UnitTo know the three elementary particles which

compose atoms.To understand the development of the atomic

model.

Let’s take this one step further…

The Atom Today

Over time, the model of the atom evolved.Two early models we saw were:

Thomson’s Plum Pudding Model Rutherford’s Nuclear Atom

The Atom Today

The model proposed by Niels Bohr was the Planetary Model The central nucleus (like the sun) surrounded by orbiting

electrons (like the planets) Explained that electrons don’t fall into nucleus because they

have fixed energy

The Bohr model was an improvement, but was replaced by the…

Quantum Mechanical Model

Modern model of the atomNo longer are electrons treated like they travel like

“classical” particles (bowling balls)Focus is on electronsElectrons are found not in orbits but in “clouds”

Based on probabilities Area where there is a 90% chance of finding an electron

Electron clouds Are 3-dimensional Come in several shapes Are tied to specific energies Energy levels fill in special order

Organizing Electrons

Electrons in an atom are organized into different energy levels1. Principal energy levels have sublevels2. Sublevels take the form of atomic orbitals3. Orbitals “contain” electrons

Principal Energy Levels (n)

Principal energy levels (n) are numberedMaximum number of electrons in a level is 2n2

n = 1 2 x 12 = 2 electronsn = 2 2 x 22 = 8 electronsn = 3

Each principal level has a number of sublevels equal to the level numbern = 1 1 subleveln = 2 2 sublevels

Sublevels

Each sublevel contains a certain number of atomic orbitalsOrbitals are regions where it is likely an

electron will be foundEach orbital can hold a maximum of 2 electronsLetters are used to denote orbitalsOrbitals have characteristic shapes

Atomic Orbitals

Orbital How many per sublevel?

Shape

s 1 spherical

p 3 peanut

d 5 4 are double-peanut1 is a donut-ringed peanut

f 7 flower

Orbitals are regions in Space

S orbital is a sphere

The energy level of an electron is the region around the nucleus where the electron is likely to be moving.

P orbitals  

                                             

Each P orbital can hold two electrons, but they need to have opposite spins

The P sublevel holds 6 electrons

D level orbitals

                                                 

Each The D sublevel can hold 10 electrons.

Each orbital holds 2 electrons with opposite spins

The F SublevelThe F sublevel has

7 orbitals

Each orbital can hold 2 electrons with opposite spin

The F sublevel holds 14 electrons

Atomic Orbitals

s

p

d

f

Electrons can change orbitals

Electrons can change orbital, by absorbing energy. When an electron absorbs a quantum of energy, it moves up to a higher orbital.

When the electron falls from a high orbital to a lower orbital, energy is released, and we see light.Wintergreen mint is an exampleWe will also see this in our

spectroscopy and flame test labs!

Atomic Orbitals

Electron configurationsA series of numbers and letters to show which orbitals

contain electrons for a given elementBefore we proceed…

Aufbau PrincipleElectrons enter orbitals of the lowest energy first

Pauli Exclusion PrincipleNo two electrons can have the same “state”

• State = electron’s orbital and its spin• Electrons in the same orbital will have opposite spin

Hund’s RuleWhen electrons occupy orbitals of equal energy, one

electron enters each orbital until all orbitals contain one electron with spin in the same direction

Aufbau Diagram

Orbital Diagrams

A box stands for one orbitalAn arrow stands for one electron

Each box (orbital) can only hold 2 arrows (electrons)

or to show direction of spin

*Note: There is some overlap of energy levels, so sublevels do not fill in exact numerical order.

Orbital Diagrams

Orbital Diagram for A Nitrogen Atom

N

1s 2s 2p 3s

Orbital Diagram for A Fluorine Atom

F

1s 2s 2p 3s

Orbital Diagram for A Magnesium Atom

Mg

1s 2s 2p 3s

Learning Check O1

Write the orbital diagram for the electrons in an oxygen atom.

Solution O1

Write the orbital diagram for the electrons in an oxygen atom.

1s 2s 2p 3s

Learning Check O2

Write the orbital diagram for the electrons in an iron atom.

Solution O2

Write the orbital diagram for the electrons in an iron atom.

1s 2s 2p 3s 3p

3d

Electron Configuration

Once electrons have been placed in boxes, the electron configuration is written.Write the energy level and letter for every sublevel

that holds electronsUse a superscript to indicate the number of electrons

in each sublevelExample: for Mg – 1s2 2s2 2p6 3s2

Mg

1s 2s 2p 3s

Exceptional Electron Configurations

Filled energy levels are the most stableHalf filled energy levels are the second

most stableCopper, Chromium and Silver are very

important examples of how “exceptions” to filling the Aufbaum diagram leads to understanding the charge of transition metals

Examples

Write electron configurations for: P, Cr, SnNow write the electron configuration without the

boxes: Mg, NiWhat element has the following electronic

configuration:1s2 2s2 2p6 3s2 3p1

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2

For fun at home:http://www.lon-capa.org/~mmp/period/electron.htm