ELECTRONS IN THE ATOM

ELECTRON CONFIGURATIONS

“THE ADDRESSES OF ELEMENTS”

EQ’s

• What do the chemical properties of atoms depend on?

• What is the quantum mechanical model?

• How is the quantum mechanical model organized?

• What is the Aufbau Principle?

ELECTRONS:

The chemical properties of atoms,

ions,

and molecules

are related to the arrangement of electrons.

EVOLUTION OF THE ATOM

To understand this concept, let’s take a look at the history of the atomic models.

Dalton – solid indivisible mass

EVOLUTION OF THE ATOM

Thomson - The atom is a ball of positive charge with electrons stuck into the ball.

Electron

Positive charge

Rutherford – Most of an atom’s mass is concentrated in the small, positively charged nucleus. The electrons surround the nucleus and the rest of the atom is empty space.

EVOLUTION OF THE ATOM

Bohr – Electrons are arranged in concentric circular paths around the nucleus.

EVOLUTION OF THE ATOM

Quantum Mechanical Model – Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space.

EVOLUTION OF THE ATOM

90% probability of finding the electron within this

space

WHERE ARE THE ELECTRONS?The quantum mechanical model is a theoretical mathematical approach to the study of atomic and molecular structure – a very complex theory! So let’s not go there. Instead we will learn some of the basic concepts using a visual that we can all relate to: an apartment building.

Vs.

ELECTRON APARTMENTS

The apartment building has different floors [principal energy level], different apartments on each floor [sublevel], and rooms [orbitals] within each apartment.

PRINCIPAL ENERGY LEVELS

There are seven “floors” in our building. Each of these “floors” is assigned a number. These are called the principal quantum numbers (n). Principal energy levels are assigned values in order of increasing energy: n = 1, 2, 3, 4, and so forth.

PRINCIPAL ENERGY LEVELSTake a look at your periodic table. How many periods are there?

Hummmm – do you think there may be a connection here? Seven levels, seven periods!

So floor 2 (n = 2) would be the second period;

Floor 5 (n = 5) would be the 5th period

SUBLEVELS

The “apartments” (sometimes called blocks or shells) within our Principal Energy Levels are identified with a letter: s, p, d or f.

If you are looking for apartment 6s, it

would be found here

Apartment 4p, would be found

Principal energy level n = 1 is a bit strange, because the sublevel s is

split into two areas.

ORBITALS

Each sublevel (apartment) contains a very specific number of rooms (orbitals):

s – blocks contain 1 orbital

p – blocks contain 3 orbitals

d – blocks contain 5 orbitals

f – blocks contain 7 orbitals

Each orbital can contain a maximum of 2 electrons.

Orbitals are difficult to show until we learn a couple of principles and one rule.A good way to remember the number of orbitals is to count (horizontally) the number of elements in a block and divide that by 2.

s-block; 2 elements = 1 orbital

p-block; 6 elements = 3 orbitals

d-block; 10 elements = 5 orbitals

f-block; 14 elements = 7 orbitals

ELECTRON CONFIGURATION

The electron configuration actually gives us the location of any element on the periodic table. We simply have to be able to count as we fill in boxes!

The way we read the configuration is to account for every electron in the atom – time to remember that as elements progress across the periodic table, the number of protons and electrons increase by one.

A little practice is all it takes.

Hydrogen: s1

Chlorine: 1s22s22p63s23p5

Iron: 1s22s22p63s23p63d6

Helium:1s2

PAULI EXCLUSION PRINCIPLE

An atomic orbital may describe at most two electrons.

To occupy the same orbital, two electrons must have opposite spins.

Arrows are used to indicate the electron and its direction of spin (↑ or ↓).

An orbital containing paired electrons is written as ↑ ↓

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.

p orbitals

When the 4th electron is needed, it will occupy the first orbital and so on - - -

AUFBAU PRINCIPLE

Electrons enter orbitals of lowest energy first7p

6d

5d

4f

4d

3d

1s

2s

2p

3s

3p

4s

4p

5s

5p

6s

6p

7s5f

Incr

easi

ng

Ene

rgy

Hydrogen (H):

Helium (He):

Beryllium (Be):

Aluminum (Al):

Sulfur (S):