Electron Configuration of the Elements

Electron Configuration of the Elements

Hydrogen Emission SpectrumWhen hydrogen gas (H2) is placed in a CRT and a high voltage electrical current passed through it, the tube glows a violet colour.

Johann Balmer

When viewed through a spectroscope (prism), we observe four discrete linesand NOT a continuous spectrum:

When an electron in the ground state of H absorbs energy, it gets promoted into a higher energy level.

The electron is unstable in this higher energy level.

When the electron falls back to the ground state, energy is given off.

This explains the bands of light emitted from a hydrogen discharge tube.

Heres another way to look at it:

Hydrogen Emission SpectrumElectrons can only exist in certain energy levels (n)n = 1, n = 2, n = 3, n = 4, etcEnergy levels in atom are quantized.This means that only certain E levels are allowed.

Each E level has one or more sublevels called orbitals

An orbital is a region of space where there is a high probability of finding an electron

Each orbital can hold a maximum of two electrons.

Electrons in an orbital will have opposite spin, designated (clockwise spin) or (counterclockwise spin).

high probability ? Heisenberg Uncertainty Principle

We cannot simultaneously know the position and the momentum of an electron

Back to orbitals . . . For n = 1 there is only one sublevel, called an s orbital.Since this orbital is in the first energy level, it is called a 1s orbital.s orbitals are spherical.

For n = 2 there are two sublevels:2s orbital (one of these)2p orbital (three of these)a p orbital looks like this A set of three p orbitals looks like this

We refer to the individual p orbitals as px, py, pz.

Lets put these orbitals together . . .

For n = 3 (the third energy level) there are three sublevels:3s orbital (one of these)3p orbital (three of these)3d orbital (five of thesesee next slide)

NB. Each orbital holds a maximum of 2 electronsThe d-orbitals

A funky look at d-orbitals

Your bottom line with d-orbitals:There are five of them in each set.eg. there are five 3d orbitals; five 4d orbitals, etc 2 electrons in each, for a maximum of 10 electrons How many columns are in the Transition Metal block (d-block) in the periodic table?10 columns in the transition metals (5x2).For n = 4 (the fourth energy level) there are four sublevels:

4s orbital (one of these)4p orbital (three of these)4d orbital (five of these)4f orbital(seven of thesesee next slide)

f-orbitals

How do electrons fill orbitals?Aufbau Principleaka Building-up PrincipleElectrons occupy orbitals beginning from the lowest energy orbital (i.e. the orbital closest to the nucleus) Start by filling 1s orbitalHow many electrons per orbital?Each orbital can hold a maximum of two electronsof opposite spin, dont forgetHere is the order in which orbitals are filled . . .

Note the peculiarity . . . 3s is followed by3p, which is followed by4s, which is followed by3d.

There are others . . . (help is on the way)

How do the electrons of 7N fill the orbitals?1s22s22p3

Overall for 7N: 1s2 2s2 2p3

Hunds RuleMore stable than . . .

Hunds RuleWhen filling p, d, f orbitals, pair electrons only when necessary

Aufbau Principle Mnemonic Device

Lets write some electron configurations . . . 1H 1s12He 1s23Li 1s2 2s14Be 1s2 2s25B 1s2 2s2 2p16C 1s2 2s2 2p2 10Ne 1s2 2s2 2p6

11Na 11Na 1s2 2s2 2p6 3s112Mg 1s2 2s2 2p6 3s213Al 1s2 2s2 2p6 3s2 3p1 18Ar 1s2 2s2 2p6 3s2 3p619K 1s2 2s2 2p6 3s2 3p64s120Ca 1s2 2s2 2p6 3s2 3p64s221Sc 1s2 2s2 2p6 3s2 3p64s2 3d1In the Periodic Table, what is the connection between the outermost electron configuration and family (column)?Alkali metalsend in s1Alkali earth metalsend in s2Halogens end in p5Noble Gasesend in p6

Groups (families) in PT

Putting it all together . . . To write the electron configuration of any element, use the periodic table (play Battleship) and the Aufbau Principle mnemonic device.

Write the complete electron configuration for

1s2 2s2 2p6 3s2 3p64s2 3d10 4p6 5s2 4d10 5p1

____________________________________[Kr] preceding noble gas only

49In [Kr] 5s2 4d10 5p1

PracticeWrite the electron configuration for each of the following15P15P 1s2 2s2 2p6 3s2 3p333As [use noble gas core abbreviated form]33As [Ar] 4s2 3d10 4p3

more practice . . .13Al13Al 1s2 2s2 2p6 3s2 3p126Fe26Fe 1s2 2s2 2p6 3s2 3p6 4s2 3d644Ru [ ]44Ru [Kr] 5s2 4d652Te [ ]52Te [Kr] 5s2 4d10 5p4

Exceptional Electron Configurations1. Write the expected electron configuration of 24Cr[Ar]4s2 3d4

Actual electron configuration is[Ar]4s1 3d5Special stability associated with half-filled p, d, f orbitals

Now write the electron configuration for 42Mo42Mo [Kr] 5s14d5

Notice any similarity with Cr?

2. Write the expected electron configuration of 29Cu: [Ar]4s2 3d9Actual electron configuration is[Ar]4s1 3d10In this way Cu has completely filled 3rd energy level (Copper is a very stable metal)

Now write the electron configuration for silver (47Ag) and gold (79Au). Use the noble gas core abbreviated forms.

47Ag [Kr] 5s1 4d1079Au [Xe] 6s1 4f14 5d10