1 Electrons in Atoms. 2 Have you even wondered why different atoms absorb and emit light of...

Electrons in Atoms

Have you even wondered why different atoms absorb and emit light of different colors?

The transition of electrons within sublevels releases an amount of energy. If this energy corresponds to the visible section of the spectrum, then we observe colors.

Gamma rays is the most energetic radiation and shortest wavelength. Gamma rays are produced by the sun, by the stars, by some unstable atomic nuclei on earth. Human exposure to gamma rays are dangerous because of the high energy, they can damage biological molecules.

X-rays can also damage biological molecules, but it requires excessive exposure to that radiation.

Ultraviolet light is an important component of sunlight. It is not as energetic as gamma or X-rays, but still carries enough energy to damage biological molecules. Excessive exposure to ultraviolet light increases the risk of skin cancer and cataracts.

Infrared light – the heat that you feel when you place your hand near a hot object is infrared light. All hot objects including the human body emit infrared light. It is invisible to our eyes, but infrared sensors can detect the infrared light and these sensors are often used in night vision to “see” in the dark.

Visible light the only light detected by the human eye.

Microwave light – lower energy but it is absorbed by water and therefore heat substances that contain water. For this reason substances that contain water, such as food, are warmed in a microwave oven, but substances that do not contain water such as a plate are not.

The longest wavelengths, are used to transmit signals responsible for AM and FM radios, cellular phones, television and other forms of communication.

Radiowaves

Parts of a wave

Wavelength

Wavelength = distance between two consecutive crests. Measure in meters, cm, nmRecall 1 m = 1x109 nmFrequency = number of cycles in one secondMeasured in hertz 1 hertz = 1 /second

Frequency = n

• The longer the wavelength (λ), the shorter the frequency (ν).

• The relationship is expressed as:

• c = ν λ

• where c = speed of light (constant) = 3.0x108 m/s

What is the wavelength of light with a frequency 5.89 x 105 Hz?

What is the frequency of blue light with a wavelength of 484 nm?

Understanding the Spectrum

The Hydrogen Line Emission Spectrum

The lowest energy state of an atom is its ground state.

A state in which an atom has the highest potential energy than it has in its ground state is an excited state.

The Hydrogen Line Emission Spectrum

Absorption and Emission Spectra

•http://www.flinnsci.com/atomicspectrum

Energy is Quantized Planck found DE came in “chunks”

with size hn DE = hn and h is Planck’s constant h = 6.626 x 10-34 J s these packets of hn are called

quantum See Planck’s ideas

• A photon is a particle of electromagnetic radiation with zero mass and carrying a quantum of energy.

• The energy of a particular photon depends on the frequency of the radiation:

Ephoton = h ν

Niels Bohr Developed the planetarium model of the atom. He said the atom was like a solar system where

electrons rotate around the nucleus like planets around the sun.

According to the model, the electron can circle the nucleus only in allowed paths or orbits.

The energy of the electron is higher when the electron is in orbits that are successively farther from the nucleus

Bohr’s model

The Bohr Model of the Atom Energy must be added to an atom in

order to move an electron from a lower energy level to a higher energy level. This is absorption.

When an electron fall to a lower energy level, a photon is emitted and the process is called emission.

The Bohr Model of the Atom

Visual concept

When energy is put into an atom, the absorbed energy allows the electron to reach higher energy levels. The electron will be in an excited state.

n=6n=7

rgy Only certain energies

are allowed, so in order for the electron to “jump” to a higher energy level, it will have to absorb the energy equal to the energy gap between the energy levels.

n=6n=7

ΔE=E4-E1

rgy When an excited

electron drops to a lower energy level, the atom emits energy. The emitted energy is what makes the emission spectra.

n=6n=7

rgy The energy released by

the electron is directly proportional to the frequency (ν) and inversely proportional to the wavelength (λ)of the radiation.

n=6n=7

rgy This transition will

show a line in the spectra in a different position (wavelength) as the transition n=7 n=1

n=6n=7

The Quantum Mechanical Model A totally new approach.

Schröedinger’s Equation Developed an equation that treated electrons in atoms as

waves. The wave function is a F(x, y, z) Electrons do not travel around the nucleus in neat orbits,

like Bohr postulated. Instead, the exist in certain regions called orbitals. Orbital is a three dimensional region around the nucleus

that indicated the probable location of an electron.

The Heisenberg Uncertainty Principle

It is impossible to determine simultaneously both the position and the velocity of an electron or any other particle.

Both, the Heisenberg Uncertainty Principle and the Schrödinger wave equation laid the foundation for modern quantum theory.

Heisenberg Principle

Quantum theory

Quantum numbers and orbitals

• The solutions to the Schröndinger wave equation are called quantum numbers and they describe the probability of finding the electron around the nucleus.

S orbital

P orbitals

D orbitals

F orbitals

•Electron spin quantum number (s)

•Can have 2 values.

•either +1/2 or -1/2

Energy level 1

Energy level 2

Energy level 3

Energy level 4 4 sublevels

3 sublevels

2 sublevels

1 sublevel

4f (14 electrons)

3p (6 electrons)

3s (2 electrons)

2p (6 electrons)

2s (2 electrons)

1s (2 electrons)

4s (2 electrons)

4p (6 electrons)

4d (10 electrons)

3d (10 electrons)

2(1)2= 2

2(2)2= 8

2(3)2= 18

2(4)2= 32

Aufbau Principle Aufbau is German for building up.

Electrons enter orbitals from low energy to high energy.

The order of orbitals based on their energies is:

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p …

Pauli Exclusion Principle

No two electrons in the same atom can have the same set of quantum numbers.

Even if two electrons are located in the same energy level and same orbital, they must be different in the spin number.

H with 1 electron

He with 2 electrons

Li with 3 electrons

Be with 4 electrons

B with 5 electrons

C with 6 electrons

N with 7 electrons

O with 8 electrons

F with 9 electrons

Ne with 10 electrons

Na with 11 electrons

Mg with 12 electrons

Ar with 18 electrons

K with 19 electrons

Ca with 20 electrons

Sc with 21 electrons

Mn with 25 electrons

Ga with 31 electrons

Orbital Notation see PPT Valence electrons- the electrons in

the outermost energy levels (not d). Core electrons- the inner electrons. Hund’s Rule- The lowest energy

configuration for an atom is the one have the maximum number of unpaired electrons in the orbital.

C 1s2 2s2 2p2

• Orbital notation or box notation allows to consider Hund’s rule and the spin of the electron.

For example: 8O 1s2 2s2 2p4

• With orbital notation, the orbitals are represented by boxes (or some authors use circles). That way the s orbital will only be represented by 1 box, p orbitals by 3 boxes, and so on.

1s2 2s2 2p4

• Oxygen is paramagnetic with 2 unpaired electrons

25Mn1s2 2s2 2p6 3s2 3p6 4s2

Manganese is paramagnetic with five unpaired electrons.

Paramagnetism is the property of substances of being attracted by an external magnetic field. It is present in all substances with at least 1 unpaired e-.

30Zn1s2 2s2 2p6 3s2 3p6 4s2

Zinc is diamagnetic with no unpaired electrons.

26Fe1s2 2s2 2p6 3s2 3p6 4s2

Iron is paramagnetic with 4 unpaired electrons

Details Elements in the same column have

the same electron configuration. Elements in same groups have similar

properties because of similar electron configuration.

Noble gases have filled energy levels. Transition metals are filling the d

orbitals.

s block

d block

p block

f block

1 Electrons in Atoms. 2 Have you even wondered why different atoms absorb and emit light of...

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