GSCI 163

Lecture 3

Review:Structure of the atom

• Atoms are made of 3 elementary particles

• The mass of an atom is primarily the sum of the masses of their protons and neutrons

Electron Negative charge

0.000549 (u) Considered a point mass

Proton Positive charge

1.0073 (u) Composed of quarks

Neutron Neutral charge

1.0087 (u) Composed of quarks

Elements

• What gives the chemical properties and characteristic of elements is the number of protons that compose the nucleus of the elements. The number of protons is called the atomic number. (In the periodic table, the elements are arranged by their atomic number)

Examples: – Hydrogen has one proton (atomic number 1) – Helium has 2 protons (atomic number 2)– Carbon has 12 protons (atomic number 12) – Gold has 79 protons (atomic number 79)

Ions…

Whereas the number of protons of an element is always the same, the number of electrons and neutrons may vary.

• Ions: atoms of an element with a surplus (cations) or deficiency (anions) of electrons– Sodium Na+ 11 protons and 10 electrons– Chlorine Cl- 17 protons and 18 electrons

…and Isotopes

• Isotopes: atoms of an element with a surplus or deficit of neutrons– Carbon: 11C, 12C, 13C, 14C with 5, 6, 7 and 8 neutrons respectively.

(12C, 13C are stable, abundance 99%, 1%)

– Hydrogen: 1H (protium), 2H (deuterium), 3H (tritium) with 0, 1 and 2 neutrons.(1H (protium), 2H (deuterium) are stable, abundance 99.99%, 0.01%)

The atomic weight in the periodic table is typically the weighted average of the masses of the elements’ isotopes.

Today’s class

• What is light?

• Where does light come from?

• Emission from atoms.

Ancient concepts of light

• Is light a particle or wave?Evidences that light is a wave:

Diffraction and interference

What kind of wave is light?

When a charged particle accelerates it emits energy in the form of electric and magnetic waves (variations on the E&M field) just like a moving source produces mechanical waves in the surface of water.

Oscillating charge

Characteristics of waves

• Wavelength – distance between crests

• Amplitude – related to intensity

• Period – time for one full oscillation

• Frequency – number of oscillations per second

Relations for light

• Speed of light, c = 300,000 km/s or 3 x 108 m/s

• Frequency and wavelength or

Spectrum: range of frequencies of a wave

c

f fc

The EM spectrum

Peak emission of a hot object

At any given temperature every object emits electromagnetic radiation. As the temperature increases, the range of energy increases and so does the distribution of frequencies (which is somewhat bell-shaped).

• At room temperature the peak is at infra-red frequencies (we can’t see)

• At 700 K the object has dull red glow• 1700 K is the temperature for a candle flame• 3000 K is aprox. the temperature of incandescent light• Sun light is emitted at about 6000 K (temperature of the surface

of the sun)

Incandescent light is inefficient because most of the energy is wasted in the invisible part of the spectrum.

Spectrum

• When light that is produced by a hot object goes through a prism, it is decomposed into many colors (white light is the sum of many frequencies).

Light emission from atomic vapor

• So far we saw how light is emitted from a hot object.

• But light can also be emitted from an incandescent gas. In this case the spectrum is different (see next slide). Why?

Light also behaves as a particle

• Photoelectric phenomena:

Light can be imagined as smalls particles that carry energy called photons.

A bright and intense light has many photons, whereas dim light has only few.

In the photoelectric phenomena, they interact with matter giving off all its energy to electrons, which make the electrical current.

Packets of energy• Max Plank 1900 came up with the idea that vibrating molecules

can only have energy in multiples of energy in certain amounts he called quanta (plural of quantum, or discrete).

• In 1905 Albert Einstein applied Plank’s concept to the photoelectric phenomena with success. (He won the Nobel prize for this work, not for relativity!)

• In summary, he found that the energy of the photon is

E = hf

where h is the value of Plank’s constant (6.63x10-34 Js)and f is the frequency of light.

Problem

• What is the energy of a photon of red light with a frequency of 4.00x1014 Hz?

• What is the energy of a photon of x-rays with a frequency of 3x1018 Hz?

Back to spectral lines

• Every element has its own spectral lines. – It is like its fingerprint. – This how scientists can tell which elements are

present in a given celestial body.• Change in temperature only change the

intensity of the lines of the colors.• The line spectra may extend beyond the

visible range.

The hydrogen spectrum

In 1885, J. J. Balmer noticed that the line spectrum of Hydrogen satisfied the following equation:

Thus, there must be some regularity in the atom. It was also known that H had only one electron.

22

1

2

11

nR

Where, R = 1.097x107 1/m

Bohr atomic model (1913)

• Allowed orbits– Electrons can revolve around an atom only in specific

allowed orbits• Radiation orbits– An electron in an allowed orbit does not emit radiant

energy, as long as it remains in the orbit.• Quantum leaps– Electrons gains or loses energy only by moving from

one allowed orbit to another. Electrons jump up when absorb energy, and jump down by emitting energy.

How much energy?

• In the simpler case of hydrogen, the energy of each of the allowed orbits can be found by:

Where E1 = -13.6 eV

21

n

EEn

Problem

• An electron in a Hydrogen atom jumps from the exited energy level n=4 to n=2. What is the frequency of the emitted photon? To what wave length (color) does this photon correspond?

Back to spectral lines

• Note that the spectrum of the sun there are some black lines. Why?

Absorption spectrum

• The colder gas in the sun absorb photons emitted by the hot gases.

Energy levels

• Electrons do not share the same energy level (orbits). Think as if the electron is a selfish particle…

• But an experiment showed that there is a way to fit 2 electrons in one energy level: if they have different spins, up and down. Like two pair of shoes in a box.

Atom shells

• The “electronic closet” of the atom is divided in:– Energy level (shoe box)– Shells (closets)– Orbitals (shelves within the closet)

• Energy levels are spin up and down• Shells are 1,2,3, etc• Orbitals are lettered “s”, “p”, “d”, “f”, “g”

Next class

Atomic shells and atomic bonding

Assignment:– Read handout day-4, Filling shells.– Quiz on the topic of Atomic structure, energy

levels.– First Teaching project: Kim on the periodic table.