A Brief Review of “Matter”

Atomnucleuselectron

e-

(proton,neutrons)

p+n

● 10,000,000 atoms can fit across a period in your textbook.● The nucleus is nearly 100,000 times smaller than the entire atom (if atom filled

the classroom auditorium, the nucleus would be barely visible at its center).● Although it is the smallest part of the atom, most of the atom’s mass is

contained in the nucleus.

Number of protons

determines the element

Number of neutrons

determines the isotope

Incorrect view

better view

Electrons do not “orbit” the nucleus; they are “smeared out” in a cloud which give the atom its size.

The number of protons determines the type of element

Hydrogene-

p+

atomic number (protons) = 1

atomic mass number (protons + neutrons)= 1

Helium

e-

p+

n

e-

np+

atomic number (protons) = 2

atomic mass number (protons + neutrons)= 4

Atomic Number Element1 Hydrogen (H)2 Helium (He)3 Lithium (Li)4 Beryllium (Be)5 Boron (B)6 Carbon (C)7 Nitrogen (N)8 Oxygen (O)

Relative abundances of elements in the universe

Every element has different “isotopes”

same number of protons, different number of neutrons)

Hydrogen

e-

p+

n

(Deuterium)

isotopeof hydrogen

atomic number (protons) = 1

atomic mass number (protons + neutrons)= 2

Hydrogen

e-

p+

n

(Tritium)

isotopeof hydrogen

atomic number (protons) = 1

atomic mass number (protons + neutrons)= 3

n

Every element has multiple isotopes (same number of protons, different numbers of neutrons) some of which may not be stable (“radioactive”)

Carbon-14 half-life = 5,730 yrs

Unstable (“radioactive”) isotopes “decay”, producing a new type of atom, i.e., an atom of a different element, or a different isotope of the original element.

One half of the atoms of an unstable isotope decay in one “half-life” of that isotope.

Three isotopes of Carbon, two stable, one unstable.

5730 yrs

14C 14N + electron + antineutrino + energy

Initial Mass (14C) > Final Mass (14N + electron + antineutrino)

difference in mass is converted into energy: E = mc2

p+

n

e-

np+

What if an electron is missing?

ion

He+1

atomic number (protons) = 2

atomic mass number (protons + neutrons)= 4

What if two or more atoms combine to form a particle?

p+ p+

8p+

8n

molecule H2O (water)

Sharing of electrons (chemistry) is involved in the construction of

molecules

The type of interaction is determined by characteristics of

the “matter” and by the wavelength of light.

Emission, absorption, transmission,

reflection

Light Interacts with Matter in one of four ways:

Light as Information Bearer

The spectrum of an object can reveal the object’s:

Composition

Temperature

Velocity

Spectrum: light separated into its different wavelengths.

Spectroscopy: The quantitative analysis of spectra

Spectra manifest themselves in three ways:

Continuous spectra

Absorption spectra

Emission line spectra

1. A luminous dense object produces a continuous spectrum

2. A low-density, hot gas emits a series of bright emission lines

3. A cool, thin gas absorbs certain wavelengths from a continuous spectrum, producing absorption lines

Kirchhoff's Laws 

In 1859 German physicist Gustav Kirchhoff summarized the observed relationships among these three types of spectra (continuous, emission line, and absorption line)

Continuous spectra are usually related to the temperature of an object that is emitting radiation.

Absorption & emission line spectra are related to the composition of the material absorbing or emitting radiation.

A dense object (solid or gas) emits a continuous spectrum.

THERMAL EMISSION

1. Hotter objects emit more total radiation per unit surface area.

2. Hotter objects have their peak radiation at shorter wavelengths (they will appear “bluer”)

Rules for Thermal Emission

The sun emits its peak radiation in the yellow portion of the visible spectrum

At “room temperature”, or “body-temperature”, an object emits its peak radiation in the infrared.

Absorption & Emission Line spectra

Interactive displays of the spectra of most elements can be seen at

http://jersey.uoregon.edu/vlab/elements/Elements.html

Electron Energy Levels

● Electrons in atoms cannot have just any energy while orbiting the nucleus.

● Only certain energy values are allowed (like the floors of an aprtment building).

● Electrons may only gain or lose certain specific amounts of energy (equal to differences in energy levels).

Electron Orbits / Absorption & Emission● Electrons can gain or lose energy while they orbit the nucleus.

● When electrons have the lowest energy possible, we say the

atom is in the ground state.

● When electrons have more energy than this, we say the atom is

in an excited state.

● When electrons gain enough energy to escape the nucleus, we

say the atom is ionized.

● Since energy must be conserved, transitions between energy

levels involve the absorption or emission of energy

Emission/Absorption Spectra

• Each electron is only allowed to have certain energies in an atom.

• Electrons can absorb light and gain energy or emit light when they lose energy.

• Only photons whose energies (colors) match the “jump” in electron energy levels can be emitted or absorbed.

Hydrogen

Energy levels of

Hydrogen.

1 eV = 1.60 x 10-19 joules

A hot, low density gas emits light of only

certain wavelengths, determined by the

composition of the gas.

Emission line spectrum.

Absorption line spectrum.

When light with a continuous spectrum passes through a cool gas, dark lines appear in the continuous spectrum at wavelengths determined by the composition of the absorbing gas.

Absorption Spectra

• If light shines through a gas, each element will absorb those photons whose energy match their electron energy levels.

● The resulting absorption line spectrum has all colors minus those that were absorbed.

• We can determine which elements are present in an object by identifying emission & absorption lines.

Molecules have rotational & vibrational energy levels

(less energetic than electron energy levels, energies correspond with infrared, microwave, and radio radiations)

The Doppler Shift:

A shift in wavelength due to a wave emitter moving towards (shorter wavelength) or away (longer wavelength) from an observer.

v c

=

The Doppler Effect

1. Light emitted from an object moving towards you will have its wavelength shortened.

2. Light emitted from an object moving away from you will have its wavelength lengthened.

3. Light emitted from an object moving perpendicular to your line-of-sight will not change its wavelength.

BLUESHIFTBLUESHIFT

REDSHIFTREDSHIFT

Measuring Radial Velocity● We can measure the Doppler

shift of emission or absorption lines in the spectrum of an astronomical object.

● We can then calculate the velocity of the object in the direction either towards or away from Earth. (radial velocity)

v c

=

Measuring Rotational Velocity