; Formulas_ Rutherford_ and the Wave Mechanical Model
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Formulas_ Rutherford_ and the Wave Mechanical Model


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									Formulas, Rutherford, and the Wave Mechanical Model

But first! A quick review of Dalton’s atomic theory!

• Elements are made of tiny particles called atoms. • All atoms of a given element are identical.
Fe Fe Fe


• The atoms of a given element are different from those of any other element.
Fe Cu S


• Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms.

• Atoms are indivisible in chemical processes. That is, atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.



Formulas of Compounds
Compound - a substance that is composed of the atoms of two or more elements and always contains exactly the same relative masses of these elements.  Chemical Formula - a shorthand method used to show the number and type of atoms present in the smallest representative unit of a substance. Example - The formula for water is H2O. Each molecule of water contains two hydrogen atoms and one oxygen atom.

Rules for Writing Formulas
 


Each atom present is represented by its element symbol. The number of each type of atom is indicated by a subscript written to the right of the element symbol. When only one atom of a given type is present, the subscript 1 is not written.

When you write the formulas, you can imagine what they would look like.  For example: H2O, NH3, CO2, and CH4  Try self check 3.1 on page 55

The Structure of the Atom

Over the last 200 years the model of how we think of atoms has changed significantly four times.

The Structure of the Atom
In 1897 a English physicist J.J. Thompson introduced the first model of the atom to include negatively charged particles called electrons.  note: (Later, a different Thompson named William Thompson reasoned that the atom might be a uniform positively charged "pudding" like form with electrons scattered around like the way raisins were distributed in plum pudding.)

Plum Pudding model of an atom.

Rutherford’s Experiment

In 1909 Ernest Rutherford, at physicist originally from New Zealand, showed the atom had a dense region with a positive charge. The region was called the nucleus.

Gold Foil Experiment
Ernest Rutherford studied, among many other things, alpha (a) particles. Alpha particles are made of two protons and two neutrons. They can be emitted by radio active material and fly through the air.  In Rutherford's experiment he bombarded a thin foil of gold with these α particles. a particles are invisible to the human eye but Rutherford surrounded gold foil with a screen that detects a particles.

Figure 3.5: Rutherford’s experiment.

Gold Foil continued

He found that most α particles passed through the gold foil but some were mysteriously deflected by something in the gold.

Figure 3.6: Results of foil experiment if Plum Pudding model had been correct.

Figure 3.6: Actual results.

Rutherford’s Conclusions
Rutherford concluded there must be a positive nucleus in the center of the gold atoms that deflected the α particles.  Rutherford's experiment showed that atoms had a nucleus. Later Rutherford also theorized that a nucleus is made of two particles:  Proton - a positive particle with an equal but opposite charge of an electron. Also significantly more massive than an electron.  Neutron - a particle with no charge but similar in mass to a proton

The Next Proposed Structure
In 1913 Niels Bohr, a Danish physicist introduced a model of the atom where the nucleus has both protons and neutrons inside.  Electrons in the Bohr model are found in orbits outside the nucleus.

The Wave Mechanical Model

Today the probable locations of electrons around an atom are represented by clouds or orbital maps.

On the left is the probability map showing where hydrogen's e- is most likely to be. The darker it is, the higher the probability. The orbital is defined as the area where the electron will be 90% of the time. Often it is shown simply as a sphere.

Difference from Bohr

Note: An orbit in the Bohr model is a fixed circular path. An orbital is a cloud like map representing the likely hood of the position of an electron.

Wave Mechanical Model




Today scientists believe light is created when electrons drop from one orbital level to a lower orbital level. From an energy point of view, an electron is said to be in different energy levels when it is in different orbitals. The electrons in an atom normally occupy the lowest energy levels available. An electron can be boosted by various means to a higher energy level. As electrons return to lower levels, photons (light waves) are emitted.

Atoms that have electrons in higher than normal orbital levels are said to be in an Excited State.  Atoms that have electrons in the lowest orbital levels are said to be in at Ground State.

The Energy Levels of Hydrogen
If energy is added to a group of Hydrogen atom some atoms will absorb the energy.  Electrons will raise to higher orbitals in those atoms that absorbed energy. Those Hydrogen atoms are said to be in an excited state.  When their electrons fall back down to normal orbital levels energy is released in the form of light waves called photons.

What do you mean light is a wave??

Figure 11.3: The wavelength of a wave.

Figure 11.5: Electromagnetic radiation.

Figure 11.6: Photons of red and blue light.

Figure 11.14: Continuous and discrete energy levels.

Figure 11.15: The difference between continuous and quantized energy levels.

The color of light emitted depends on the energy change of the atom  A larger energy change may correspond to a blue photon.  A smaller energy change may produce a red photon.

Another Difference from Bohr
The Bohr Model of the Atom could not explain the photons emitted by Hydrogen  Four different colors of photons are emitted from excited Hydrogen atoms implying Hydrogen has four different quantized orbital energy levels for it's electrons. The Bohr Model of Hydrogen contained only three possible electron orbits.

The Bohr model and its orbits has been replace with the Wave Mechanical Model and its orbitals.

Quantized orbital energy levels

Violet Blue
Green Red

Photons emitted by excited Hydrogen atoms.

Introduction to the Modern Atomic Structure

symbol charge location mass compared to the mass of one electron

neutrons protons electrons

n0 p+ e-

0 +1 -1

nucleus nucleus orbital cloud

1836 1836 1

A X Z
Atomic Number (Z) - the number of protons  identifies the element (all atoms of the same element have the same number of protons)  found on the periodic table (look in the back cover of your book)  equal to the number of electrons in a neutral atom

A X Z
What accounts for most of the mass of an atom? protons and neutrons  Mass Number (A) - the sum of the p+ and n0 (protons and neutrons)  Are all atoms of the same element identical?  No, isotopes exist.


Atoms of the same element that have different numbers of neutrons. They have identical atomic numbers but different mass numbers.

Hydrogen Isotopes
Hydrogen has three isotopes (protium, deuterium and tritium) protium deuterium tritium

1 H 1

2 H 1

3 H 1

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