Dev't of Atomic Theory

The Atom

Development of Atomic Theory

Philosophical Ideas of Atoms

Democritus called nature’s basic particle an ‘atom’, Greek word that means ‘indivisible’

Aristotle believed in the continuity of matter – there is no limit to subdividing matter.

Both remained speculations as no experimental evidence were presented until 2000 years later, scientists began to gather evidence favoring the atomic theory of matter

John Dalton - Early 1800’s

Conservation of Matter

“Mass is conserved in a

reaction. The amount

you begin with is equal to

the amount you end up

with.

Dalton’s TheoryThe “Pool Ball” (Billiard Ball) Model All matter is made up of atoms. Atoms are tiny, indivisible, indestructible, fundamental particles. Atoms cannot be created or destroyed. Atoms of a particular element are alike. Atoms of different elements are different. A chemical change involves the union or separation

of individual atoms.

Problems with Pool Ball Model Doesn’t explain bonding. Doesn’t explain ions

Michael Farraday demonstrated that some matter can be charged or can carry a charge.

Michael Faraday

When certain substances are dissolved in water, they conduct electricity.

Certain compounds decompose into their elements when current is passed through them. Atoms of some elements are then attracted to the negative electrode, others to the positive electrode.

J. J. Thompson’s “Plum Pudding” Modeln Cathode Ray Tube Experimentsn A charge was applied across a tube filled with various

gases or a vacuum. Tiny charges flowed out of the cathode (-) and traveled towards the anode(+). These particles were negatively charged. Where were they coming from?

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J.J.Thompson’s “Plum Pudding” Model

n Tiny, negatively charged particles which Thompson called “corpuscles” and are now known as electrons were imbedded in a positive mass, like raisins in a plum pudding.

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Rutherford’s Scattering Experiment

n Rutherford shot alpha particles (+ charged, 7000x the mass of an electron) at gold foil.

n Prediction - They would go straight through.n Results - The particles were scattered, sometimes at wide angles.

Rutherford’s Findings Based on his Alpha Scattering Experiment Rutherford found that the atom contains a

tiny nucleus The nucleus has all the positive charge of

the atom The nucleus has most of the mass of the

atom The diameter of the atom is 100,000 times

more than that of the nucleus (the atom is mostly empty space)

Rutherford wrote - “-----Scattered! It was as if

you fired a 15 inch shell at a piece of tissue paper, and the shell bounced back and hit you.”

Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus.

The Neutron

Discovered by James Chadwick in 1932

About the same mass as a proton

No charge (neutral)

Proton Neutron

Electron(-)

+

Inside the Atom

atoms are composed of particles like neutrons, electrons, and protons

The electron always has a "-" or negative charge. The proton always has a "+" or positive charge. If the charge of an entire atom is "0", that means there are equal numbers of positive and negative particles, equal numbers of electrons and protons. The third particle is the neutron. It has a neutral charge (a charge of zero).

Other sub-atomic particles

There are even smaller particles moving around in atoms. These super-small particles can be found inside the protons and neutrons. Scientists have many names for those pieces, but you may have heard of nucleons and quarks.

Selected Properties of the Three Basic Subatomic Particles

Name Charge

Mass(amu)

Mass(g)

Electron (e)

-1 5.4 x 10-4 9.1095 x 10-28

Proton (p) +1 1.00 1.6725 x 10-24

Neutron (n)

0 1.00 1.6750 x 10-24

There are over 100 elements in the periodic table. The thing that makes each of those elements different is the number of electrons, protons, and neutrons. The protons and neutrons are always in the center of the atom. Scientists call the center of the atom the nucleus. The electrons are always found whizzing around the center in areas called orbitals.

It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary.

Determining the Composition of an Atom Calculate the numbers of protons, neutrons, and

electrons in an atom of fluorine. The atomic symbol for the fluorine atom is 19

9F.Solution:Step 1. The mass number tells us that the total

number of protons + neutrons is 19.Step 2. The atomic number, 9, represents the

number of protons.Step 3. The difference, 19 – 9, or 10, is the

number of neutrons.Step 4. The number of electrons must be the

same as the number of protons, hence, 9, for a neutral fluorine atom.

Practice on the board

Calculate the number of protons, neutrons, and electrons in each of the following atoms:

sulphur, sodium, plutonium, potassium

Seatwork Answer the following:

2.31

2.33 a-b, 2.35 a-b, 2.36 a-b

2.41

On page 77

*Describe the properties of protons, electrons, and neutrons.

*Briefly describe Rutherford’s alpha scattering experiment – his findings and how he interpreted them.

Isotopes

Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons.

Hydrogen Deuterium Tritium

Isotopes are often written with the name of the element followed by the mass number. For example, the isotopes 12

6C and 146C may be

written as carbon-12 (or C-12) and carbon-14 (or C-14) respectively.

The existence of isotopes explains why the average masses, measured in atomic mass units (amu), of the various elements are not whole numbers. This is contrary to what we would expect from proton and neutron masses, which are very close to unity.

Uses of Isotopes

Certain isotopes (radioactive isotopes) of elements emit particles and energy that can be used to trace the behavior of biochemical systems.

What are Ions?

Electrically charged particles that result from a gain of one or more electrons by the parent atom (forming negative ions, or anions) or a loss of one or more electrons from the parent atom (forming positive ions, or cations).

Formation of Ions

199F + 1e- 19

9F-

2311Na 23

11Na + 1e-

Determining Atomic Mass

Page 45/Example 2.2 Calculate the atomic mass of naturally

occurring chlorine if 75.77% of chlorine atoms are 35

17Cl (chlorine-35) and 24.23% of chlorine atoms are 37

17Cl (chlorine-37).

Solution

Step 1. Convert each percentage to a decimal fraction.

75.77% = 0.7577 and 24.23% = 0.2423

Step 2. Multiply the decimal fraction by the mass of that isotope

0.7577 x 35 = 26.52 a.m.u

0.2423 x 37 = 8.965 amu

Step 3. Get the sum

26.52 amu + 8.965 amu = 35.49 amu

Board Exercises

2.2 on page 46 2.29 on page 77

Homework

Answer 2.16 page 77 2.26 2.27 2.30

Inadequacies of Rutherford’s Model of the Atom First, he could not explain the composition of

what he called the nucleus. Although he said that most of the atom’s mass and positive charges were to be concentrated into this very small and dense area called the nucleus, he could not explain what was in the nucleus. (The existence of protons and neutrons were not known at the time).

The biggest problem that Rutherford failed to explain was how the negative electrons could stay away from the positive nucleus without collapsing into it.

Although he proposed that the electrons should be placed around the nucleus,he did not know how exactly to arrange the electrons around the nucleus, except‘like planets around the Sun’. We will address this problem when we study Bohr’s atom.

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Dev't of Atomic Theory