ATOMS

HISTORY, DALTON’S THEORY, SUBATOMIC PARTICLES, ISOTOPES AND AVERAGE ATOMIC MASS

ATOMS

4oo B.C. Democritus- called nature’s basic principal an “atom” which translates as “indivisible”

Elements- up until the 1700’s though they could not be broken down any further.

1808: English schoolteacher John Dalton reasoned that elements were composed of atoms and that only whole numbers of atoms can combine to form compounds

Ex: H2O as opposed to HO1/2

NEW SYSTEM OF CHEMICAL PHILOSOPHY SYMBOLS BY DALTON

In this work, Dalton revived the atomic theory of matter, adding to it the key idea that different elements had different characteristic atomic weights.

Also present in this work is one of the earliest depictions of molecular structure, in which the atoms in a molecule are arranged in space in a specific fashion.

Dalton's chemical atomic theory was the first to give significance to the relative weights of the ultimate particles of all known compounds, and to provide a quantitative explanation of the phenomena of chemical reaction.

Dalton believed that all matter was composed of indestructible and indivisible atoms of various weights, each weight corresponding to one of the chemical elements, and that these atoms remained unchanged during chemical processes.

Dalton's work with relative atomic weights prompted him to construct the first periodic table of elements (in Vol. 1, pt. 1), to formulate laws concerning their combination and to provide schematic representations of various possible combinations of atoms.

Of the eight copperplates of this work, plate 4 is especially significant since it gives Dalton's symbols for the atoms of various elements and their compounds.

On account of these symbols and the theory which they accompany, Dalton is known as the "father of modern chemistry."

1) All matter is composed of atoms

DALTON’S PRINCIPLES OF ATOMIC THEORY:

1) All matter is composed of atoms

2) Atoms if a certain element are identical in size, mass and other properties and atoms of different elements differ in size, mass and other properties.

DALTON’S PRINCIPLES OF ATOMIC THEORY:

1) All matter is composed of atoms

2) Atoms if a certain element are identical in size, mass and other properties and atoms of different elements differ in size, mass and other properties.

3) Atoms cannot be subdivided, created, nor destroyed.

DALTON’S PRINCIPLES OF ATOMIC THEORY:

1) All matter is composed of atoms

2) Atoms if a certain element are identical in size, mass and other properties and atoms of different elements differ in size, mass and other properties.

3) Atoms cannot be subdivided, created, nor destroyed.

4) Atoms combine in simple whole number ratios to form chemical compounds

DALTON’S PRINCIPLES OF ATOMIC THEORY:

1) All matter is composed of atoms

2) Atoms if a certain element are identical in size, mass and other properties and atoms of different elements differ in size, mass and other properties.

3) Atoms cannot be subdivided, created, nor destroyed.

4) Atoms combine in simple whole number ratios to form chemical compounds

5) In chemical reactions, atoms are combined, separated or rearranged.

DALTON’S PRINCIPLES OF ATOMIC THEORY:

1) All matter is composed of atoms

2) Atoms if a certain element are identical in size, mass and other properties and atoms of different elements differ in size, mass and other properties.

3) Atoms cannot be subdivided, created, nor destroyed.

4) Atoms combine in simple whole number ratios to form chemical compounds

5) In chemical reactions, atoms are combined, separated or rearranged.

DALTON’S PRINCIPLES OF ATOMIC THEORY:

*** #3- atoms are divisible; it is the ONLY principal that has been proven wrong since 1808!!!!!

EOC- RELEASED TEST QUESTION: Based on his observations, the English chemist John Dalton formulated an atomic theory: Dalton’s Atomic Theory

1. All elements are made up of tiny indivisible particles called atoms.

2. Atoms of the same element are identical. The atoms of one element are different from the atoms of another element.

3. Atoms of different elements chemically combine to form chemical compounds.

4. During chemical reactions, atoms are rearranged. Atoms of one element cannot be changed into atoms of a different element as a result of a chemical change.

In 1897, J. J. Thomson showed that negative charges could be made to move from one end of a cathode ray tube to another, causing the tube to glow. Because of this, Thomson is credited with the discovery of the electron. Based on this information, which part of Dalton’s atomic theory conflicted with Thomson’s new data? _________________

ATOMS:Smallest particle of an element that retains the chemical properties of that element

2 regions:

Nucleus: center, small dense, + protons, ₒ nucleus

Surrounding Nucleus: - electron cloud

Subatomic particles: proton, neutron and electron

1800’SMany experiments passing electrical current through various gases at low pressure in cathode ray tubes.

Scientist noticed that when current was passed through the Cathode Ray Tube, the surface directly opposite the cathode glowed.

The glow must be a stream of particles called a Cathode Ray

The particles moved AWAY from the negative side

Therefore the Cathode Ray must be made up of negative particles

1897:

Joseph John Thomson named these particles: electrons

1909:

Robert Millikan determined the mass of an electron to be 1/2000 the mass of H, the smallest known nonmetal atom= 9.109 x 10-31 kg

These 2 men’s experiments lead to the following two conclusions:

1) Because atoms are electrically neutral, they must contain the same number or positive protons and negative electrons.

2) Because electrons do not weigh much the mass must be located somewhere else

1911:Rutherford discovered the proton

He bombarded a thin piece of gold foil with fast moving alpha particles (positively charged particles with 4x the mass of H)

Most went straight through; implying the surrounding was negative if the positive alpha particles are passing straight through

However, in the middle, they deflected implying that there was a middle, it was very dense and it was positive

The mass of the proton is 1.673 x 10-27 kg (1836x heavier than the electron)

1932: CHADWICKFigured the math did not add up:

the number of electrons and the number of protons did NOT equal the weight of the atom

Found the neutron- NO charge and the same weight as the proton

ATOMIC NUMBER (Z): # PROTONS= # ELECTRONS

MASS NUMBER (A): # PROTONS + # NEUTRONS

To find the neutrons: subtract the mass from the atomic number

Element Atomic Number Mass Number Protons Electrons Neutrons

 C

         

O 

         

Na 

         

 Ne

         

ISOTOPES:

Atoms of the SAME element occur that have different masses because they have a different number of neutrons

Most elements occur naturally as a mix of isotopes

Explains why the atomic mass is NOT a whole number

235 92 U

How many protons, electrons and neutrons are in the above example of Uranium?

How many protons, electrons and neutrons are there in chlorine-37?

How many protons, electrons and neutrons are there in Bromine-80?

Write the nuclear symbol for carbon-13

AVERAGE ATOMIC MASS (AAM)

***Progress Report Activity Prior to these notes

Weighted average of the atomic masses of naturally occurring isotopes of an element

Suppose you have a box with 2 types of marbles: 25% of the marbles weigh 2.00 g each and 75% weigh 3.00 g each. How is the weighted average calculated?

Suppose you have a box with 2 types of marbles: 25% of the marbles weigh 2.00 g each and 75% weigh 3.00 g each. How is the weighted average calculated?

1st step: turn the % into decimals (either divide by 100 or move the decimal 2 places to the left)

25%= 0.25 and 75%=0.75

Suppose you have a box with 2 types of marbles: 25% of the marbles weigh 2.00 g each and 75% weigh 3.00 g each. How is the weighted average calculated?

1st step: turn the % into decimals (either divide by 100 or move the decimal 2 places to the left)

25%= 0.25 and 75%=0.75

2nd Step: Multiply the decimal by its mass

(0.25 x 2.00 g)=0.50 g

(0.75 x 3.00 g)=2.25 g

Suppose you have a box with 2 types of marbles: 25% of the marbles weigh 2.00 g each and 75% weigh 3.00 g each. How is the weighted average calculated?

1st step: turn the % into decimals (either divide by 100 or move the decimal 2 places to the left)

25%= 0.25 and 75%=0.75

2nd Step: Multiply the decimal by its mass

(0.25 x 2.00 g)=0.50 g

(0.75 x 3.00 g)=2.25 g

3rd step: Add up the answers you got

0.50 g + 2.25 g = 2.75g

Example: Naturally occurring copper consists of 69.17% copper-63, which has an atomic mass of 62.929 g/mole and 30.83% copper-65, which has an atomic mass of 64.927 g/mole

 

Example: Chlorine has an atomic mass of 34.969 and a percent abundance of 75.77 as well as an atomic mass of 36.966 and a percent abundance of 24.23

Example: Silicon is found in nature combined with Oxygen to give sand, quartz, agate and similar materials. The element has 3 stable isotopes. Calculate the average atomic mass of Si

Exact mass Percent Abundance

27.977 92.23

28.977 4.67

29.974 3.10

Example: 3 isotopes of Argon occur in nature: 36 18 Ar 0.337%,

38 18 Ar 0.063%, and 40 18 Ar 99.600%. Calculate the average atomic mass of argon to the thousandths place, given the relative atomic masses and abundances of each.

Which is more abundant: Carbon 12, Carbon-13 or Carbon-14?

Which is more abundant Hydrogen-1 or Hydrogen-2?

Which is more abundant: Oxygen-16, Oxygen-17 or Oxygen-18?