Daily science- pg. 30 Create a table like the following on page 30 in your

notebook. Label this page History of the Atom. Leave a few lines for each scientist.

Scientist/Philosopher

Theory Year Model/Experiment

Democritus

Aristotle

John Dalton

J. J. Thompson

Ernest Rutherford

Niels Bohr

James Chadwick

New Atomic Theory

THE STRUCTURE OF ATOMS

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What makes up an atom?

Atom- smallest unit of an element that still retains that element’s properties

Atoms are made of subatomic particles. The three subatomic particles are the

proton, electron, and neutron.

Structure of an atom

The nucleus is located at the center of the atom.

It contains positively charged particles, called protons, and neutral particles, called neutrons.

Most of the mass is located here.

Structure of an atom

Electrons, or negatively charged particles, exist outside of the nucleus in the electron cloud.

Subatomic Particles chart

Particle Charge Mass Location

Proton + 1 1.67 x 10 -27 In the nucleus

Neutron 0 1.67 x 10 -27 In the nucleus

Electron -1 9.11 X 10 -31 Outside the nucleus

Atomic Number

The periodic table can be used to find the number of protons, electrons, and neutrons.

The atomic number of an element (labeled Z) tells you how many protons there are.

On the periodic table, the elements are listed as neutral elements so the number of electrons=the number of protons.

Atomic number

Z

Mass Number

The mass number (labeled A) is the number of neutrons + protons.

Mass number (if # neutrons is unknown)=atomic mass rounded to the nearest whole number

The atomic mass is located under the element symbol.

You can find the number of neutrons by subtracting the Atomic number (Z) from the Mass number (A)

A-Z = # of neutrons

Mass Number

Writing Atomic number and mass number

There is a way to represent atomic mass and number

Steps: 1-start with the element symbol 2- place the mass number on the top left

side of the symbol 3-place the atomic number on the bottom

left side of the symbol

Symbol

Al27

13

Isotopes

An isotope is an element that has the same number of protons but a different number of neutrons.

Ex. Carbon has 2 common isotopes: Carbon-12 Carbon-14

The numbers represent the mass number

Determining the number of neutrons Ex. Carbon 12 and Carbon 14

COUNTING ATOMS

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Atomic Mass

Atomic mass units (amu) are used to express an atoms mass

Atomic mass units are based off the element Carbon

Found under the element symbol

Average atomic mass

Average atomic mass is a weighted average of the isotopes of a given element.

More common isotopes have a greater effect on the average atomic mass than do less common isotopes.

Ex. Chlorine- 37 and Chlorine- 35 (average atomic mass is 35.45)

Percent Abundance

The atomic mass can be calculated by summing the products of each isotope’s percent abundance and that isotope’s atomic mass.

Mass contribution tells how much of each isotope contributes to the total atomic mass.

Mass contribution = (% abundance) x (atomic mass)

Percent Abundance example Chlorine 35 has an atomic mass of

34.969 amu and its percent abundance 75.770%. What is its mass contribution?

Chlorine 37 has an atomic mass of 36.966 amu and its percent abundance is 24.230%. What is its mass contribution.

Using the two previous answers, what is the average atomic mass of Chlorine?