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Atomic Structure. Mass Number. Actual masses of atoms are frustrating to work with.Therefore we use relative comparisons of the masses of atoms. We use the atomic mass unit (amu) which is one-twelfth the mass of a carbon atom that contains six protons and six neutrons.

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Atomic Structure

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Mass Number

  • Actual masses of atoms are frustrating to work with.Therefore we use relative comparisons of the masses of atoms.

  • We use the atomic mass unit (amu) which is one-twelfth the mass of a carbon atom that contains six protons and six neutrons.

  • Therefore one proton or one neutron weighs one (1) amu.

  • Hydrogen has a mass number 1,Helium has a mass number of 4

  • Page 76, Complete Problem 3.




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Imagine This!

  • If an atom were the size of a football stadium, the nucleus would be the size of a marble!

  • If people were the size of atoms, all the people living on the earth today would fit on the head of a pin!f


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Chemistry in the Environment

  • The processing of uranium ore for nuclear fuel produces large amounts of radioactive mill tailings. Until the late 1970’s disposal of the radioactive tailings was not controlled. As a result , millions of tons of these radioactive tailings are present in unmarked sites across the United States and Canada.


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Consumer Chemistry

  • In the late 1980’s, tests showed traces of dioxins and furans in paper products such as coffee filters, paper towels and disposable diapers. These compounds form when pulp paper is bleached with chlorine. Dioxins and furans are considered potentially dangerous environmental pollutants.


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Consumer Chemistry

  • Dioxins and furans have also been identified as probable carcinogens and poisons which affect the reproductive and immune systems at low doses.

  • What is cheaper? Bleached or unbleached?

  • Why do we bleach paper products?

  • What does this mean to you?


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Atomic Theory

  • John Dalton and His Atomic Theory

  • In 1803, John Dalton (1766–1844) forcefully revived the idea of atoms. Dalton linked the existence of elements, which cannot be decomposed chemically, to the idea of atoms, which are indivisible.


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John Dalton’s Theory

  • Compounds, which can be broken down into two or more new substances, must contain two or more different kinds of atoms. Dalton went further to say that each kind of atom must have its own properties,including mass.This idea allowed his theory to account quantitatively for the masses of different elements that combine chemically to form compounds.


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Electrons

  • Electrons are subatomic particles found rotating around the nucleus of every atom.The number of electrons outside the nucleus is always the same as the number of protons in the nucleus. This makes the atom electrically neutral.


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Arrangement

  • The electrons which move around the nucleus of an atom have different quantities of energy. Those restricted to being closest to the nucleus have the least energy while those with the ability to be furthest away from the nucleus have the most energy.


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Electron Arrangement

  • The electrons can therefore be regarded as occupying different spherical volumes of different sizes, these levels usually being referred to as shells.It is worth noting that within each shell the electrons are not orbiting the nucleus at a fixed distance but can travel anywhere within the spherical shape of that shell.


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Neutrons

  • Neutrons are subatomic particles found in the nucleus of every atom.The number of neutrons in the nucleus does not follow a regular pattern.The number of neutrons can vary even for atoms of the same element.(Isotope)

  • Neutrons have no charge.


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Neutrons

  • Discovery:

  • Neutrons were the last of the three main subatomic particles to be discovered, being first recorded in 1932.


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Dalton’s Theory

  • 1.All matter is composed of tiny indivisible particles called atoms.

  • 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element.


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Dalton’s Theory

  • 3.Atoms of different elements can combine with one another in simple whole number ratios to form compounds.

  • e.g. H2OCO2HCl

  • 4.Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element are not changed into atoms of another by a chemical reaction.


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Protons

  • Protons are stable subatomic particles found in the nucleus of every atom.

  • The number of protons in the nucleus is the proton number of that atom, which tells which element the atom belongs to.

  • The proton number determines the relative position of the element on the periodic table.


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Proton: Discovery

  • Two scientists, W. Wien in 1898, and J.J.Thomson in 1910, concluded that a positive particle existed and had a mass equal to that of a hydrogen atom.

  • In 1919, E.Rutherford demonstrated that particles were emitted when nitrogen was bombarded with alpha particles. In 1920 he proposed that these hydrogen nuclei were fundamental particles and he named them PROTONS.


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Isotopes

  • Remembering back to Dalton's atomic theory:

  • He stated that all atoms of the same element are identical.

  • That is not true.

  • The nuclei of atoms must contain the same number of protons in order to maintain the elements characteristics.


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Isotopes

  • Symbolizing the composition of an isotope:

  • The symbol is written.

  • The mass number is superscripted to the left.

  • The atomic number is subscripted to the left.

  • He

  • H

  • The larger number is always on top.


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Isotopes

  • Atoms that have the same number of protons but different numbers of neutrons are called isotopes. However, the number of neutrons may vary


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Atomic theory Review

  • (1) The idea of atoms was put forward thousands of years ago.

  • (2) Initially, atoms were considered to be small solid spheres.

  • (3) Most discoveries about atoms were made in the twentieth century.

  • (4) Atoms are now known to be made of even smaller particles.


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Atomic Theory Review

  • (1) Atoms are too small to be seen.

  • (2) Models help to visualise the behaviour of atoms.

  • (3) The Rutherford/Bohr model of the atom is useful to explain some atomic behaviour.

  • (4) Modern models are still based on the Rutherford/Bohr ideas but electron behaviour is represented in a different way.


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Atomic Theory Review

  • (1) Atoms are made of many different particles.

  • (2) There are three main particles called protons, neutrons and electrons.

  • (3) Protons and neutrons form the main part of the centre of the atom.

  • (4) The centre of the atom is called the nucleus.

  • (5) Electrons orbit the nucleus.

  • (6) Most of the volume of the atom is empty space.

  • (7) Most of the mass of the atom is in the nucleus.


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Atomic Theory Review

  • There are two types of charge: positive and negative.

  • (3) Protons have a positive charge.

  • (4) Electrons have a negative charge exactly equal in size, but opposite to the charge of a proton.

  • (5) Neutrons have no charge; they are neutral.

  • (6) Atoms are electrically neutral because they have the same number of protons and electrons.

  • (7) The number of protons gives the proton number (atomic number) of the atom.


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Atomic Theory Review

  • (1) Electrons have different amounts of energy.

  • (2) The amount of energy of an electron determines its movement around the nucleus.

  • (3) The different energy levels of the electrons are called shells.

  • (5) There is a maximum number of electrons that fit into each shell.

  • (6) One shell does not have to be full before electrons appear in a higher shell.

  • (7) Atomic mass is determined by the total number of protons and neutrons in the nucleus.


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Atomic Theory Review

  • (1) Atoms bond together in different ways to form a piece of the element.

  • (2) The appearance and properties of the same element can be very different depending upon how the atoms are joined.

  • (3) The different forms of the same element, caused by different atomic arrangements, are called allotropes.


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