Year 9 Chemistry

Models of the atom400BC Democritus

Atomic Model

Matter is made up of indivisible particles

Limitation: No scientific evidence to support this theory

Models of the atom1803 DaltonBilliard BallExperimental evidence of conservation of massAll atoms of a given element are identical, but different to the atoms of another element.Compounds form from atoms of more than one element and only whole atoms can combine.In a chemical reaction, atoms can be separated or combined but never destroyedLimitation: No mention of sub-atomic particles

Models of the atom1898 Thompson

Plum pudding

Evidence of two types of sub-atomic particles.Negatively charged electrons (plums) were embedded in a positively charged pudding

Limitation: Arrangement of sub-atomic particles incorrect

Models of the atom1910 RutherfordNuclear

Positive protons concentrated in small nucleus.Mass concentrated in nucleus.Atom consists mainly of empty spaceVolume due to motion of negative electrons.

Limitation: Could not explain how the electrons were arranged or how they moved

Models of the atom1910 RutherfordNuclear

Positive protons concentrated in small nucleus.Mass concentrated in nucleus.

Models of the atom1913 Bohr

Planetary

Electrons found in fixed orbits.Electrons with lower energy found in orbits closer to nucleus.

Limitation: Could not explain small energy differences between electrons in the same energy level

Models of the atom1932 Chadwick Discovered neutrons – important in accounting for mass of nucleus, and unstable atoms 

THE STRUCTURE OF THE ATOMThe atom consists of a central nucleus containing protons and neutrons. Orbiting around the nucleus are electrons.

THE STRUCTURE OF THE ATOMProtonsThese are positively charged particles found in the nucleus

They have a mass of 1,67 x 1027 kg. Since this is such a small number, a new unit of measurement is used; the Atomic Mass Unit (u). One proton has a mass of 1u.

Atoms of the same element will always have the same number of protons and an element can be identified by the number of protons found in its atoms

THE STRUCTURE OF THE ATOMNeutronsThese are neutral particles found in the nucleus of the atom

They have the same mass as the proton ie: one neutron has a mass of 1u

The number of neutrons can vary from one atom to the next within the same element (see Isotopes)

THE STRUCTURE OF THE ATOMElectronsThese are negatively charged particles found orbiting the nucleus

They are extremely tiny, with a mass of 1/1840u

They can be added to or removed from atoms to form ions

They make up the volume of the atom in a neutral atom, there are the same number of electrons as protons

THE STRUCTURE OF THE ATOMAtomic Number (Z)is the number of protons found in the nucleus.  Atomic Mass Number (A)is the total number of (protons + neutrons) found in the nucleus

(Therefore the number of neutrons can be found by subtracting the Atomic Number from the Atomic Mass)

Molecules are particles that can move independently of each other.

Lattices are 3 dimensional arrangements of atoms or ions

Above: NaCl , common salt

Left: SiO2 , sand

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HALF LIFEWhat is Half-life?

1. Half-life is the time taken for half of the radioactive nuclei to decay

2. Half-life is the time taken for the count rate to fall to half its original reading. 

HALF LIFEWhat is Half-life?

An Explanation of Half-life.A radioactive material will have some nuclei that are stable and some that are unstable. The stable nuclei don’t change, that is what stable means. In the picture below, the unstable nuclei (shown as brown balls) will change into stable nuclei (shown as purple balls) and emit radioactivity. 

HALF LIFEWhat is Half-life?

An Explanation of Half-life.Half-life is a measure of the time taken forthe unstable nuclei to change into stable nuclei.

Different substances do this at different rates.

HALF LIFESome do it very quickly and half of the unstable nuclei decayin less than one second. For example, lithium-8 has a half-life of only 0·85 seconds.

Some do it very slowly and half of the unstable nuclei take billions of years to decay.For example, uranium-238 has a half-life of 4·51 billion years.

 

HALF LIFE

 

‘Dating’ MethodsCarbon 14 dating – relies on the half life of C-14, 5730 years.Useful for dating once living relics, less than 50000 years old. Useful for archeologists and anthropologists.C-14 is produced in the atmosphere when cosmic rays strike N-14 atoms.All living things take in C-14 as they live, the level stays relatively constant, When the organism dies, no new C-14 is taken in, and the C-14 remaining in its body decays. It is like a chemical ‘hourglass’.

 

‘Dating’ MethodsCarbon 14 dating – relies on the half life of C-14, 5730 years.Useful for dating once living relics, less than 50000 years old. Useful for archeologists and anthropologists.C-14 is produced in the atmosphere when cosmic rays strike N-14 atoms.All living things take in C-14 as they live, the level stays relatively constant, When the organism dies, no new C-14 is taken in, and the C-14 remaining in its body decays. It is like a chemical ‘hourglass’.

 

‘Dating’ MethodsBelow is a list of signs and symptoms likely to occur when a human is exposed to acute radiation (within one day), in mSv:0 to 250 mSv - no damage250 to 1,000 mSv. Some individuals may lose their appetites, experience nausea, and have some damage to the spleen, bone marrow and lymph nodes.1000 to 3000 mSv - nausea is mild to severe, no appetite, considerably higher susceptibility to infections. Injury to the following will be more severe - spleen, lymph node and bone marrow. The patient will most likely recover, but this is not guaranteed.3,000 to 6,000 mSv - nausea much more severe, loss of appetite, serious risk of infections, diarrhea, skin peels, sterility. If left untreated the person will die. There will also be hemorrhaging.6,000 to 10,000 mSv - Same symptoms as above. Central nervous system becomes severely damaged. The person is not expected to survive.10,000+ mSv - Incapacitation. Death. Those who do survive higher radiation doses have a considerably higher risk of developing some cancers, such as lung cancer, thyroid cancer, breast cancer, leukemia, and cancer of several organs.

 

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Geographic Region

Q1 Q2 Q3 Q4

United States 1254 1254 1254 1254

Europe and Asia 324 324 324 324

Australia 32 32 32 32

South America 2 2 2 2

Canada 1 1 1 1

Mexico 1 1 1 1

TOTALS 1614 1614 1614 1614

Quarter North America

Asia Europe Australia

Q1 2009 123 34 45 10

Q2 2009 134 35 44 12

Q3 2009 150 45 50 14

Q4 2009 201 55 60 18

Q1 2010 175 44 47 13

Q2 2010 180 48 52 14

Q3 2010 204 55 60 16

Q4 2010 250 62 70 20

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Quarter North America

Asia Europe Australia

Q1 2009 123 34 45 10

Q2 2009 134 35 44 12

Q3 2009 150 45 50 14

Q4 2009 201 55 60 18

Q1 2010 175 44 47 13

Q2 2010 180 48 52 10

Q3 2010 204 55 60 4

Q4 2010 250 62 70 1

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