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Year 9 Chemistry. Models of the atom. 400BC Democritus Atomic Model Matter is made up of indivisible particles Limitation: No scientific evidence to support this theory. Models of the atom. 1803 Dalton Billiard Ball Experimental evidence of conservation of mass

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Models of the atom
Models of the atom

400BCDemocritus

Atomic Model

Matter is made up of

indivisible particles

Limitation: No scientific evidence to support this theory


Models of the atom1
Models of the atom

1803Dalton

Billiard Ball

Experimental evidence of conservation of mass

All 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 destroyed

Limitation: No mention of sub-atomic particles


Models of the atom2
Models of the atom

1898Thompson

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 atom3
Models of the atom

1910Rutherford

Nuclear

Positive protons concentrated in small

nucleus.

Mass concentrated in nucleus.

Atom consists mainly of empty space

Volume due to motion of negative electrons.

Limitation: Could not explain how the electrons were

arranged or how they moved


Models of the atom4
Models of the atom

1910Rutherford

Nuclear

Positive protons concentrated in small

nucleus.

Mass concentrated in nucleus.


Models of the atom5
Models of the atom

1913Bohr

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 atom6
Models of the atom

1932Chadwick

Discovered neutrons – important in accounting for mass of nucleus, and unstable atoms


The structure of the atom
THE STRUCTURE OF THE ATOM

The atom consists of a central nucleus containing protons and neutrons. Orbiting around the nucleus are electrons.


The structure of the atom1
THE STRUCTURE OF THE ATOM

Protons

These are positively charged particles found in the nucleus

They have a mass of 1,67 x 1027 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 atom2
THE STRUCTURE OF THE ATOM

Neutrons

These 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 atom3
THE STRUCTURE OF THE ATOM

Electrons

These 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 atom4
THE STRUCTURE OF THE ATOM

Atomic 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)



Lattices are 3 dimensional arrangements of atoms or ions

Above: NaCl, common salt

Left: SiO2 , sand


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A Two Column Page Layout

A Second line of text can go here.


A two column page layout1
A Two Column Page Layout

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Half life
HALF LIFE

What 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 life1
HALF LIFE

What 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 life2
HALF LIFE

What is Half-life?

An Explanation of Half-life.

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

Different substances do this at different rates.


Half life3
HALF LIFE

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



Dating methods
‘Dating’ Methods

Carbon 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 methods1
‘Dating’ Methods

Carbon 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 methods2
‘Dating’ Methods

Below 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 damage

250 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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