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The Atom, Bohr’s Theory and Elements. Bohr’s Theory of the Atom. When looking at the light of different elements, we notice they are not the same. Thus, each element emits a very specific pattern of wavelength.

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bohr s theory of the atom
Bohr’s Theory of the Atom

When looking at the light of different elements, we notice they are not the same.

Thus, each element emits a very specific pattern of wavelength.

By noticing these patterns we create the emission spectrum – a pattern of wavelengths emitted by different elements.

bohr s theory of the atom1
Bohr’s Theory of the Atom

Rutherford was very close to finalizing what the atom looked like. It was up to one of his students, Niels Bohr that took Rutherford’s model and perfected it.

Bohr examined the way light was emitted from different elements. He noticed that if he excited an electron and made it move a different light was produced.

He then concluded that he was moving electrons from one orbit to another orbit and thus deduced there are multiple orbits that surround the nucleus of an atom.

slide4

Light Emission

+

ElectronExcitation

n=1

n=2

n=3

bohr s theory of the atom2
Bohr’s Theory of the Atom
  • Bohr Theory on Electron Orbits:
    • Electrons are located in defined shells, which are located a certain distance from the nucleus.
    • Electrons cannot exist between the shells.
    • Electrons can gain energy to move up to a higher shell, or they lose energy to move down to a lower shell.
    • Electrons are more stable (have less energy) when they are closer to the nucleus.
bohr s atomic theory
Bohr’s Atomic Theory
  • Bohr’s idea helped us understand the modern periodic table.
    • The first row of the periodic table has 2 electrons in the first orbit.
    • The second and third row of the periodic has 8 electrons in the second and third orbit.
    • The fourth and fifth row of the periodic table has 18 electrons in the fourth and fifth orbit.
    • The sixth and seventh row of the periodic table has 32 electrons in the sixth and seventh orbit.
  • This isn’t quite accurate, but it was what Bohr concluded and he was VERY close to what we really have. (You will learn more about this in grade 10 and 11 Chemistry)
bohr s atom
Bohr’s Atom

Potassium example.

Potassium has an atomic number of 19

Observe there are 19 electrons… ohhhhahhhhh!

defining an element
Defining an Element
  • An element has three important factors
    • Atomic number – which states how many protons are in an element.
    • The Mass number – is the total number of protons and neutrons in an atom.
    • The Chemical symbol – is the symbol we use to denote the chemical –Cl is Chlorine, C is Carbon, H is hydrogen and so forth.
defining an element1
Defining an Element

This is written in Standard Notation

Information about an element includes the mass number, the atomic number and the chemical symbol.

A chemical is written with the chemical name and after it will be the mass number. Example Chlorine - 35

Standard Notation will tell us that Chlorine - 35 has 17 protons, 18 neutrons17 electrons and a mass of 35 g.

defining an element2
Defining an Element

Elements tend to have a few different atomic masses, but have the same amount of protons and electrons.

Example Carbon-12 and Carbon-13.

Hmmm what is going on here?

Carbon 12 – has 6 proton, 6 neutrons and 6 electrons and a mass of 12.00. Standard Notation - 126 C

Carbon 13 – has 6 protons, 7 neutrons and 6 electrons and a mass of 13. Standard Notation - 136 C

isotopes
Isotopes

Even though Carbon-12 and Carbon-13 have different masses because of an extra neutron, THEY HAVE THE SAME CHEMICAL PROPERTIES.

These elements are called Isotopes. A lot of element on the periodic table tend to have quite a few isotopes.

ion s
Ion’s

When we charge and atom we call it an ion.

When there are more electrons than protons it has a negative charge and will have a notation of 1-, 2-, 3- and so forth.

When there are more protons than electrons it has a positive charge will have a notation of 1+, 2+, 3+ and so forth.

If you see just a ‘–’ or a ‘+’ symbol that represents 1- or 1+.

ion s1
Ion’s

Standard Notation for a Chlorine Ion that has a negative one charge

Standard Notation for a Magnesium Ion has a positive 2 charge.

periodic table 7 3
Periodic Table 7.3

The negative ions are on the right side of the zigzag line of the periodic table up to the noble gases.

The positive ions are on the left side of the zigzag line of the periodic table.

the element on the periodic table
The element on the Periodic Table

On the periodic table each element has listed: the atomic number, the density at standard temperature and pressure, the most common ion charge, symbol of the element, name of the element and the atomic mass.

atomic mass
Atomic Mass

The atomic mass is the average of the mass numbers of particular isotopes of an element.

You can determine the mass number of the element by rounding the atomic mass number to the nearest whole number.

Aluminum’s atomic mass is 26.98 (this is the average weight of all its isotopes) and but if we round it to 27 we get its mass.

calculating the number of electrons
Calculating the Number of Electrons

If we look at Aluminum – it has an atomic number of 13. Thus it is supposed to have 13 protons, 13 neutrons and 13 electrons.

But since Aluminum has an ionic charge of 3+ it must have three more protons than electrons.

Since electrons are easier to move than protons, Al3+ will have 13 protons, 13 neutrons and 10 electrons.

assignment
Assignment

Pg 212 – Questions – 3, 5 (a, c, f, h), 6 (b, d, f), 7, 8, 10

Pg 217 – Questions 1 – 5, 7 – 10, 12, 14, 16, 17