The Atom, Bohr’s Theory and Elements

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 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.

Bohr’s Theory of the Atom

+

n=1

n=2

n=3

Electron Excitation

Light Emission

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 Theory of the Atom

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 Atomic Theory

Potassium example.

Potassium has an atomic number of 19

Observe there are 19 electrons… ohhhh ahhhhh!

Bohr’s Atom

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 Element

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 Element

This is written in Standard Notation

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 - 12

6 C Carbon 13 – has 6 protons, 7 neutrons and 6

electrons and a mass of 13. Standard Notation - 13

6 C

Defining an Element

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.

Isotopes

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’s

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

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

Ion’s

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.

Periodic Table 7.3

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.

The element on the Periodic Table

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.

Atomic Mass

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.

Calculating the Number of Electrons

Element Atomic Number

Number of

Electrons in the Atom

Ion Charge

Number of

Electrons in the Ion

Aluminum 13 13 3+ 10

Oxygen 8 8 2- 10

Sodium 11 11 +1 10

Phosphorus

15 15 3- 18

Complete the Chart

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

Assignment