atom

supplemental notes

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Atom Structure

 

Within the atom, there are three major subatomic particles - protons, neutrons and electrons. Protons are positively charged and determine the atomic number. Protons have a mass number of 1. Electrons are negatively charged and have zero mass. Neutrons have no charge, although they have the same mass as a proton(1). Thus, all of the mass of an atom is due to protons and neutrons.

 

	Size	Mass	Charge
Protons	same as neutron	1	+
Electrons	1/1000 of the proton or neutron	0	-
Neutrons	same as proton	1	no charge

 

Some general observations will allow you to use the periodic table to determine the atomic structure and certain properties of each element.

 

1 H 1.008	atomic number
	element
	atomic mass

“Atomic number” is the number of protons

• protons = atomic number

Example: Hydrogen

• Atomic number of 1
• Has 1 proton

“Atomic mass” is the protons plus the neutrons

 

The atomic mass is often written in decimal form (Example: 39.10) because it is an average of the atomic mass of all of the isotopes of that element. But since we only have whole numbers of particles in any given atom, you must round the mass number to the nearest whole number when calculating the number of neutrons.

• protons + neutrons = atomic mass
  (or) 
• atomic mass – protons = neutrons

Example: Hydrogen 

• Atomic Mass of 1.008 (or 1 rounded to a whole number) 
• Has 1 proton
• 1 – 1 = 0 neutrons

Hydrogen is an interesting element because its most common isotopic form with a mass of one does not allow for any neutrons to be present. As a contrast Na (Sodium) with an atomic number of 11 and an atomic mass of 23 has one more neutron than proton.

 

Example: Sodium

• Atomic Mass of 22.99 (or 23 rounded to a whole number)
• Has 11 protons (protons = atomic number)
• 23 - 11 = 12 neutrons

Electron Placement

 

Electrons travel in "orbits" around the nucleus in energy shells. The placement of the electrons is always in an ordered fashion from the innermost shell outward. The following table can be used as a guideline to placement of the electrons in the first four shells which is all we are working with at this level. The only deviation from this table is that from the 2nd shell outward, the outermost shell can only hold 8 electrons.

 

Energy Shell	Electrons held
1st	2
2nd	8
3rd	18
4th	32

 

The following examples demonstrate the correct placement of the subatomic particles that have been discussed above.

 

 

	Hydrogen
	1 H 1.008 atomic number element atomic mass
	Atomic Mass of 1.008 (or 1 rounded to a whole number)  Has 1 proton 1 – 1 = 0 neutrons Has 1 electron (electrons = protons) The element Hydrogen contains one proton in the nucleus symbolized by the red particle and one electron in the first shell symbolized by the purple particle.

 

	Sodium
	11 Na 22.99 atomic number element atomic mass
	Atomic Mass of 22.98977 (or 23 rounded to a whole number)  Has 11 protons 23– 11 = 12 neutrons Has 11 electrons The element Sodium contains eleven protons and twelve neutrons in the nucleus and eleven electrons in the energy shells. The first shell contains the maximum of two. The second contains its maximum of eight. This leaves one electron in the outer (third) shell.

 

Now let's look at one that has a few more electrons and uses the fourth shell. Four shells are the most we will use for this class.

 

Example: Potassium

• Atomic Mass of 39.10 (or 39 rounded to a whole number)
• Has 19 protons (protons = atomic number)
• 39 - 19 = 20 neutrons
• Has 19 electrons (electrons = protons)

	Potassium
	19 K 39.10 atomic number element atomic mass
	Atomic Mass of 39.10 (or 39 rounded to a whole number) Has 19 protons (protons = atomic number) 39 - 19 = 20 neutrons Has 19 electrons (electrons = protons) The element Potassium contains 19 protons and 20 neutrons in the nucleus and 19 electrons in the shells. The first shell contains 2, second shell contains 8, the third shell contains 8 and the last shell contains one. But you might be asking why the third shell doesn't just have nine since that shell can hold up to eighteen. Remember the outer shell can only hold eight electrons, so the ninth one must go to the fourth shell.