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Group Activity: Atomic Structure
Part I


What is an atom? An atom is the basic building block of all matter. A molecule is a collection of atoms that are bound together. Since everything is made of atoms and molecules, everything in the world has to do with chemistry!

Atoms are made up of smaller particles, sub-atomic particles called protons, neutrons, and electrons. Each particle is defined by two properties: mass and electric charge. Mass just refers to how big or small the particle is. Charge comes in two vartieties: positive and negative. When two positive or negative charges meet, they feel a force pushing them apart. But when a negative and a positive charge are brought close together they feel a force pushing them together.

A proton (p+) is the most important part of an atom. The number of protons in an atom determines what kind of element it is. Protons have a mass of 1 atomic mass unit (amu) and a charge of +1. Protons are located in the atomic nucleus. Because they all have a positive charge there is a force pushing them apart. Thanks to another force, called the strong nuclear force, the protons are held together. They get a little help from neutrons. Elements are put in order of number of protons on the periodic table.

Neutrons (n0) are a necessary part of almost all atoms because they make atoms more stable by forcing the protons to stay together. They do this because they contribute to increasing the influence of the strong nuclear force. They have a mass of 1 amu and a charge of 0. That is, they are neutral. (Can you guess why they are called ‘neutrons’?) Atoms of an element can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.

Electrons (e-) balance out the electric charge of atoms because they have a charge of –1. Their mass is so small that it usually just is not counted. Electrons are located in a sort of cloud that extends away from the nucleus.

The mass of an atom is the sum of the masses of its parts. The charge on an atom is the sum of the charges of its parts.

Subatomic Particles
  Mass Charge Symbol
proton 1 amu +1 p+
neutron 1 amu 0 n0
electron 5 ×10-4 amu –1 e
Atom (5K)
Electric Forces
+ <—> +
+ >–<

  1. Which subatomic particle weighs the least?
  2. Where are protons and neutrons located inside an atom?
  3. What is the name of the force that keeps the atomic nucleus from flying apart?
  4. What two subatomic particles are most important for finding the mass of a whole atom?
  1. What two subatomic particles are most important for finding the electric charge of a whole atom?
  2. An atom has 6 p+ and 7 n0. What is its mass in amu? What element is it?
  3. An atom has 7 p+, 7 n0. What is its mass in amu? What element is it?
  4. An atom has 11 p+, 12 n0. What is its mass in amu? What element is it?

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A graphic showing schematic diagrams of the structure of atoms.

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Atomic Symbol Mo with A and Z positions indicated atomic symbol of molybdenum (Mo) showing the atomic number and mass number of molybdenum-96

Z = 42 & A = 96
Z = the number of protons
Z = the atomic number

A = the atomic mass
A = the atomic mass number
Z + n0 = A or
A - Z = n0
Reading the periodic table:
An example of how to read the periodic table.

OK, now for some problems. Fill in the table showing Z and A in the symbol, like in the example of molybdenum given above. Since these atoms are neutral they have the same number of electrons as protons. So the overall charge for all of them is 0.

For these problems you need to know how to read the names of isotopes.
In aluminum-27 the word gives you the name of the element. The number gives you the mass of the isotope. Find the element on the periodic table. The atomic number (see key above) tells you how many protons that element has. The mass tells you the sum of protons plus neutrons for that isotope.

Name Symbol Z
No. of p+
A - Z
No. of n0
No. of e Overall
nitrogen-15 157N 7 8 15 7 0
oxygen-16           0
oxygen-18           0
magnesium-26           0
magnesium-24           0
phosphorous-31           0
calcium-40           0
calcium-42           0
silicon-28           0
silicon-30           0
neon-22           0
Draw each of the atoms from the table above in a way that clearly shows the number of protons, neutrons, and electrons. Put the electrons in circles around the nucleus:
2 in the first circle
8 in the next circle
18 in the next circle
32 in the next circle (if needed)
For this packet to be complete, print out one copy of the periodic table for each student.
Last updated: Nov 5, 2006        Home