Atomic Mass Model

& Background In this activity, pennies will be used to model protons and neutrons in an atom. A heads-up penny will represent a proton, and a tails-up penny will represent a neutron. All protons are identical to each other, and all neutrons are identical to other neutrons. Pennies minted after 1982 are all made the same way from the same materials, like protons and neutrons have almost exactly the same mass from one to the other. Neutrons are very slightly heavier than protons, but this difference is difficult to notice without the most advanced instruments. For all practical purposes, one proton and one neutron have the same mass.

 Materials ƒ60 Pennies ƒDigital balance

Procedure

Find carbon on the Periodic Table of the Elements. Carbon’s atomic number is _____________. The atomic number is the number of protons in an element, and identifies the element. The number of protons determines the element as listed on the Periodic Table of the Elements. How many protons does every carbon atom have? _____________ The mass listed for carbon is ___________ amu. When rounded to the nearest whole number, carbon’s mass number is ____________.

The mass of an atom is the sum of the number of protons and number of neutrons. If every carbon atom has ________ protons, and the mass of carbon on the periodic table is _________, the number of neutrons in most carbon atoms is _________. Using pennies to represent protons (heads) and neutrons (tails), create a carbon nucleus. With all the protons and neutrons for the carbon atom lying flat on the table, move them together in a tight circle. Draw a diagram of the carbon nucleus in the box. Label the protons and neutrons with P’s and N’s instead of heads and tails. Carbon Atom Nucleus

How many total protons and neutrons are in the carbon nucleus you made? __________ This isotope of carbon is called carbon-12 because it has 12 total protons and neutrons. Which other question above was answered with “12”? ________ This is not a coincidence! Atomic mass on the periodic table is given in units of atomic mass units. One atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom.

9. Using the digital balance, measure the mass in grams of all the pennies (protons and neutrons) making up your carbon-12 nucleus. _____________ g 10. Divide the total mass by 12 to get one-twelfth the mass of your carbon-12 nucleus. __________ g

11. Using the digital balance, measure the mass of one of the pennies from your nucleus. __________ g

 Conclusion How does the measurement in #11 compare to the calculation you did in #10 above? What does the mass tell you about an atom?