# The Atom - California State University, Northridge

The Atom How big is an Atom? The average size atom (Al) is about 0.00000003 cm. It would take a stack of 50,000 aluminum atoms to equal the

thickness of a sheet of aluminum foil. Thats Small!!!! Whats Inside an Atom? An

atom is made up of a team of three players: protons, neutrons, and electrons They each have a charge, mass, and a location Protons... Charge:

positive Mass: 1 amu (atomic mass unit) Location: Nucleus Neutrons... Charge: none

Mass: 1 amu (atomic mass unit) Location: Nucleus Electrons... Charge: negative Mass: almost zero Location: electron clouds

Electrons vs Protons The electrons negative charge is the same size as a protons positive charge. They cancel each other out: +1 -1 = 0 Atoms like to have 0 charge.

Questions... What particles are in the nucleus? Explain why atoms are neutral. Why do scientists say that most of the mass of an atom is located in the nucleus?

How Do Atoms of Different Elements Differ? Lets compare a Hydrogen atom and a Helium atom

Hydrogen... Draw one proton. Draw a circle around the proton. Place one electron on the circle. You have just created a hydrogen atom! Hydrogen has no neutrons.

Helium Draw two protons and two neutrons all together. Draw an electron orbit around them. Place two electrons on the orbit. You have just created a helium atom!!!

What is different? The two different elements have different numbers of electrons, protons, and neutrons. OK...

Are all atoms of an element the same? Draw another Hydrogen atom, but this time add a neutron. It is still Hydrogen because it has only one proton, but it has more mass in the nucleus. It is called an isotope.

What is an Isotope? Isotope - atoms that have the same number of protons but have different numbers of neutrons. Some isotopes are radioactive. Otherwise they are mostly the

same as the regular atom. isotope from another? By their mass numbers. mass number = protons + neutrons.

Different isotopes will have different mass numbers because they have different numbers of neutrons. Lets see... Hydrogen-1

has 1 proton and 0 neutrons. Mass Number = 1 Hydrogen-2 has 1 proton and 1 neutron. Mass Number = 2 Questions List

the charge, location, and mass of each particle. Determine the number of protons, electrons, and neutrons in aluminum-27.

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