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The Atom. What to look for: Three Essential Questions E. Q. #1 = What are the parts of an atom? E. Q. #2 = In what manner can subatomic particles be differentiated? E. Q. #3 = How does the atomic structure impact chemical change & reactivity?. The Building Block of Matter.

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The atom
The Atom

What to look for:

Three Essential Questions

E. Q. #1 = What are the parts of an atom?

E. Q. #2 = In what manner can subatomic particles be differentiated?

E. Q. #3 = How does the atomic structure impact chemical change & reactivity?

The building block of matter
The Building Block of Matter

  • 400 BC Democritus (Greek Philosopher)

  • Atomos = “indivisible”

  • …like a brick is used to build a building; an atom is used to build all matter…

First atomic theory
First Atomic Theory

  • 1808 John Dalton (English Chemist)

  • Using experiments formulates the first atomic theory

    • All elements are composed of indivisible particles called atoms

    • Atoms of the same elements are exactly the same

    • Atoms of different elements are different

    • Two or more elements combine to form compounds

Subatomic particles
Subatomic Particles

  • The idea that the atoms was not a solid sphere, but a composition of small particles which came to be known as subatomic particles

  • Julius Plucker, William Crooks & J J Thomson

  • mid-1800’s

J j thomson
J J Thomson

  • 1897 Discovers the electron

Ernest rutherford english physicist
Ernest Rutherford (English Physicist)

  • Gold Foil Experiments

  • 1908 Discovers that the atom is mostly empty space

  • Discovers a hard, dense center of the atom called the NUCLEUS

The atom is mostly empty space
The atom is mostly empty space

  • Most of the particles passed straight through to gold foil.

  • First important discovery by Rutherford

The nucleus is discovered
The NUCLEUS is Discovered

  • 1911 Rutherford proves that the particles that do not pass through are hitting a small, dense center of the atom called the nucleus.

  • Rutherford later goes on to discover the proton

The atom


Name three major discoveries made by Ernest Rutherford & the Gold Foil Experiment:

1. _______________________________________

2. _______________________________________

3. _______________________________________

What are the two results of firing particles at atoms in the foil of the Gold Foil Experiments:

1. _______________________________________

2. _______________________________________



Who was the first to attempt to form a complete atomic theory?

1. _______________________________________

Electron energy levels
Electron Energy Levels

  • 1913 Niels Bohr (Danish physicist) discovers the electrons travel around the outside of the atom on different energy levels

  • The Bohr Model = “solar system” model

The neutron is found
The Neutron is Found

  • 1932 James Chadwick proves the existence of the neutron.

  • Only 4 years later the neutron is used to split the first atom

  • Why was the neutron’s existence more difficult to prove?

  • What evidence hinted of its existence? “the missing mass”

E. Q. #1 = What are the parts of an atom?

The subatomic particles
The Subatomic Particles

E. Q. #2 = In what manner can subatomic particles be differentiated?

Finding the number of subatomic particles in an atom
Finding the Number of Subatomic Particles in an Atom

  • Atomic Number = the number of protons in the nucleus of an atom

  • Identifies each element

  • Hydrogen is 1 and it has 1 proton in its nucleus (only hydrogen has 1 proton)

  • Helium is 2 and it has 2 protons in its nucleus (only helium has 2 protons)


  • IFthe atom is neutral (same number positive charges as negative charges) then the number of electrons will be the same as protons.

  • ION = a charged atom (unequal number of protons and electrons)

  • +1 charge = the atom LOST one electron

  • -1 charge = the atom GAINED one electron

  • +2 charge = LOST 2 electrons

  • -2 charge = GAINED 2 electrons and so on

Example of ions
Example of Ions

  • Sodium will lose one electron when it bonds

  • Na+1 or Na+

  • Sodium’s atomic number is 11, so it has 11 protons.

  • The +1 charge means Na+1 has 10 electrons

Another example
Another Example

  • Chlorine will gain an electron when it bonds

  • Cl-1 or Cl-

  • Chlorine’s atomic number is 17, so it has 17 protons

  • The -1 charge means Cl-1 has 18 electrons

Mass number
Mass Number

  • The two subatomic particles that have mass?

  • Proton and neutron, both found in the nucleus

  • Mass Number = # of Protons + # of Neutrons

  • Mass # - Atomic # = # of neutrons

Practice problems
Practice Problems

Do Parts of the Atom & Atomic Structure practice sheet now.


  • Atoms of the same element with a different number of neutrons.

  • Ex: carbon-12 & carbon-14

  • Carbon-12

    • 6 protons

    • 6 neutrons

    • 6 electrons

  • Carbon-14

    • 6 protons

    • 8 neutrons

    • 6 electrons

  • Isotopes of hydrogen
    Isotopes of Hydrogen

    • Hydrogen-1 (protium) 0 neutrons

    • Hydrogen-2 (deuterium) 1 neutron

    • Hydrogen-3 (tritium) 2 neutrons

    The electron cloud

    • The electron cloud is separated by different energy levels.

    • Electrons with less energy travel on the levels closest to the nucleus.

    • Energy levels are divided into sub-levels

    • Sub-levels are divided into orbitals

    • All of these levels, sub-levels & orbitals can overlap.

    Valence electrons

    • Valence electrons = the outermost electrons

    • The electrons involved in bonding

    • Determines the chemical properties of an atom

    E. Q. #3 = How does the atomic structure impact chemical change & reactivity?

    Do Practice sheets What Is This Atom Now? 1 & 2 now.

    More about electrons
    More about Electrons

    • Maximum # of electrons on each energy level

    • Level 1 = 2 electrons

    • Level 2 = 8 electrons

    • Level 3 = 18 electrons

    • Level 4 = 32 electrons

    The stable 8
    “the Stable 8”

    • Due to the overlapping levels, sub-levels and orbitals the maximum number of outer most electrons is EIGHT.

    • An atom is becomes stable when it outer-most energy levels is full of electrons

    • This explains the chemical bonding properties of the atoms

    • Atoms will do one of three things in order to fill their outer most energy levels.

      • gain electrons

      • lose electrons

      • share electrons

    Concepts coming up next
    Concepts Coming Up Next

    • The number of valence electrons an atom has determines the group it becomes to on the PERIODIC TABLE.

    • Groups or families of elements on the periodic table has similar chemical properties.

    Concepts coming up next1
    Concepts Coming Up Next

    • Valence electrons determine how atoms will bond with other atoms in order to complete their outer-most energy level.

    • The Stable 8


      • Ionic bonding (gaining/losing of electrons)

      • Covalent bonding (sharing of electrons)