Introduction to the Bohr Atom Model 1

2 Electrons that leave one orbit of an atom must move to another orbit. Electrons can only change orbits if they receive specific amounts of extra energy (quanta of energy) from the outside world. Bohr’s “Planetary” Model (1913) Electrons posses specific amounts of energy and they must stay fixed distances (orbits) from the nucleus.

Introduction to the Bohr Atom Model 3 If an electron returns to the orbit it used to reside in, it will give up the extra energy it had when it moved in the first place. The energy that electrons give up when they move back into an original orbit often shows up as a specific color of light. Bohr’s 1913” Planetary” Model (continued) If an electron receives enough energy from the outside world, it can not only leave the orbit it is in but it can leave the atom it is in.

Introduction to the Bohr Atom Model 4 This atomic arrangement of electrons about a nucleus with the lowest possible energy is known as the “ground” state for an atom. Stable atoms are arrangements of electrons around a nucleus that represent the lowest possible use of energy. Bohr’s 1913” Planetary” Model

Introduction to the Bohr Atom Model 5 3 p + This atom is in it’s “ground” state. What element does it represent? 3 p + This atom is in an “excited” state. What element does it represent? Which atomic arrangement of electrons represents an atom that has acquired more energy? Why?? Bohr’s 1913” Planetary” Model

Introduction to the Bohr Atom Model 6 The electron Millikan’s Oil Drop Experiment (1909) The nucleus Chadwick’s Beryllium Radiation Experiments (1932) Exploring the components of the atom as envisioned by Bohr.

Introduction to the Bohr Atom Model 7 to vacuum pump port used to spray oil drops into vacuum chamber Measure the time it takes for an oil drop to free fall in the vacuum chamber. 1) Determine the mass of the oil drop. 2) Remember that the speed of the falling oil drop is related to its mass by the gravitational acceleration. The Oil Drop Experiment lead to the determination of the mass of an electron. Millikan’s Oil Drop Experiment (1909)

Introduction to the Bohr Atom Model 8 to vacuum pump port used to spray oil drops into vacuum chamber Add charge to the drop as it passes through the opening. 3) Adjust a positive voltage on the bottom plate of the chamber to move negatively charged drop to top plate. 4) A drop’s upward speed is related to its mass-to-charge ratio. The electron mass is now determined using the known mass of the uncharged oil drops. to (+) terminal of power supply to (-) terminal of power supply Millikan’s 1909 Oil Drop Experiment

Introduction to the Bohr Atom Model 9 A radiation beam was given off but this stream of high energy particles was not deflected by electric or magnetic fields. Radiation beam emitted from the beryllium consisted of particles that were not positive or negative. The conclusion These particles were named neutrons, have about the same mass as protons, and lead to the development of uncontrolled and controlled nuclear reactors. The experiment The observation The impact on technology Chadwick’s Beryllium Radiation Experiment (1932) Beryllium was bombarded with alpha particles.

Introduction to the Bohr Atom Model 10 View of the Atom by early 1930’s An electron orbital structure as outlined in Bohr’s model.1) had a number of protons exactly equal to the number of electrons in the orbits outside the nucleus. 3)A center dense core called the nucleus that had a number of neutrons with each neutron weighting about the same as a proton. An atom had 2)Electrons whose mass is very small compared to the mass of protons.

Introduction to the Bohr Atom Model 11 View of the Atom by the early 1930s An atom is the smallest particle of an element that consists of a specific number of protons. The physical properties of an element are due to the electron arrangement (configuration) about the nucleus of the atoms in the element. Elements get their names from the number of protons they have in the nucleus of their atoms. The only difference between a large amount of an element and a small amount of the same element is the number of these atoms. An atom has the same number of electrons outside its nucleus as it has protons inside its nucleus.

Introduction to the Bohr Atom Model 12 e - / orbits0 amu9.11 x Electron (chemical properties) n 0 /Nucleus01 amu1.675 x Neutron p + /Nucleus+11 amu1.673 x Proton (identifies) Symbol/ Location Relative charge Relative Mass (amu) Mass (g)Particle Characteristics of basic particles in an atom ( subatomic particles ) View of the Atom by the early 1930s

Introduction to the Bohr Atom Model 13 Great Role for Periodic Table! Keeping Track of an Atom’s Subatomic Particle Information HHe LiBeBCNOFNe Symbols for first 10 elements of the periodic table View of the Atom by the early 1930s

Introduction to the Bohr Atom Model 14 Element Symbol H Keeping Track of an Atoms Subatomic Particle Information Atomic Number Atomic Mass in atomic mass units (amu) (the number of protons in the nucleus) (indicates the name of the element) (approximate mass of protons plus the mass of neutrons) (the number of electrons in the atom) Remember (2) The mass of a proton equals the mass of a neutron (1) The mass of a proton almost equals 1 amu (3) The number of neutrons equals the nearest whole number difference between the atomic mass and the atomic number (4) The mass of the electron is small compared to the mass of the protons and neutrons.

Introduction to the Bohr Atom Model 15 K Mass number Atomic number #p + =#e - = 19 #n 0 =39 – 19 = 20 Shorthand Notation Remember Keeping Track of an Atoms Subatomic Particle Information Element symbol

Introduction to the Bohr Atom Model 16 Li 7 3 #n 0 =7 – 3 = 4 3 p + 4 n o “Ground” state electron configuration 2 different electron configurations for a Lithium atom #n 0 =7 – 3 = 4 “Excited” state electron configuration 3 p + 4 n o Li 7 3 Keeping Track of an Atoms Subatomic Particle Information

Introduction to the Bohr Atom Model 17 Beryllium 4 p + 5 n o Be 9 4 Is this a “ground” state or an “excited” state electron configuration for Beryllium? Keeping Track of an Atoms Subatomic Particle Information

Introduction to the Bohr Atom Model 18 Boron B 11 5 Is this a “ground” state or an “excited” state electron configuration for Boron? 5 p + ? n o Keeping Track of an Atoms Subatomic Particle Information

Introduction to the Bohr Atom Model p + 16 n o Phosphorus P “Ground” state electron configuration Keeping Track of an Atoms Subatomic Particle Information

Introduction to the Bohr Atom Model Maximum # of electrons = 2(n 2 ) Energy Level (n) An atom is in its “ground” state electron configuration when the electrons fill the orbits in the following order: The value of “n” indicates the orbit position. orbit closest to the nucleus View of the Atom by the early 1930s

Introduction to the Bohr Atom Model 21 Electrons that leave one orbit of an atom must move to another orbit. Electrons can only change orbits if they receive specific amounts of extra energy (quanta of energy) from the outside world. Electrons posses specific amounts of energy and they must stay fixed distances (orbits) from the nucleus. Bohr’s atom Summary

Introduction to the Bohr Atom Model 22 If an electron receives enough energy from the outside world, it can not only leave the orbit it is in but leave the atom it is in. If an electron returns to the orbit it used to reside in, it will give up the extra energy it had when it moved in the first place. The energy electrons give up when they move back into an original orbit often shows up as a specific color of light. Bohr’s atom Summary

Introduction to the Bohr Atom Model 23 This atomic arrangement of electrons about the nucleus is known as the “ground” state for an atom. Stable atoms are arrangements of electrons around the nucleus that represent the lowest possible use of energy. Bohr’s atom Summary

Introduction to the Bohr Atom Model 24