Introduction to Atoms Chapter 14 Section 1

History of Atom All atoms share the same basic structure All atoms share the same basic structure During past 200 years, scientists have proposed different models During past 200 years, scientists have proposed different models

Dalton’s Model Based on experiments, Dalton developed a theory of structure of matter Based on experiments, Dalton developed a theory of structure of matter 4 main concepts: 4 main concepts: All matter is composed of tiny, indivisible particles called atoms All matter is composed of tiny, indivisible particles called atoms Atoms of each element are exactly alike Atoms of each element are exactly alike Atoms of different elements have different masses Atoms of different elements have different masses Atoms of different elements can join to form compounds Atoms of different elements can join to form compounds

Dalton’s Model

Thomson’s Model End of 1800s End of 1800s Thomson discovered that atoms were not simple, solid spheres Thomson discovered that atoms were not simple, solid spheres Atoms contained subatomic particles Atoms contained subatomic particles Very small, negatively charged Very small, negatively charged Called them electrons Called them electrons

Thomson’s Model Also knew that atoms were electrically neutral Also knew that atoms were electrically neutral Must contain enough positive charge to balance negative charge of electrons Must contain enough positive charge to balance negative charge of electrons Developed model where electrons were stuck into a positively charged sphere Developed model where electrons were stuck into a positively charged sphere Like chocolate chips in cookie dough Like chocolate chips in cookie dough

Thomson’s Model

Rutherford’s Model By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons By early 1900s, scientists knew that positive charge of atom comes from subatomic particles called protons 1911—Rutherford begins to test theory 1911—Rutherford begins to test theory His experiments led him to believe that protons are concentrated in a small area at center of atom His experiments led him to believe that protons are concentrated in a small area at center of atom Called this area the nucleus Called this area the nucleus

Rutherford’s Model Rutherford’s model describes an atom as mostly empty space, with a center nucleus that contains nearly all the mass Rutherford’s model describes an atom as mostly empty space, with a center nucleus that contains nearly all the mass Like the pit in a peach Like the pit in a peach

Bohr’s Model Modified Rutherford’s model in 1913 Modified Rutherford’s model in 1913 Proposed that each electron has a certain amount of energy Proposed that each electron has a certain amount of energy Helped electron move around nucleus Helped electron move around nucleus Electrons move around nucleus in region called energy levels Electrons move around nucleus in region called energy levels Energy levels surround nucleus in rings, like layers of onion Energy levels surround nucleus in rings, like layers of onion

Bohr’s Model Has been called planetary model Has been called planetary model Energy levels occupied by electrons are like orbits of planets at different distances from the sun (nucleus) Energy levels occupied by electrons are like orbits of planets at different distances from the sun (nucleus)

Electron Cloud Model Model accepted today Model accepted today Electrons dart around in an energy level Electrons dart around in an energy level Rapid, random motion creates a “cloud” of negative charge around nucleus Rapid, random motion creates a “cloud” of negative charge around nucleus Electron cloud gives atom its size and shape Electron cloud gives atom its size and shape

Electron Cloud Model

Modern Day Atomic Structure

ELEMENTS INFORMATION For any element: For any element: Number of Protons = Atomic Number Number of Protons = Atomic Number Number of Electrons = Number of Protons = Atomic Number Number of Electrons = Number of Protons = Atomic Number Number of Neutrons = Mass Number - Atomic Number Number of Neutrons = Mass Number - Atomic Number

Example of Krypton For krypton: Number of Protons = Atomic Number = 36 Number of Protons = Atomic Number = 36 Number of Electrons = Number of Protons = Atomic Number = 36 Number of Electrons = Number of Protons = Atomic Number = 36 Number of Neutrons = Mass Number - Atomic Number = = 48 Number of Neutrons = Mass Number - Atomic Number = = 48