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Atomic Theories (Sections 4.1 & 4.3)

Ancient Greek Models •  Democritus believed that all matter

consisted of extremely small particles that could not be divided – Atoms

• Means uncut or indivisible – Thought there were different

types of atoms with specific properties • Smooth and round for liquids,

rough and prickly for solids

Ancient Greek Models •  Aristotle didn’t think there was a

limit to the number of times matter could be divided

•  Thought that all substances were built up from only four elements – Earth – Air – Fire – Water

•  His theory was accepted for many centuries

Dalton’s Atomic Theory •  Gathered evidence by measuring

masses when elements combine and form compounds

•  Developed a theory to explain why the elements in a compound always join in the same way

•  Elements are like solid spheres – Made wooden spheres to

represent the atoms of different elements

Dalton’s Theory •  Dalton proposed the theory that all matter is

made up of individual particles called atoms, which cannot be divided – All elements are composed of atoms – All atoms of the same element have the

same mass, and atoms of different elements have different masses

– Compounds contain atoms of more than one element

– In a particular compound, atoms of different elements always combine in the same way

Thomson’s Model of the Atom •  Used an electric current to learn more

about atoms –  Electric current is a flow of charged

particles –  Objects with like charges repel and

objects with opposite charges attract •  Used a sealed tube of gas with two

oppositely charged disks on either side –  Cathode Ray Tube –  A glowing beam appeared between the

two disks –  The beam bent toward a positively

charged plate outside the tube

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Thomson’s Model •  The particles inside the

beam were negatively charged and came from inside atoms

•  Thomson’s experiments provided the first evidence that atoms are made of even smaller particles

•  Revised Dalton’s theory to account for these subatomic particles

•  Thomson’s Model – “Plum Pudding” model – Negative charges are

evenly scattered throughout an atom filled with a positively charged mass of matter

– Like chocolate chip ice cream

Rutherford’s Atomic Theory •  Gold Foil Experiment

– Used alpha particles and a screen of gold

– Found that many particles were deflected more than 90 degrees, some straight back

Rutherford’s Model •  Found that the positively charge

of an atom is not evenly spread throughout the atom – It is concentrated in a very

small central area called the nucleus • Nucleus is a dense, positively

charged mass located in the center of the atom

•  According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus

Bohr’s Model of the Atom •  Focused on the electrons •  In Bohr’s model, the electrons

move with constant speed in fixed orbits around the nucleus – Like planets around the sun

•  Each electron has a specific amount of energy and if it gains or loses energy the energy of the electron can change

•  Energy levels - the possible energies that electrons in a atom can have

Energy Levels •  Like steps in a staircase

–  As you move up or down you can measure how many steps you took

–  You can take one step up or jump up two steps down

–  Up or down you can only move in whole-step increments, never between steps

•  An electron in an atom can move from one energy level to another when the atom gains or loses energy –  Can move up two energy levels if it gains

enough energy –  Can move down two energy levels if it loses

enough energy –  The size of the jump determines the amount

of energy gained or lost

Evidence for Energy Levels •  Scientists can measure the energy

gained when electrons absorb energy and move to a higher energy level

•  They can measure the energy released when the electron returns to a lower energy level

•  The movement of electrons between energy levels explains the light you see when fireworks explode –  Heat from the explosion causes some

electrons to move to higher energy levels, when they move back they emit some energy as visible light

–  Different elements emit different colors of light

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Electron Cloud Model •  Electrons move in a less

predictable way than Bohr thought

•  Electron cloud - a visual model of the most likely locations for electrons in an atom – Denser at locations where the

probability of finding an electron is high

•  Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus

Atomic Orbitals •  The electron cloud represents all the

orbitals in an atom •  Orbital - a region of space around the

nucleus where an electron is likely to be found

•  Electron cloud is a good approximation of how electrons behave in their orbitals

Energy Levels, Orbitals, & Electrons

•  The level in which an electron has the least energy (lowest energy level) has only one orbital

•  Higher energy levels have more than one orbital

•  The first four energy levels of an atom –  Max # of electrons in an energy

level is twice the number of orbitals –  Each orbital can contain two

electrons at most

Electron Configurations •  Electron configuration - the arrangement of electrons in the

orbitals of an atom •  The most stable electron configuration is the one in which the

electrons are in the orbitals with the lowest possible energies •  Ground state - when all the electrons in an atom have the lowest

possible energies •  Excited state - when an electron is in an orbital with a higher

energy level, less stable than the ground state