chp 4.1 and 4.3 atomic theory - weebly
TRANSCRIPT
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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