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* Studying Atoms * How is it possible for us to study something we can’t see? * Need indirect evidence * Atoms are so small that even with the most powerful microscopes we can’t see them * *Democritus believed that all matter consisted of small particles that couldn’t be divided. He called these atoms from the Greek word atmos which means uncut or indivisible *Democritus thought there were different types of atoms with different types of properties: atoms in liquids were round and smooth, atoms in solids were rough and prickly * *Aristotle didn’t think there was any limit to the number of times matter could be divided *Aristotle thought all matter was built up from four basic substances; earth, air, fire, and water. *This idea was accepted up until the 1800’s (over 2000 years) *By the 1800 evidence to support an atomic model began to appear * * * Dalton was interested in predicting the weather so he studied the behavior of gases in air * Predicted that a gas consisted of individual particles because of the pressure they exert * * When elements combine the ratio of the masses of the elements in the compounds is always the same * Compounds have a fixed composition * * 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 (is this always true?) * In a particular compound, atoms of different elements always combine in the same way * *His model of the atom pictured the atom as a solid sphere with different masses *Reminder: A theory explains the results from many experiments *Was widely accepted for a long time *Eventually scientists found that not all of Dalton’s ideas were completely correct *This lead to a revision of the theory * * *Used an electric current to learn more about atoms *Vacuum tube attached to an electric current: a glowing beam appears *Beam contained charged particles *Used charged plates on either side of the beam: beam attracted to positive plate and repelled from the negative plate * * Particles in the beam had a negative charge * His experiments showed that atoms are made of smaller particles (Contrary to Dalton’s idea of a solid ball) * * Atoms are basically neutral – no positive or negative charge * Negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter.. * Plum pudding model (Chocolate Chip Ice Cream model) * * * Discovered that uranium emitted fast-moving particles that had a positive charge (He called them alpha particles) * Gold Foil Experiment * Aimed the alpha (positive particles) at a thin gold foil * They did not behave as Rutherford expected * * * Rutherford proposed that the positive charge was concentrated in a small area which he called the nucleus * Nucleus: a dense, positively charged mass located in the center of the atom * * All of an atoms positive charge is concentrated in the nucleus * Through experimentation they discovered that the total volume of an atom is about a trillion (1,000,000,000,000) times the volume of the nucleus. * * * Rutherford saw the existence of two subatomic particles and predicted a third. * Protons * Electrons * Neutrons * *Rutherford concluded the the amount of positive charges varied from element to element *Each nucleus must contain at least one positive particle which he called a proton *Has a +1 charge *Some nuclei contain more than 100 protons * * An electron is a negatively charged subatomic particle found in the space outside the nucleus * Electrons have a charge of -1 * *Neutrons were shown to exist by James Chadwick in 1932 *Charged objects did not deflect their paths *A neutron is a neutral subatomic particle that is found in the nucleus of an atom *It has a mass almost exactly equal to that of a proton (See page 109 in your textbook) * * All three subatomic particles can be distinguished by mass, charge, and their location in an atom * All of this information is based on how particles behave. We still don’t have any instrument that can see inside an atom. * * Atomic Number is the number of protons in the nucleus of an atom * Atoms of different elements have different atomic numbers * The atomic number also equals the number of electrons. Why? * *The mass number is the sum of the number of protons plus the number of neutrons *You can figure out the number of neutrons by subtracting the atomic number from the mass number *Al = 27 How many neutrons? *Carbon = 12 How many neutrons? *Argon = 40 How many neutrons? * *Every atom of the same element does not have the same number of neutrons *Isotopes are atoms of the same element that have different numbers of neutrons and therefore different mass numbers *Radioactive elements typically are isotopes *Some others are oxygen and carbon *All of the elements have isotopes * * *Nanotechnology *Using atoms and molecules as construction materials to build very small objects *1/1,000,000,000 of a meter = 1 nanometer *Human hair = 80,000 nm *Can build from the top down or from the bottom up *Nanogears *Nanorobots * * * * Model looks like planets orbiting around a sun in a solar system * Bohr agreed with Rutherford about the nucleus * Focused on the electrons * A description of the arrangement of the electrons is the main part of modern atomic theory * * *Electrons move with constant speed in a fixed orbit around the nucleus *Each electron has a fixed amount of energy *Electrons can gain or lose energy *Energy levels are the possible energies that an electron can have *Like steps – you can’t be between – you must be on one level or the other – each step would represent an energy level *No two elements have the same set of energy levels * * Electrons move from one energy level to another when they gain or lose energy * Can move more than one energy level * Evidence: the light given off by fireworks * Raising energy levels and light given off when they return to their original energy level * Each element has a unique color spectrum they give off because each element has a different set of energy levels * Move from the ground state – get excited – return to the ground state and give off unique light signature * * An improvement on Bohr’s model * Electrons move in a less predictable way * Electron Cloud: a visual model of the most likely locations for the electrons of an atom to be around the nucleus * Cloud is denser where the electrons are most likely to be – based on probability * * *An orbital is a region of space around the nucleus where an electron is likely to be found *These orbitals have unique shapes and can hold 2 electrons each * s, p, d, and f * s orbitals (there are 1) – can hold 2 electrons * p orbitals (there are 3) – can hold 6 electrons * d orbitals (there are 5) – can hold 10 electrons * f orbitals (there are 7) - can hold 14 electrons * Energy Level Maximum # of Electrons 1 Number of Orbitals Possible 1 2 4 8 3 9 18 4 16 32 2 *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 orbitals with the lowest possible energies (we call this the ground state) *If an electron absorbs energy it is said to be in an excited state which is less stable (light emission) *