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 LevelNumber of Orbitals Possible Maximum # of Electrons

* 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)