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

Thomson’s Model Thomson’s Model “Plum Pudding” 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 cookie

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

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 Example - your location at school If you placed a dot on a map every 10 minutes throughout the school day Your dots would create a model of your orbital at school The places with more dots are the places you are most likely to be The places with few dots are the places you are least likely to be

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