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CHAPTER 4 Atomic Structure Atoms • Atoms– the smallest particle of an element. Named by Democritus, a Greek philosopher. What are atoms? Atoms are the basic building blocks of matter that make up everyday objects. A desk, the air, even you are made up of atoms! There are 90 naturally occurring kinds of atoms. Scientists in labs have been able to make about 25 more. Dalton’s Atomic Theory 1. All elements are composed of atoms. Atoms can not be broken into smaller pieces. They are extremely small. 2. In any element, all the atoms are the same. 3. Atoms of 2 or more elements can combine to form compounds. Compounds contain atoms of more than one element. Dalton’s Atomic Theory cont. 4. In a particular compound, atoms of different elements always combine in the same way. The ratio of their masses is always the same for that compound. EX: H 2O H-O 2:1 same ratio in all samples of water. Dalton’s Atom Dalton’s Model of an atom showed an atom as a solid sphere without subatomic particles Thomson’s model of the Atom Thomson, used an electric current to learn more about atoms. Thomson’s experiments provided the first evidence that atoms are made of even smaller particles, subatomic particles. In Thomson’s model of the atom, the negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter. Rutherford’s Atomic Theory • Through his testing using a Gold Foil Experiment Rutherford concluded that the positive charge of an atom is not evenly spread throughout the atom. The positive charge is concentrated in a very small, central area that Rutherford called the nucleus. • The 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. Sec 4.2 The Subatomic Particles of an Atom Protons- Positively charged subatomic particle that is found in the nucleus of an atom. Electrons- Negatively charged subatomic particle that is found in the space outside the nucleus. Neutrons- 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. Each positive charge in an atom is balanced by a negative charge because atoms are neutral. (# of electrons = # of protons) Protons, electron, and neutrons can be distinguished by mass, charge, and location in an atom. (this is how we can tell them apart) The protons and the neutron have almost the same mass. An electron has far less mass than either a proton or a neutron. About 2000 electrons equal the mass of one proton. ATOMIC NUMBER– the number of protons in the nucleus of the atoms in an element. Atoms of different elements have different number of protons. ATOMIC MASS NUMBER–The atomic mass of an atom is the sum of the protons and neutrons in the nucleus of that atom. Electrons have a much smaller mass than protons and neutrons therefore most of the atom’s mass is in the nucleus. # of neutrons + # of protons (atomic number ) = atomic mass number Isotopes of an element have the same atomic number ( same # of protons) but different mass numbers because they have different numbers of neutrons. Example: C-12 C- 14 14 total mass Carbon 12 Total mass Atomic # (6 protons in nucleus) 12 6 C ! 12 total mass Carbon 14 ________________ atomic mass -6 protons ---------6 neutrons -6 protons 14 6 6 protons C 8 neutrons in nucleus hyperlink to Isotopes http://www.colorado.edu/physics/2000/isotopes/index.html Hydrogen Hydrogen-1 Deuterium Hydrogen-2 Tritium Hydrogen-3 The most common isotope of hydrogen has no neutrons at all. There is also a hydrogen isotope called deuterium with one neutron and tritium, with two neutrons. Sec 3 Bohr’s Model of the Atom • Scientist, Bohr focused on the electrons of an atom. • Bohr believed that electrons moved with constant speed in fixed orbits around the nucleus, like planets around the sun. • If an atom gains or loses energy, the energy of an electron can change. The possible energies that electrons in an atom can have are called energy levels. • An electron in an atom can move from one energy level to another when the atom gains or loses energy. (when an electron moves to a higher energy level the electron has gained energy) Bohr’s Model of an Atom The region of space around the nucleus where an electron is likely to be found is called an Electron Orbital. Bohr’s Model of an atom shows subatomic particles where the electrons are in fixed circular orbitals about the nucleus. Evidence of Energy Levels The movement of electrons between energy levels explain the light you see when fireworks explode. Light is a form of energy. Heat from the explosion causes some electrons to move to higher energy levels. When they move back to lower energy levels they give off energy as light. Because no 2 elements have the same set of energy levels, different elements emit different colors of light. An electron configuration is 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. When all the electrons in an atom have the lowest possible energies, the atom in said to be in its ground state. If an atom gains enough energy, one of its electrons can move to an orbital with a higher energy level. This configuration is referred to as an excited state. At an excited state the atom is less stable than at ground state. Eventually the electron loses energy and the atom returns to the ground state. Electron Cloud Model • Today, we know that electrons move in a less predictable way. • An electron cloud is a visual model of the most likely locations for electrons in an atom. • Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus. • The electron cloud represents all the orbitals in an atom. Electron Cloud Model In the Electron Cloud Model the moving electrons are represented by a cloud. • The level in which an electron has the least energy is the lowest energy level, next to the nucleus. It has only one orbital in it. Higher energy levels have more than one orbital. • Each orbital can only contain 2 electrons. • Therefore the maximum number of electrons in an energy level is twice the number of orbitals. Energy Level Number of Orbitals Maximum Number of Electrons 1 1 2 2 4 8 3 9 18 4 16 32 Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. ATOMIC ORBITALS Each orbital has a name s orbital p orbitals d orbitals One of the 7 f orbitals sublevel orbital maximum # of electrons s 1 2 p 3 6 d 5 10 f 7 14 An electron configuration is the arrangement of electrons in the orbitals of an atom. To show the electron orbitals you can write it as: Examples of Electron Configurations Element # of Electrons in Element Electron Configuration He 2 1s Li 3 1s Be 4 1s O 8 Cl 17 1s 1s Democritus Aristotle (said matter is made up of tiny particles called ATOMS that cannot be divided) (said matter made up of earth air, fire, water) + - -+ + + + Thomson Dalton (solid sphere, no subatomic particles, had a predictable mass for a given element) -- (discovered negative charges-thought they were scatter throughout atom) + - Bohr’s Model -electrons in fixed orbitals like plants around sun Rutherford (said positive charge was concentrated in the center of atom— called it the nucleus Electron Cloud Model- used today. Electron location by probability—where it is most likely to be located + - + - + + + + - • Dalton Model • Thomson Model • Electron Cloud Model • Bohr Model • Rutherford Model