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Cornell Notes Topic:___________________________________ Questions Notes The Guide to Atomic Structure What is an atom? Atomic Structure An atom is the smallest part of an element that still has the element’s properties. Atoms are the building blocks of molecules. What are the two regions of the atom? Atomic Structure Nucleus: At the center of each atom, small volume, large density, positive electric charge. – made of protons and neutrons – contains 99.99% of the atom's mass Electron cloud: contains electrons, outside the nucleus, negative charge, large volume, low density, divided into energy levels. What are the particles in the atom? Atomic Structure Atomic Structure All atoms are made of the same 3 subatomic particles in different Copy this table amounts: Symbol Charge Mass Actual Mass Number Proton p+ Neutron n Electron o e- +1 0 -1 1 1.673* 10-27 kg 1 1.675* 10-27 kg 0 9.109* 10-31 kg What is the charge of an atom? Atoms are Neutral • Atoms are electrically neutral because they contain equal numbers of protons and electrons. • A proton has a positive charge equal in magnitude to the negative charge of an electron. 8 + protons 8 – electrons = neutral atom Ions are Atoms with a Charge An ion is an atom that has lost or gained electrons and has a negative or positive charge. Atoms that lose electrons have a positive charge. Atoms that gain electrons have a negative charge. If an atom is an ion the charge & number will follow the element symbol. O -2 What holds the particle in the nucleus together? Forces in the Nucleus • When two protons are near each other, there is a strong electromagnetic force pushing them apart. • Like charges repel. Forces in the Nucleus • The force that hold the protons together is the strong nuclear force. • The strong nuclear force is provided by the neutrons and only acts over very short distances. Neutron hold Strong Nuclear Force How are atoms measured? The Sizes of Atoms • The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud. • Atomic radii are measured in picometer, pm. • 1 pm = 1 × 10−12 meter • Mass of atoms is measured in atomic mass unit, or amu. • 1 amu = 1.660 540 × 10−27 kg Counting Atoms Mass Number • The mass number of an atom is the sum of protons & neutrons in the nucleus of an atom. • The electrons are too small to add any measurable mass to the atom. What are isotopes of an atom? Counting Atoms Isotopes • Isotopes are atoms of the same element that have different masses. • The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. • Most of the elements consist of mixtures of isotopes. The 3 Isotopes of Hydrogen: Copy this diagram Counting Atoms Average Atomic Masses of Elements • Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element. • The average atomic mass is listed on the periodic table- it is an average of many • The mass number is the mass of a specific isotope- it is the exact mass for that isotope How are different isotopes of an atom written? Counting Atoms Designating Isotopes • Hyphen notation: The mass number is written with a hyphen after the name of the element. uranium-235 • Nuclear symbol: the mass number on top and the atomic number on bottom followed by the element symbol. 235 92 U How can you determine the structure of an atom? Counting Atoms • number of protons = the atomic number from the periodic table • number of electrons = the number of protons in neutral atoms Lithium-7 Atomic number is 3 3 p+ 3 e- Counting Atoms • number of neutrons = the mass number minus the atomic number mass number atomic number = number of neutrons Li-7 7 3 protons = 4 neutrons Counting Atoms • number of electrons in an ion = the atomic number minus the ion charge & number atomic number ion charge & number = number of electrons O-2 8 (-2) = 10 electrons +2 Cu 29 (+2) = 27 electrons Watch this video on Calculating Atomic Structure Click the button Atomic Structure Video Or copy & paste this address into your web browser: http://www.youtube.com/watch?v=CokAnD1uKvA How can you draw the structure of an atom? Placing the Particles in the Atom - - Draw the protons & neutrons in a tight bunch in the center + - o o + - - Copy this diagram - - Start with the first energy level & add up to 2 electrons The second energy level holds up to 8 electrons