Tiny Particles of Matter Atoms Tiny Particles of Matter

Start of the atom Dalton 1808 atomic theory Moseley 1st used atomic number 1913 Bohr used planetary model of atom

Atoms- Early Models Democritus- first suggested that there was tiny particles- atoms Dalton- provide scientific basis for atoms, and their chemical reactivity (1808

Atomic Theory Play atomic theory video clip

Dalton’s Atomic Theory 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) Chemical reaction occurs when atoms separate, or rearrange. Atoms never change into atoms of another element as a result of a chemical reaction

Ernest Rutherford’s Gold Foil Experiment - 1911 Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded

Rutherford’s problem: In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? Target #2 Target #1

The Answers: Target #1 Target #2

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!” Conclusions: The nucleus is small The nucleus is dense The nucleus is positively charged

The Rutherford Atomic Model Based on his experimental evidence: The atom is mostly empty space All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” The nucleus is composed of protons and neutrons (they make the nucleus!) The electrons distributed around the nucleus, and occupy most of the volume His model was called a “nuclear model”

Empty Space? Activity: Jump up and down Why don’t we fall through the floor?

Bohr Model Niels Bohr in 1913 came up with the Bohr model to explain how electrons are arranged around the nucleus of an atom. He showed that electrons move around the nucleus of an atom in an orbit Like planets around the sun

Bohr Model

Timeline In 1700 there were 13 elements In 1869 there were 26 elements Now there are 118

Mass of Atom The mass of 1 amu is about 1.67 x 10-24 grams. The proton is 1.0073 amu and the neutron is 1.0087 amu, which is essentially equal in mass. The mass of the electron is 0.000549u, or about 1/2000 the mass of a proton.

Size of Atoms If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long A scanning tunneling microscope allows scientist to see atoms These are nickel atoms from STM

Subatomic particles Electrons, protons and neutrons The nucleus of the atom contains protons and neutrons Electrons revolve around the nucleus

Protons and Neutrons 1886 Protons found Mass = 1 amu Positive charge Neutrons found in 1932 Neutral charge cathode ray video

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

Modern Cathode Ray Tubes Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Mass of the Electron Mass of the electron is 9.11 x 10-28 g The oil drop apparatus 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

Electrons Electrons are negatively charged Discovered in 1897 Found by passing electrical current through gas Size 1/2000 the size of Hydrogen atom Today: size 1/1840

# protons in an atom = # electrons Atomic Number Atoms are composed of identical protons, neutrons, and electrons How then are atoms of one element different from another element? Elements are different because they contain different numbers of PROTONS The “atomic number” of an element is the number of protons in the nucleus # protons in an atom = # electrons

Atomic Number Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # (Z) Carbon 6 Phosphorus 15 Gold 79

Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope: Mass # = p+ + n0 Nuclide p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

Complete Symbols Contain the symbol of the element, the mass number and the atomic number. Mass number X Superscript → Atomic number Subscript →

Br Symbols 80 35 Find each of these: number of protons number of neutrons number of electrons Atomic number Mass Number 80 Br 35

Symbols If an element has an atomic number of 34 and a mass number of 78, what is the: number of protons number of neutrons number of electrons complete symbol

Symbols If an element has 91 protons and 140 neutrons what is the Atomic number Mass number number of electrons complete symbol

Symbols If an element has 78 electrons and 117 neutrons what is the Atomic number Mass number number of protons complete symbol

Isotopes Elements have different versions Each version has a different number of neutrons so different mass Same element different # neutrons Keeps same chemical properties Play elements and Isotope video

Isotopes Dalton was wrong about all elements of the same type being identical Atoms of the same element can have different numbers of neutrons. Thus, different mass numbers. These are called isotopes.

Isotopes Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

We can also put the mass number after the name of the element: Naming Isotopes We can also put the mass number after the name of the element: carbon-12 carbon-14 uranium-235

Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2

Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons.

Atomic Mass How heavy is an atom of oxygen? It depends, because there are different kinds of oxygen atoms. We are more concerned with the average atomic mass. This is based on the abundance (percentage) of each variety of that element in nature. We don’t use grams for this mass because the numbers would be too small.

Measuring Atomic Mass Instead of grams, the unit we use is the Atomic Mass Unit (amu) It is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 chosen because of its isotope purity. Each isotope has its own atomic mass, thus we determine the average from percent abundance.

To calculate the average: Multiply the atomic mass of each isotope by it’s abundance (expressed as a decimal), then add the results. If not told otherwise, the mass of the isotope is expressed in atomic mass units (amu)

Composition of the nucleus Atomic Masses Atomic mass is the average of all the naturally occurring isotopes of that element. Isotope Symbol Composition of the nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 7 neutrons 1.11% Carbon-14 14C 8 neutrons <0.01% Carbon = 12.011

Electrons Electrons determine the physical and chemical properties of an element P.T. classifies elements by electron configuration (4 groupings) Noble gases Representative elements Transition elements Inner transition elements Look at bottom of pg. 395

Electrons Play electron video clip

Energy Levels Electrons in a particular path have a fixed energy Electrons don’t lose energy so they don’t fall into the nucleus The energy level is the region around the nucleus where electrons are likely to be found

Energy Levels Like the rungs of a ladder Electrons close to the nucleus have less energy For electrons to move from energy level to the next it must gain or lose the right amount of energy The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

Energy Levels The P.T. can help determine electron levels Valence electrons are outer electrons- affect reactions Oxidation numbers follow Group numbers

Energy Levels Like the rungs of a ladder Electrons close to the nucleus have less energy For electrons to move from energy level to the next it must gain or lose the right amount of energy The farther away the less force the nucleus has on the electron, so it is easier for the electron to leave the atom

Quantum Mechanical Model Just like the Bohr model but Electrons don’t have set orbits It uses probability to show where an electron could be located. Math based Electron cloud Areas of high probability

Electron Cloud

Energy Levels Principal energy level= major levels Sublevels, each principal level has a set # of sublevels that coincides with the number of principal level 1 = 1 sublevel 2 = 2 sublevels

Energy Levels Orbitals are areas where electrons are likely to be found Letters denote the orbitals S= spherical p= dumbbell shape d and f

Energy levels The number of electrons in a principal energy level is based on 2n2 1 2 3 4 p.level 2 8 18 32 sub

Orbitals s= 1 orbitals P= 3 orbitals D= 5 orbitals F= 7 orbitals

Electron Configuration 3 rules pg.367-368 Electrons enter orbitals of lowest energy 1st An atomic orbital describes at most two electrons When electrons occupy orbitals of equal energy, one electron enters each orbital until they are all full.