Holt McDougal Chapter 4 Section 2 Atoms

The Development of Atomic Theory

NGSSS: SC912P8.4 Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their mass, electrical charges and locations within the atom

Questions you should be able to answer by the end: What scientists helped to develop atomic theory? What part of atoms did Thomson discover? What part of atoms did Rutherford discover?

Origins of Atomic Theory Democritus Greek philosopher in 4th century BCE Tiny, indivisible units and void Atomos  unable to be cut or divided No evidence; all conjecture and theory

Dalton’s Contribution 1808, British schoolteacher John Dalton performed experiments Atoms DO exist! Atoms could form compounds Law of Definite Proportions

What’s the Answer? How was Dalton’s theory different from that of Democritus? Carbon makes up 27% of the mass of CO2. How many grams of C are there in 88g of CO2?

Thomson Contributes to the Development of Atomic Theory 1897, British scientist J.J. Thomson working with cathode rays Discovered electrons, negatively charged particles Plum Pudding by another Thomson called Lord Calvin

Can you Explain? What was the Greek perspective of atoms? How did Dalton support and detract from the Greek perspective of atoms? How did JJ Thomson discover electrons?

Rutherford Breaks Away from the Pack! Was a student of Thomson Tested the idea that the mass of an atom was spread evenly over its volume Gold Foil experiment Bounce back indicates a concentrated center How would test that idea?

Gestation of The Planetary Model Begins . . . Rutherford concluded that atom’s positive charge is concentrated at the center Nucleus  dense positively charged core of atom So tiny that like a marble in a football stadium Negative electrons orbit the nucleus

Rutherford’s Atomic Model No specific orbits Only established the existence of the positive nucleus

Who did what? Scientist Contribution First proposed that the universe is made up of tiny, indivisible units called atoms Carried out scientific experiments that showed that atoms exist Showed that atoms contain smaller particles called electrons Showed that the positive charge in an atom is concentrated in a nucleus at its center

The Structure of Atoms

NGSSS: SC912P8.4 Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their ass, electrical charges and locations within the atom

By the end of the lesson, you should be able to answer: What do atoms of the same element have in common? What are isotopes? How is an element’s average atomic mass calculated?

Who did what? Rutherford Dalton Thomson Democritus Scientist Contribution First proposed that the universe is made up of tiny, indivisible units called atoms Carried out scientific experiments that showed that atoms exist Showed that atoms contain smaller particles called electrons Showed that the positive charge in an atom is concentrated in a nucleus at its center Democritus Dalton Thomson Rutherford

Bohr and Chadwick: The next steps in Atomic Theory Development 1913, Danish physicist Neils Bohr Fixed orbits for the electrons James Chadwick, Bohr’s student Found the neutron in the nucleus Led to discovery of fission and the atomic bomb

Electron Cloud Model Established by 1926 Area where electrons are most likely to be found 100,000 times larger than the diameter of the nucleus Impossible to accurately describe path and location at any given time Orbital  a region in which an electron is most likely to be

What is Inside an Atom Particle Charge Mass (kg) Location Proton +1 1.67 x 10 -27 Inside the nucleus Neutron Electron -1 9.11 x 10 -31 Outside the nucleus

What Do Atoms of the Same Element Have in Common? The number of protons is the same!!! # of protons identify the atom and element

Balancing Charge with Electrons Protons  positive Electrons  negative 1 proton + 1 electron = neutral charge Gain/lose electron = ion (ionized atom) Ions are NOT neutral

The Electric Force Opposites attract Protons and electrons attract each other Farther apart = less attraction

What is an Atomic Number? Atoms of same element = same # protons Atomic # = # of protons Neutral atoms means the atomic # = protons = electrons (i.e., net charge = zero)

Practice Atomic Number Find the atomic number and name for the following elements: H Sc Ta Bh F Sn U Hydrogen  1 Scandium 21 Tatalum  73 Bohrium  107 Flourine  9 Tin  50 Uranium  92 How many electrons does each of the listed elements have?

Atomic Mass Number Most atoms have protons and neutrons Atomic Mass # = protons + neutrons Atomic Mass = Atomic Number + neutrons How can atoms of the same element have different mass #’s when they all MUST have the same atomic #?

Reminding Yourself Which two types of subatomic particles are located in the nucleus? How does the mass of an electron compare with the mass of a proton? Why are atoms neutral? What force holds atoms together?

Isotopes Atoms of an element with different mass #’s Remember, the AN (# of protons) is ALWAYS the same # of neutrons is different from one isotope to another

Let’s Peer at H Isotopes . . . Protium Deuterium Tritium What is the atomic number, atomic mass, and electrical charge for each?

How Common Are Different Isotopes? Some more than others (e.g., O has AM of 16 usually; about 0.2% have mass of 18) Some isotopes are unstable and decay How can decayed isotopes form new/different isotopes of the same element (even though they are breaking apart)?

How Can You Show an Atom’s Atomic and Mass Numbers Cl Chemical symbol represents the element AMN is written on top AN is written on bottom How can you compute the number of neutrons? 35 17

More About Atomic Mass Too tiny  1 atom of F has mass (kg) of less than one trillionth of one billionth of one gram What do you think that number looks like? Write it down and compare with your neighbor 1 1,000,000,000,000,000 X 1,000,000,000,000

Unified Atomic Mass Unit Describes the masses of atoms SI standard of measure for atoms A AMU = 1/12 mass of C-12 atom Mass number = protons + neutrons (i.e., whole number) Atomic mass = mass of single atom of element (i.e., usually a decimal)

How’s Your Memory? What is the AN of most atoms of O? What is the AMN of most atoms of O? How many protons are in the nucleus of an atom of Cl? What unit do scientist use to measure the masses of atoms?

The Behavior of Electrons