Models of the Atom Ms Fedor Arts and College Preparatory Academy

ChemCatalyst The model shown here is a tiny cube of gold. 1. What do you think a scientific model is? 2. The spheres in this model represent atoms. What do you think atoms are?

Agenda 1. Collect Assignments 2. Notes: Atomic Structure 3. Activity: Atomic Pudding HW: Students who have completed quiz 2: study and schedule Quiz 3 Students who have completed Quiz 3: work on organizing your notebook, finishing your preplanning for your element project Objectives: describe the structure of an atom, understand how atomic theory developed over time

What I Need From You Supplements for Quiz 2 ◦ Mole practice problems ◦ Chemical formulas/reactions Activities: ◦ A New Language ◦ Copper Cycle Lab ◦ What Goes Around Comes Around

Earliest Ideas About Atoms Democritus – 460 BCE Philosopher All matter is made up of indivisible particles Atoms means “indivisible” No experimental evidence to support idea

Democritus Greek banknote with Democritus and drawing of atom Where did Democritus get his ideas for small, indestructible, indivisible atoms?

Atoms are small… new wedding ringsold wedding ring EVIDENCE: Old wedding rings get worn away slowly, and get thinner and thinner, but you never see the gold atoms on your finger, so gold atoms must be very small!

Atoms are indestructible… Atoms are indivisible… EVIDENCE: The Earth has been around a long time. Even though mountains get washed away, the rocks continue to exist. New plants grow where old plants die. EVIDENCE: None. This was a HYPOTHESIS made by Democritus to explain nature as he saw it.

Aristotle had other ideas ARISTOTLE: Famous Greek philosopher, born 384 BCE. He was a student of Plato (another famous philosopher) and the teacher of Alexander the Great, who later conquered the world. Greek banknote and coin picturing Aristotle

Aristotle’s idea of matter Aristotle did not believe that Democritus’s idea of atoms was correct. He believed all matter was made from four elements: Earth Air Fire Water

Aristotle’s idea of matter Aristotle was more famous than Democritus, so people believed him, even though he was wrong. So Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803.

Then in 1803… John Dalton, an English chemistry teacher, made several conclusions about atoms from his experiments: 1. All elements are composed of tiny particles called atoms, and the atoms of different elements are different from one another. 2. Atoms of different elements combine to form new substances in whole number ratios. 3. Chemical reactions occur when atoms are rearranged so that they attach to different atoms.

Dalton’s Chemical Symbols We use different symbols today.

Overarching Question

Activity: Atomic Pudding 1. Examine and compare the different models 2. Use new evidence to determine the correct sequence of the models 3. Identify the parts of the atom Objectives: I can interpret the results of each major experiment in the development of atomic theory I understand and can describe the structure of an atom, and am able to explain the differences between atoms of different elements

Atoms are the smallest units of an element Sulfur (S)

Average Atomic Mass and Reactivity

ChemCatalyst Which model represents Dalton’s idea that atoms are indivisible? Which model suggests that there are negative particles in an atom?

Atomic Theory All matter is made of atoms

Solid sphere model, 1803 Plum pudding model, 1897 nuclear model, 1911 Solar system model, 1913 cloud model, quantum model, first proposal of the atom, 442 BC

How did Thomson know to run an electrical current through matter? Michael Faraday, English chemist, born In 1834 he learned that matter would interact with electricity. He realized that electricity had to me made up of particles that could be counted, but he did not know what they were. (They were electrons.)

ChemCatalyst What are the three particles that make up an atom? What is the charge of each particle?

J.J. Thomson Discovered electron 1897 ◦ Cathode Ray Experiment: electric charges make materials lose negatively charged particles. ◦ Atoms contain negatively charged particles Plum Pudding model 1904 ◦ Electrons in a soup of positive charges

More About Charge… Opposite charges attract, alike charges repel (push away).

Cathode Ray Experiment

Thomson’s “Plum pudding” atom – raisin RULE: Matter is NEUTRAL, so for every electron (one negative charge) they must also have a proton (one positive charge) to balance the math to zero charge (“neutral”). ENGLISH PLUM PUDDING – electron

Ernest Rutherford Gold foil experiment: ◦ Shot alpha particles (positively charged particles) at gold atoms ◦ Most went through, but some bounced back Nuclear model 1911 ◦ An atom’s mass is mostly in the nucleus ◦ The nucleus has a positive charge ◦ Electrons in fixed orbit

Rutherford’s Gold Foil Experiment

Rutherford gold foil experiment Rutherford expected the  particles to pass straight through, like this… But what he got was most  particles went straight through, but a few were deflected, like this…

Problem with Rutherford’s Model But there also was a problem with Rutherford’s nuclear atom! To prevent the negative (-) electrons from being attracted to the positive (+) nucleus, the electrons would have to orbit the nucleus like the Earth orbits the sun. Why don’t the electrons crash into the nucleus?

Niels Bohr Each element emits specific set of wavelengths of light (colors) ◦ Wavelength = amount of energy ◦ Electrons of those elements have different energy levels Solar system Model 1913 ◦ Nucleus surrounded by orbiting electrons at different energy levels ◦ Electrons have definite orbits

Bohr’s Quantized Energy Levels Bohr’s new atomic model had quantized energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.). They absorb energy when they jump out, and emit (send out) energy when they jump in.

Ernest Rutherford Alpha particles can knock positively charged particles out of the nucleus. ◦ Nucleus contains positively charged particles called protons Proton model 1918 ◦ Central nucleus made of protons ◦ Tiny orbiting electrons (negatively charged) ◦ Most atom is empty space

ChemCatalyst Who discovered the electron? What did Bohr’s research change about the atom?

Ernst Schrödinger Quantum Mechanical Model 1926 ◦ Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed (Heisenberg uncertainty principle) Werner Heisenberg

Cloud/Quantum mechanical model

James Chadwick Discovered Neutron 1932 Neutrons hold positively charged nucleus together

The Complete Modern Atom An atom is mostly empty space with a nucleus containing protons and neutrons which contain most of the mass of an atom. Electrons are in energy levels around the nucleus, and are very tiny. Electrons jump between levels, emitting and absorbing energy as they jump. We often refer to these energy levels as “orbitals”, but real orbitals are much more complicated…

Simple atomic model Includes: 1. Rutherford’s discovery of the proton 2. Chadwick’s discovery of the neutron 3. Bohr’s discovery of electrons in orbits

Regions of an atom Nucleus: center of the atom Makes up most of its mass Electron cloud: Mostly empty space Take up most of the radius of the atom

Electron Cloud Nucleus Subatomic Particles Neutrons (Neutral) electrons (-) Protons (+)

Atomic Particles ParticleChargemassRelative mass proton x g 1 amu neutron01.67 x g 1 amu electron9.11 x g 1/1840 amu * amu = atomic mass units A neutral atom has no net charge: number of protons is equal to the number of electrons. #P + #e = 0 A neutral atom has no net charge: number of protons is equal to the number of electrons. #P + #e = 0

Images of atoms Scanning tunneling electron microscope 1981 Bombards surface with electrical current (electrons) Read changes in current, translate into image