Studying Atoms Lecture 4.1

Objectives: Describe ancient Greek models of matter List the main points of Dalton’s atomic theory and describe his evidence for the existence of atoms Explain how Thomson and Rutherford used data from experiments to produce their atomic models

Do you know what makes up the parts of an atom? Daily Question Do you know what makes up the parts of an atom?

Overview Most natural materials are mixtures of pure substances “Pure” substances are either elements or compounds

Ancient Greek Models Compounds always contain the same elements in the same proportions Law of Constant Composition Democritus (460-370 B.C.): was the first to use the word “atomos”( which meant atoms); stated that matter is composed of atoms; atoms are indestructible and indivisible

Ancient Greek Models Aristotle (384-322 B. C.): said that empty space could not exist; matter is made of earth, fire, air, and water

Dalton’s Theory Dalton proposed that all matter is made up of individual particles called atoms, which cannot be divided.

Dalton’s Atomic Theory (1808) 1) Elements are made of tiny particles called atoms 2) Atoms of any element are identical 3) Atoms of an element are different from atoms of any other element 4) Atoms of elements combine to form compounds Always the same numbers and types of atoms

Dalton’s Atomic Theory (1808) 5) Atoms are indivisible Cannot be created or destroyed Atoms rearrange during chemical reactions

Importance of Dalton’s Theory It explained many important observations Conservation of mass Constant composition

Atoms and Conservation of Mass + A B 1 mass unit 3 mass units 4 mass units A B C + 4 mass units 5 mass units 2 mass units 7 mass units

Thomson Thomson took Dalton’s model, and expanded on it. His experiments provided the first evidence that atoms are made of even smaller particles.

Importance of Thomson’s Experiments Confirmed existence of electrons Implied existence of protons

Schematic of a cathode ray tube.

Cathode Ray Experiments Proved the existence of electrons Sir Joseph John Thomson Mass of the electron (9.109 x 10-28 g) Approximately 1/1800 the mass of the simplest hydrogen atom The electron is the smallest possible negative charge All other negative charges are whole-number multiples of the electron

Plum Pudding model of Thompson pg 114

Models of the Atom THOMSON “Plum Pudding” - - - DALTON Indivisible -

The Nucleus A dense, positively charged mass located at the center of the atom Central portion of the atom Contains most of the atom’s mass, but . . . Occupies only a tiny fraction of its volume The plural form of nucleus is nuclei

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Earnest Rutherford (1899) Discovered that uranium emits fast-moving particles with positive charges He wanted to test Thompson’s Plum pudding model He predicted that most of the particles would go straight through According to Rutherford’s model, all of the atom’s positive charge is concentrated in its nucleus

Rutherford’s Gold Foil experiment.

Results of foil experiment… If the Plum Pudding model had been correct.

Actual results

Rutherford’s Model of an Atom

Niels Bohr (1885-1962) Worked with Rutherford and agreed with Rutherford’s model But his model focused more on the electrons which surrounded the nucleus

Bohr’s Model Electrons move with constant speed in fixed orbits around the nucleus Like planets around a sun Each electron in an atom has a specific amount of energy Electron can gain or lose energy

Bohr Model Picture

Evidence of Energy Levels Movement of electrons between energy levels explains the light you see from fireworks Light is form of energy No two elements have same set of energy levels , so different elements emit different colors of light

Firework Colorants Color Compound Red strontium salts, lithium salts lithium carbonate, Li2CO3 = red strontium carbonate, SrCO3 = bright red Orange calcium salts calcium chloride, CaCl2 calcium sulfate, CaSO4·xH2O, where x = 0,2,3,5 Gold incandescence of iron (with carbon), charcoal, or lampblack Yellow sodium compounds sodium nitrate, NaNO3 cryolite, Na3AlF6 Electric White white-hot metal, such as magnesium or aluminum barium oxide, BaO Green barium compounds + chlorine producer barium chloride, BaCl+ = bright green Blue copper compounds + chlorine producer copper acetoarsenite (Paris Green), Cu3As2O3Cu(C2H3O2)2 = blue copper (I) chloride, CuCl = turquoise blue Purple mixture of strontium (red) and copper (blue) compounds Silver burning aluminum, titanium, or magnesium powder or flakes Firework Colorants

Electron Cloud Scientists use the electron cloud model to describe the possible locations of electrons around the nucleus Electrons move in a less predictable way

Orbital Is a region of space around the nucleus where an electron is likely to be found The electron cloud represents all of the orbitals in an atom

Figure 15 pg 117 Energy Level # of orbitals Max. # of electrons 1 2 4 8 3 9 18 16 32

Electron Configuration arrangement of electrons in the orbitals The most stable electrons in orbitals with lowest possible energies Electron moving up an higher energy orbital= configuration is in excited state

Subatomic particles Particle Symbol Location Charge Rel. mass Actual mass electron e- Space around nucleus 1- 1/1840 9.11 x 10-28 Proton p In nucleus 1+ 1 1.673 x 10-24 Neutron n In nucleas 1.675 x 10-24

4.2 Structure of the Atom

The Nucleus is: Positively charged Very Dense the center portion of the atom Contains most of the atom’s mass, but . . . Occupies only a tiny fraction of its volume

Composition of the Nucleus: Proton found in the nucleus of an atom Positive charge Mass of 1.673 x 10-24 grams or 1amu Neutron found in the nucleus of an atom. No electrical charge (neutral) Mass approximately equal to the proton

Composition of the Nucleus: Electron moves around the nucleus Negative charge Very small compared to proton and neutron 1/1800 relatively far away from the nucleus Most of the atom is empty space Atom Electron Nucleus

Protons, Neutrons, and Electrons Relative Mass Charge proton 1 1/1800 +1 –1 neutron electron

Atomic number The atomic number (“Z”) = number of protons in an atom The atomic number identifies element elements have different numbers of protons

Mass number Mass number (“A”) =protons + neutrons. Mass number identifies the particular isotope In an atom, electrons equal the number of protons makes atoms electrically neutral

Nuclear Symbol Mass Number 65 Cu 29 Atomic Number (Z)

Cu 65 29 Protons Remember!! Atoms are neutral!! Protons + Neutrons 29 Electrons 65 Cu 36 Neutrons 29 Protons Remember!! Atoms are neutral!!

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Why do atoms have different properties? Chemical properties are related to the number and arrangement of electrons Electrons “intermingle” when molecules form

Isotopes Isotopes are atoms of the same element with different mass numbers Same atomic number Different mass numbers because they have different numbers of neutrons

2. The Hydrogen Atom An element is identified by the number of protons in the nucleus Hydrogen exists in three forms Protium One p+, one e– Most abundant form Deuterium One p+, one n0, one e– Tritium One p+, two n0, one e–

Two isotopes of sodium.

How to calculate the number of neutrons in an atom? Number of neutrons= mass number – atomic number

Practice Write the nuclear symbol for bromine-80 How many protons, electrons, and neutrons are in this atom? Write the hyphen notation for the element that has 6 protons and 7 neutrons How many electrons does this atom have? Write the nuclear symbol for this atom