Chapter 4 & 5

Atomic Models and Structure

1. Atomic TheoryA. General

i. as early as 400BC, a few people believed in an atomic theory

a. the idea that matter is made of tiny particles called atomsb. Democritus was one of the first supporters of this idea

ii. until recently scientists had no evidence of atoms

2. Dalton's Atomic Theory

A. Generali. in 1808, John Dalton developed an atomic theory

a. he believed that a few kinds of atoms made up all matter, that elements are composedof one kind of atom and compounds are made from 2 or more kinds of atomsb. he also reasoned that only whole numbers of atoms could combine to form compounds

B. Dalton's Atomic Theory contains 5 principlesi. all matter is composed of extremely small particles called atoms which cannot be subdivided, created or destroyedii. atoms of a given element are identicaliii. atoms of different elements are differentiv. atoms of different elements combine in simple, whole-number ratios to form compoundsv. in chemical reactions, atoms are combined,

separated, or rearranged but never created or destroyed

-today, we have revised his theorya. we know now that atoms can be divided into

smaller particles, many atoms of the same element combine to form molecules, and all atoms of the same element have the same number of protons but they

can have different numbers of neutrons

-Subatomic Particles1. General

A. in the mid 1800's scientists discovered that atoms were made of smaller particles, called subatomic particles

i. the three subatomic particles are the electron, proton, and the neutron

2. Electrons were discovered by using cathode rays A. J.J. Thomson (1897) pumped most of the air out of a glass tube and applied an electrical current to it

i. the positive end was called the anode and the negative end was called the cathodeii. he observed a glowing beam in the tube that came from the cathode and struck the anode, so he called them cathode rays and the tube was called a cathode ray tube

3. An electron has a negative chargeA. Thomson knew the rays must have come from the cathode which had a negative charge, so he reasoned that the rays were negatively chargedB. To test this hypothesis, he placed a magnet near the tube

i. the beam deflected (see fig. 6, p.80)C. He also placed a paddle wheel in the path of the beam, and it turned

D. These 2 experiments showed that electrons are indeed particles and they have a negative chargeE. An electron has a negative charge, but an atom is neutral. Also, electrons have much less mass than that of an atom. That means there must be other particles in an atom and some must have positive charges that balance out the negatives F. we can use the symbol e- to represent an electron

*Millikan discovers the charge of an individual electron and calculates the mass of an electron using an oil drop experiment

-relative charge = -1-mass – 1/1840 the mass of a hydrogen atom

Goldstein discovers protons (1886)-he found rays in a cathode ray tube that traveled in the opposite direction then the cathode rays so they must be positively

charged

Chadwick discovered neutrons (1932)-subatomic particles with no charge (see next few slides)

Physicists think protons and neutrons are made of even smaller particles called quarks

4. Rutherford Discovered the Nucleus A. Thomson proposed that the electrons of an atom were embedded in a positively charged ball of matter

i. this is called the plum pudding modelB. In 1909, Ernest Rutherford (one of Thomson's former students) performed experiments that

disproved the plum pudding modeli. the experiment is referred to as "the Gold Foil" experiment because; Rutherford directed a beam of small, positively charged particles (called alpha particles) at a thin sheet of gold foil ii. the results of the experiment showed that most of the alpha particles went right through the foil, with a few alpha particles being deflected (some even went backwards)

C. The fact that most of the alpha particles went straight through the foil shows that atoms are made mostly of empty spaceD. The fact that a few of the alpha particles were deflected backwards shows that there is a very tiny positively charged core to the atom

i. this small positively charged core is called the nucleus

ii. the nucleus contains almost all of the atoms mass and all of its positive charge, but only a very small fraction of its volumeiii. the nucleus is made up of proton and neutrons

5. Protons and Neutrons Compose the NucleusA. The nucleus of an atom is very small compared to the overall size of the atom

i. if you think of the nucleus as the size of a marble, than the entire atom would be the size of a football field

B. The positively charged particles are called protons i. they have a charge opposite but equal to the charge of an electron ii. their mass is about 2000 times greater that the mass of an electron

C. Since protons and electrons have opposite but equal charges and the overall charge of the atom is neutral, atoms must contain equal numbers of protons and electrons

D. The mass of an atom is still greater than the mass of the protons and electrons combined

i. this means that there is another subatomic particleE. About 30 years after the discovery of the electron, Irene Joliot-Curie discovered that when alpha particles hit a sample of beryllium, a beam that could penetrate almost anything was produced. James Chadwick found that this beam was not deflected by electric or magnetic fields.

i. that means the beam had no charge, it was neutralii. these neutral particles are called neutrons and they are found in the nucleus of all atoms except most hydrogen

F. The number of protons in an atom is called the atomic number and it determines the identity of the element

i. each element has a specific number of protons in its nucleus

6. Protons and Neutrons form a stable nucleus

-Atomic Number and Mass Number1. General

A. all atoms are made of protons and electrons and most have neutronsB. protons and neutrons make up the nucleusC. the electrons occupy the space surrounding the nucleusD. elements differ from each other in the number of the protons their atoms contain

2. Atomic Number is the Number of Protons of the Nucleus A. the atomic number of an atom is its number of protonsB. the atomic number is always a whole number, atoms do not contain partial protonsC. no 2 elements have the same number of protons or atomic numberD. we can use the number of protons (the atomic number) to identify an elementE. To date, scientists have identified 118 elements F. Since the number of protons in a neutral atom is equal to the number of electrons, the atomic number also tells us the number of electrons in a neutral atom

3. Mass Number is the Number of Particles in the NucleusA. mass number is equal to the number of protons

plus the number of neutronsi. therefore, to calculate the number of neutrons in an atom, subtract the atomic number from the mass number

*mass number - atomic number = number of neutronsB. unlike the atomic number, mass number can vary

among atoms of the same elementi. all atoms of the same element have the same number of protons, but they can have different numbers of neutronsii. atoms of the same element with different numbers of neutrons (different mass numbers) are called isotopes

C. Now we know how to find the number of protons, electrons and neutrons in an atom

5. Atomic Structure Can Be Represented by SymbolsA. Each element has a name and the same name is given to all of the atoms of that elementB. Each element also has a symbol

i. the symbol is taken from the name of the element

a. it is usually the first letter of the elements name (ex. Oxygen - O)b. if more than one element begins with the same letter, the symbol can be the first and second letter or the first and some other letter in the elements name (Helium - He)c. in some cases the symbols come from the latin or greek form of the elements name (ex. Iron - Fe)-these symbols do not use any letters from the elements name

C. The elements atomic number and mass number are sometimes written with an elements symbol

i. the atomic number is the smaller whole numberii. the mass number is the larger of the two and it may have a decimal in itiii. both numbers can be written with the symbol, or one, or neither

Isotopes of an element have the same atomic number

A. all atoms of the same element have the same number of protons, but they can have different numbers of neutrons and electrons

B. Remember, atoms of the same element with different numbers of neutrons (different mass numbers) are called isotopes

-Atomic MassA. Masses of Atoms are Expressed in Atomic Mass units

1. atoms are so small that they do not have much mass, this means that the gram is not a very

convenient unit for measuring their masses. 2. therefore we use a special mass unit to express atomic mass (the mass of an atom)

i. this unit has 2 names: the atomic mass unit (amu) and the Dalton (Da)ii. we will use amu

3. you can find the atomic mass for an element by checking the periodic table

i. the mass number on the periodic table is an average atomic mass of all the isotopic forms of the element

a. remember, isotopes are atoms of the same element with different masses due to different numbers of neutronsb. the mass listed on the periodic table takes all of the isotopic masses into accountc. to calculate the average atomic mass for an element given the isotopic forms and their percentages:

-change all percentages to decimals-multiply the masses by their relative percentages-add all of the answers together

d. this is the reason that the mass numbers given on the periodic table have decimals in them

Atomic Models1. General

A. models help scientists imagine what may be happening at the microscopic levelB. models have limitations and may need to be revised or replaced as new information arises

2. Rutherford's Model Proposed Electron OrbitsA. Rutherford's goldfoil experiment led to the

replacement of Thomson's plum-pudding model of the atom

B. Rutherford suggested that electrons are around the nucleus C. 2 years later, Niels Bohr revised Rutherford's model by explaining that the electrons orbit the nucleus in specific energy levels (planetary atomic model)

3. Bohr's Model Confines Electrons to Energy LevelsA. Bohr reasoned that electrons can be only certain distances from the nucleus

i. each distance corresponds to a certain quantity of energy that electrons can have

ii. An electron in its lowest energy level is closest to the nucleus (this is called the ground state)iii. The higher the energy level the farther from the nucleus an

electron isiv. The difference between 2 energy levels is known as a quantum of energy

B. the energy levels in Bohr's model can be compared to the rungs on a ladderC. an electron can be in only one level or another, not between levelsD. electrons can move between levels by gaining or giving off energy, but while remaining in the same level electrons do not change energy

4. Electrons Act Like Both Particles and WavesA. Thomson's experiments demonstrated that

electrons act like particles that have massB. Louis de Broglie pointed out that the electrons in Bohr's model behave as waves with certain

frequencies that correspond to the energy levels in which electrons are found

C. This is called the wave-particle duality of nature (electrons have properties of both waves (energy) and particles)

D. the present day model of the atom takes into account both the wave and particle properties of electrons

i. in this model, electrons are located in orbitalsa. regions around the nucleus that correspond to specific energy levelsb. a region in an atom where there is a high probability of finding electrons

ii. we call this model of orbitals an electron cloud because they do not have sharp boundaries

E. when an orbital is drawn it shows the area an electron is likely to be, but electrons can be found elsewhereF. ex. the blades of a fan, when the blades are spinning, you know that each blade is within the spinning image but you cannot tell exactly where one blade is

-Electrons and Light1.General

A. Albert Einstein proposed that light has the properties of both waves and particles

i. wave properties - light has frequencies, wavelengths and speeds

-wavelength is the distant between wave peaks & frequency is the number of wave that pass a certain point in a given time period-the wave length of light can vary , the broad range of light wavelengths makes up the electromagnetic spectrum

ii. particle properties-when light strikes a metal, electrons are released (this is called the photoelectric effect)

a. to remove an electron, a particle has to have at least a minimum energy and therefore a minimum frequency

B. According to Einstein, light can be described as a stream as particles, the energy of which is determined by the lights frequency

2.Light is an Electromagnetic WaveA. the electromagnetic spectrum is made of waves with many different wavelengths and frequenciesB. the visible spectrum is only a small section of the electromagnetic spectrumC. some of the other waves in the spectrum include X-rays, Ultraviolet, Infrared, Microwaves, and Radio waves D. All of these waves are referred to as light, even though we can't see themE. The frequency and wavelength of a wave vary inverselyF. The visible spectrum consists of ROYGBIV

i. red has the longest wavelength in the visible spectrum and violet has the shortest

3. Light EmissionA. Every element contains a certain # of

electrons, those electrons can move from 1 energy level to the next

B. they can move to higher levels by absorbing energy and back to lower levels by releasing energy

C. This energy is released as light that has a specific wavelength

D. movement between different levels releases light of a different wavelength and these different lights make what is called and line-emission spectrum E. each element has its own specific line-emission spectrum and we can use them to identify elements

4.Light Provides Information About ElectronsA. the lowest energy that an electron can occupy is called the ground stateB. when an electron absorbs energy it moves to a higher energy and it is in an excited state

-Quantum Numbers

1.the present day model of the atom where electrons are located in orbitals is also known as the quantum model2.to describe the region in which the electrons can be found around the nucleus, scientist have assigned 4 quantum numbers to each electron

A. these numbers describe the properties of an electron and no 2 electrons in the same atom can

have the same set of numbers because they cannot be in the exact same energy state

i. this is known as the Pauli Exclusion Principle

B. the four quantum numbers are as follows:i. the principal quantum number, symbolized by n, indicates the main energy level an electron occupies

a. as n increases, the electron's distance from the nucleus increases and so does its energy

b. this can indicate the size of the electron cloudii. the angular momentum quantum number, symbolized by l, represents the sublevel an electron occupies

a. whatever the level number you are in tells you how many sublevels are in that level (until past the 4th level)

-ex. If you are in level 3, there are 3 sublevelsb. there are 4 possible sublevels, they are referred to as the s, p, d, f sublevels c. this can indicate the shape or type of the orbital

-see page 131

iii. the magnetic quantum number, symbolized by m, indicates the orbital (subset of the sublevel)

a. indicates the number and orientations of the orbitals around the nucleus

b. each sublevel can hold a specific number of orbitals-s=1, p=3, d=5, f=7

c. each orbital can hold 2 electronsiv. the spin quantum number, symbolized by an up or down arrow

a. indicates the orientation of an electrons magnetic field b. in order for 2 electrons to occupy the same orbital they must have opposite spins

-Electron Configuration1.General

A. Remember, no 2 electrons in an atom can have the same set of 4 quantum numbersB. The arrangement of electrons in an atom is called its electron configurationC. electrons tend to assume arrangements that have the lowest possible energyD. different sublevels have different numbers of

orbitals with different shapesi. see page 131

2.An Electron Occupies the Lowest Energy Level AvailableA. The aufbau principle states that the structure of each successive elements is obtained by adding one proton to the nucleus and one electron to the lowest energy orbital that is available

i. electrons fill orbitals that have the lowest energy firstii. they start in the lowest level, the lowest sublevel etc.

B. we use arrow diagrams to show electron configurations for elements

i. in these diagrams, we begin by drawing the levels, sublevels and orbitals for the electrons

ii. the orbitals are represented by circles, and each circle can hold 2 electrons that are represented by arrows

a. the arrows must go in the circles with one pointing up and one going down, this shows that they have different spins and therefore different quantum numbers (which agrees with the Pauli Exclusion Principle)

iii. the arrows begin filling in the first level in its only sublevel and orbital, then they go to the next level and fill in the first sublevel in its only orbital before going to the next sublevel with its 3 orbitals. These three orbitals are filled in with one electron at a time and then the electrons go back and double up in the orbitals. Etc.

C. the order for filling in the 1st 18 electrons is straight forward

i. the electron fill in the 1s, 2s, 2p, 3s, & 3p sublevelsD. the order after the first 18, electrons begins to jump around, but there is a pattern that is usually followed

i. see arrow diagram E. there are elements that do not follow the pattern, or can have more than one configuration

C. remember the p, d, and f sublevels have more than 1 orbital. Electrons in these sublevels fill in the orbitals one at a time before backtracking to double up in the orbitals

i. this is known as Hund's rule which states: for an atom in the ground state, the number of unpaired electrons is the maximum number possible and these unpaired electrons have the same spinii. think of these orbitals as seats on a bus. Students fill in the bus seats singly until there are no empty seats left. Then they go back and double up

3. An electron configuration is a shorthand notationA. we can take our arrow diagram and use it to name the electron configuration of an element

i. this "name" shows which levels and sublevels contain electrons and how many are in each ii.1s2 2s2 2p4

B. each elements configuration builds on the previous elements configuration, so to save space when writing these configurations we can write the symbol of the previous noble gas (last row of elements on the

periodic table, they have a full outer level) and then write the additional electrons in the configuration out (ex. [Ne]3s2)