Nomen ___________________________________________Dies ___________________________________________

Atomic History

Scientist Theory ExperimentDemocritus First person to

hypothesize that there were atoms, small indivisible particles of matter

N/A

Dalton 1. Matter is composed of indivisible particles (atoms)

2. All atoms of the same element are identical in size, mass, and chemical property.

3. Atoms of specific elements are structurally different from atoms of a different element.

4. Atoms combine in certain whole-number ratios to form compounds

5. In a chemical reaction, atoms are separated, combined, or rearranged.

N/A

Thought the atom was like a marble

J.J. Thompson Discovered the first subatomic particle, the electron.

Thought the atom was positively charged with negative electrons stuck in it, like raisin in pudding. The “plum pudding” model.

Cathode Ray Experiment: Gas is excited and emits cathode rays. These rays move in a straight line. When these rays are passed through an electromagnetic field (i.e. a field with a + and an – side), the rays bend towards to positive side. Since opposite sign attract, they must be negative. Electrons.

Rutherford Atom is mostly empty space. It contains a small,

Gold Foil: Radon is used to shoot positive alpha

dense, positively charged nucleus.

Planetary model – dense, positive nucleus at the center with electrons in orbits like planets around it

particles at thin gold foil. Most particles go straight through (atom mostly empty space) but some deflect and a few bounce straight back (due to dense positive nucleus)

Bohr Why don’t the negative electrons crash into the positive nucleus?

Electrons are in distinct energy orbitals around the nucleus. Each orbital has a different amount of energy. Electrons “jump” from one orbital to the other.

N/A

Schrodinger Orbitals are not circular orbits where electrons travel. They are defined as the area that has the highest probability where the electron can reside. “Clouds”. Electrons move so fast have the properties of waves. Wave-Mechanical Model.

Mathematical model only. Equation that defines the probable areas where electrons can be found.

Heisenberg Uncertainty Principle: You can never know exactly where an electron is AND how fast it is moving.

Atomic Structure

Proton Neutron ElectronLocation Nucleus Nucleus In a cloud outside

the nucleusMass 1 AMU 1 AMU 1/1600 AMUCharge +1 0 -1Extra Info Proton number is

the atomic number. This defines the element.

Neutrons contribute to the mass and the stability of the nucleus of an atom

Electrons account for the chemical reactivity of the atom. Found in orbitals outside the nucleus.

Atomic Number = # of Protons

Mass Number = # of protons + # of neutrons = Atomic Number + # of neutrons

AMU: 1 amu is defined as 1/12 the mass of a Carbon-12 atom.

***If you are given the atomic mass, you can subtract the atomic number to find the number of neutrons!***

In a neutral atom, i.e. an atom that does not have a charge overall, the number of protons equals the number of electrons!!

Isotopes are atoms of the same element (i.e. have the same atomic number, same number of protons) that have different masses.

The mass number in the periodic table is the average mass number, i.e. the weighted average of all naturally occurring isotopes of that element. When doing problems with isotopes, always use the atomic mass given to you not the average mass.

Solving Average Atomic Mass Problems:

Atomic Mass (amu) Percent Abundance Total (Multiply AM by %) (amu)

14 0.04 0.5612 0.95 11.411 0.01 0.11

ADD 12.07

Ways to right isotopes:

14C (mass written in the upper left hand side)Carbon-14 (mass written after name)

Number of protons can always be found on the periodic table.

Nuclear Chemistry – Review

Radioisotope – an isotope of an element whose nucleus is unstable; emits radiation to become more stable.

Two reason nuclei could be unstable:

(1) Ratio of neutrons to protons is outside of belt of stability (2) Too large (> 83 protons)

For smaller nuclei, atoms tend to want a 1:1 ratio of neutrons to protons. For large nuclei, atoms tend to want a ration of 1.5:1 neutrons to protons.

Penetration power – gamma > beta > alpha

Symbol Charge Penetration Power

Alpha

He24

+2Low, paper stops it

Beta

e

-1Medium, clothes

stop it

Gamma 0High, only metal

stops it

Positron 0+1e−

+1 Medium, similar to beta particles

Neutron 1 n0

0 (Used in artificial transmutation)

Balancing Nuclear Reactions

Type of Decay

Particle Emitted

Change in Mass

Change in Atomic #

Alpha Helium nucleus

Decrease by 4

Decrease by 2

Beta Beta or an electron

No change Increase by 1

Gamma X-Ray Photon

No change No change

Positron Beta plus or positron

No change Decrease by 1

Electron Capture

Beta or an electron

No change Decrease by 1

Balancing Nuclear Equations:(1) Atomic masses on both sides MUST balance(2) Atomic numbers on both sides MUST balance

Emission: particle on the right (product)Capture: particle on the left (reactant)

Half-Life

Radioactive substances decay at constant rateThe rate of decay is measured in half-lives

Example: The half life of Strontium-90 is 29 years. How much is left in a 10g sample after four half lives?

**Two ways to solve!**

Way 1: Set up chart

Example:Number of Half Lives Elapsed Time (years) Amount of

Strontium-90 Left (grams)

0 0 101 29 52 58 2.53 87 1.254 116 0.625

Steps:

1. Set up three columns – Number of Half Live, Elapsed Time, and Amount Remaining

2. Place a zero (O) in the first row of # of half-lives and elapsed time. This row is for your original amount!

3. In the next row, put 1 for #of half-lives and look up the half-life in Table N. Put the time in “Time”. This is the amount of time that has passed after one half-life.

4. In the “Amount Remaining” column, divide by 2 every time you move down a row (i.e. another half-life has passed) and multiply by two as you move up (in the case where you are finding the original amount).

5. In the “Time” column, you add the half-life amount as you go down (DO NOT MULTIPLY!!!). I.e. if the half life is 2 years, your column should go 0yr, 2 yr, 4yr, 6yr, 8yr NOT 0yr, 2, 4, 8, 16, etc.

6. Write “X” in the row you are trying to solve for. Use the rules above to solve.

Way 2: Formula!

N = N0

(1/2)

(t/t1/2)

Where:

N = amount remainingNo = original amoundT = time T1/2 = half-life

**Nota bene: To find original amount and time, you would need to be able to solve exponential equations.

Transmutation: When atoms of one element gain or lose subatomic particles to become atoms of another element, i.e. the atomic number changes!Natural Transmutation: Unstable radioisotope decays. Ie. Alpha, beta, gamma, electron capture, positron decay

Artificial Transmutation: High speed particle hits nucleus and causes decay. I.e. fission, fusion, etc.

Fission: Heavy nuclei split into lighter nuclei with the release of energy. Some mass converted into energy. Used in nuclear reactions

Fusion: Light nuclei slammed together to create heavier elements. Happens in sun and the stars.

Applications

Electron Configurations

Bohr Model:

Each row on the periodic table represents a different energy shell in the Bohr model.

(1) The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus.

(2) Bohr used the term energy levels (or shells) to describe these orbits of differing energy. He said that the energy of an electron is quantized, meaning electrons can have one energy level or another but nothing in between.

(3) The energy level an electron normally occupies is called its ground state. But it can move to a higher-energy, less-stable level, or shell, by absorbing energy. This higher-energy, less-stable state is called the electron’s excited state.

(4) After it’s done being excited, the electron can return to its original ground state by releasing the energy it has absorbed, as shown in the diagram below.

(5) Sometimes the energy released by electrons occupies the portion of the electromagnetic spectrum (the range of wavelengths of energy) that humans detect as visible light. Slight variations in the amount of the energy are seen as light of different colors.

**Bohr’s model only really worked for hydrogen**

N = 1 - 2 electronsN = 2 – 8 electronsN = 3 - 18 electrons

Valence electrons – Electrons in the outermost energy shell. These are the electrons involved in chemical reactions.

Led to…the Quantum-Mechanical Model

The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. This is known as the Uncertainty Principle.

The quantum mechanical model of the atom uses complex shapes of orbitals (sometimes called electron clouds), volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty.

Four numbers, called quantum numbers, were introduced to describe the characteristics of electrons and their orbitals:

Principal quantum number: nAngular momentum quantum number: lMagnetic quantum number: m(l)Spin quantum number: m(s)

Principal Quantum Number (n)

Distance away from the nucleus (higher levels equals further away, more energy)

Can have positive, whole number integers; 1, 2, 3, 4, 5, etc. Row on periodic table

Angular Momentum Quantum Number (l)

Defines the shape of the orbital. Combined with the n defines the shape and size.

0 = s (sphere)1 = p (dumbbell)2 = d (double dumbbell)3 = f4 = g

Can have value from 0 to n-1.

Example: in the 5th energy level can have 0, 1, 2, 3, 4

Magnetic Quantum Number (m(l))

Defines orientation in space. Can tell you have many orbitals for a given shape.

Range is –l to +l

Example: For p orbitals can have -1, 0 +1 (i.e. three orbitals oriented in different directions)

Spin Quantum Number Describes electron spin. Can have one up and one down electron per orbital.

I.e. in the p orbital, there are three orbitals. For each of those you can have two electrons (one up, one down) which means 6 total.