The Atom: The Periodic Table & Electron Configuration

Moseley organized the Periodic Table based on atomic number and the physical and chemical properties of the elements.  What we know now is that those properties are caused by the element's electron configuration (how the electrons are arranged in the electron cloud).  Have you ever noticed that the Periodic Table is not an orderly box?  It seems to have a big gap in the middle or is divided into sections?

The reason the Periodic Table looks this way is because of where the last electrons are being placed into the electron cloud.  The Periodic Table is arranged by atomic number (# protons), but for neutral atoms, with each new proton comes another electron. So we can also say that as a result of the atomic number, for a neutral atom, the Periodic Table is also arranged by increasing number of electrons.

With each period, electrons are being placed in a higher energy level (Principle Quantum Number). Their location can also tell us which shape (Orbital Quantum Number) the last electron is placed.  See the picture below.

























Using the Periodic Table it is possible to determine the electron configuration of an element.

The Atom: Quantum Mechanics

Born, Planck, Heisenberg and Schrondinger built on the work of Rutherford to develop the Quantum Mechanical Model of the Atom.  Here are the primary points:

1. Energy is ultimately quantized into small discrete packet that we call photons.
2. Electrons can absorb (gain) or emit (lose) photons, but they can't use or lose a portion of a photon.
   - When an electron gains/absorbs a photon it will become excited and jump to a higher energy level.
   - When an electron loses/emits a photon it will has less energy and jump to a lower energy level.
   - Electrons DO NOT MOVE (Newtonian concept), they JUMP.  They cannot use a portion/fraction of a photon.  It's all or nothing.
   - When electrons jump from one energy level to another, it is called a Quantum Leap.  They have gained or lost a quantized amount of energy- a photon.
3. Heisenberg Uncertainty Principle- (basic) The exact velocity and location of an electron cannot be determined without changing its velocity and/or location.  We can only determine an electron's probable location in the electron cloud.  This probable position is explained by the 4 Quantum Numbers.
4. Quantum Numbers
• Principle Quantum Number describes the distance from the nucleus, or the energy level.
• Orbital Quantum Number describes the shape of the electron cloud.
• Magnetic Quantum Number describes the orientation of the electron cloud in space with respect to the 3 axis.
• Spin is a notation of up/down to represent that the electrons within a pair will always be as far apart as possible (Remember- like charges repel).
5. Pauli Exclusion Principle- No 2 electrons in the same atom can have the same 4 Quantum Numbers.  A pair of electrons in the same atom will be in the same energy level, shape and orientation, but they will always be as far apart as possible.

The Atom: Development of a Modern Model

In 1897 British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electron in a series of experiments using a cathode ray tube at Cambridge University.  He said that there were "bodies much smaller than atoms" that had a negative charge.  He called the negative particles electrons. Thomson took the model of the atom one step further with his plum-pudding model.  While you might not be familiar with plum-pudding, you are probably very familiar with a chocolate cookie.  Thomson theorized that if there were negative particles in the atom, there must also be a positive charge to make an atom neutral. His model described the atom as being basically positive (the cookie) with areas of negative that can be broken off (the chips).


One of Thomson's students, Ernest Rutherford built on his research and further developed our modern concept of the atom. One of his first discoveries was that radiation was not a single substance but three. While he was at McGill University in Canada, he found that radiation could be influenced by an electrical charge.  The radiation that was attracted to a negative charged, he called alpha (positive charge).  The radiation that was attracted to a positive charge he called beta (negative charge).  Later gamma radiation was found to be unaffected by any charge (neutral).


After receiving the Nobel Prize, Rutherford moved to Manchester and attracted many young scientists to his research group. Under his direction, they continued to study the atoms and radiation.  The Gold Foil Experiment was to become one of the most famous.  In this experiment, alpha particles (+) were fired at a very thin sheet of gold foil.  It was expected that the particles would go straight through the foil like bowling balls through tissue paper.  Imagine their surprise when some of the particles seemed to be deflected, and even bounced back off.  This observation led to the conclusion that the atom is mostly empty space, with a small, hard positively charged center.  Rutherford called this the nucleus.  This group would go on to discover the proton in 1917.


Later Rutherford and a fellow Danish physicist, Neils Bohr, developed a new model of the atom.  They were bothered by the concept that if electrons were small and negative, and the nucleus was dense and positive, why don't the electrons simply fall into the nucleus?  They developed a model that described the atom as having a small positive nucleus with the electrons orbiting the nucleus in energy levels.  In this model, the electrons are held in the atom by the electrostatic attraction between the positive nucleus and negative electrons, but are constantly being pushed outward by their angular momentum.  This is the familiar planetary model that is commonly taught to young students.


During this same time, Max Planck was studying electromagnetic radiation in Berlin. Among his many discoveries, he found that EMR is quantized. That it is carried by small, discrete packets that he called photons. These small quantities cannot be divided, they are the smallest amount of energy.  This can be very difficult for us to conceptualize.  We live in a world where we can divide amounts easily into smaller amounts.  You don't have to use an entire gallon of gas at a time, you can just use a little, then a little more.  But when you start to look at the world on the subatomic level, you reach a point where it is all or nothing.  Think of money.  We normally think in terms of dollars.  That's the level we are comfortable with, that we use everyday. Dollars can be broken down into quarters, dimes, nickels and even pennies.  We don't think of paying for everything in pennies because they are very small in relation to what we are buying- but imagine buying something very small. Can you pay for something worth a portion of a penny?  No, you either use a whole penny or not.  Photons are the pennies of the electromagnetic spectrum.


Bohr and Rutherford incorporated this concept into their model. Now an electron could not move from one level to another, it had to jump or make a quantum leap. When an electron absorbs a photon, it will jump to a higher energy level, when an electron emits a photon, it will fall to a lower energy level.  Again, this is difficult for us to imagine.  We think in terms of something orbiting, but in reality electrons don't orbit, they jump. 


Werner Heisenberg, Max Born and Wolfgang Pauli took these ideas and formed the quantum mechanical model of the atom.

The Atom: The Periodic Table

The modern concept of the atom can be traced back to Democritus in 465 BC.  Unlike most Greeks of his time, he believed that everything was made up of small, indivisible shapes that he called atomos.

For the next 2 thousand years, most thinkers, philosophers and alchemists followed the basic idea set down by Aristotle.  He argued that everything was composed of some combination of the 4 elements- fire, earth, air and water.  This belief slowly began to break down during the Renaissance. This was when belief and debate began to be replaced with experimentation and observation.

Antoine Lavoisier (1743-1794) began to pull the ideas of the previous 200 years when he clearly stated the Law of Conservation of Matter.  From his experiments, he found that even if the appearance of a substance may change, the total mass of the materials before a change will always equal the total mass after the change. Thanks to Einstein, we now realize that we can make nuclear changes that will convert matter into energy, but Lavoisier's concept is still valid for chemical changes.
Law of Conservation of Matter- matter can neither be created nor destroyed by ordinary chemical means.

In the early nineteenth century, Dalton further defined matter by clearly stating that all matter is composed of atoms. He said an atom is the smallest unit of an element that can exist alone or in combination with other atoms of the same or different elements.

1. All matter is made up of very small particles called atoms.
2. Atoms of the same element are all chemically alike: atoms of different elements are chemically different.
3. The atoms of different elements have different average masses.
4. Atoms are not subdivided in chemical reactions, they unite in simple ratios to form compounds.

Over the next century, many others made contributions to our concept of the atom. Many different types of atoms (elements) were identified and investigated.  By the end of the nineteenth century, about 60 elements had been discovered and described.  Dmitri Mendeleev was the first to publicly attempt to organize the elements and look for patterns.  He organized the known elements by atomic weight and their known properties.  He was even able to predict the properties and atomic weights of elements that had not been discovered yet, but there were problems with his table.  There were several elements that seemed to be in the wrong place.  One of the most glaring was iodine.  Iodine clearly had properties similar to bromine, but it fell in the same row with oxygen.

Henry Gwyn Jeffreys Mosley (1887-1915):  Mosley began to work on the inconsistencies found in Mendeleev's table at Oxford. He arranged the elements in order of their atomic numbers horizontally and formed columns of elements with similar properties. Tragically for the development of science, Moseley was killed in action during WW1 at Gallipoli in 1915. 

The modern periodic law states that if the elements are arranged by increasing atomic number, their properties will reoccur at regular intervals.

Isotopic Notation

Isotopes are atoms with the same number of protons but have a different number of neutrons.  They are basically different varieties of the same element.  Two of the most common isotopes that people hear about are Carbon-14 and Carbon-12.  Both isotopes are carbon because they each have 6 protons.  They are different because C-14 has 8 neutrons and C-12 only has 6.  While C-12 is the most abundant (common) isotope of carbon, both are called isotopes.

The number of protons in an atom is also called the atomic number.  The number of protons determines the identity of an atom, no matter how many neutrons or electrons are in the atom.  Atomic number is listed on the periodic table.  It is a whole number and is usually listed above the symbol for the element.  If you change the number of protons in an atom, the element has also changed.

Mass number is the total number of particles in the nucleus of an atom.  In other words, it is the number of protons plus the number of neutrons.  Isotopes have the same atomic number, but different mass numbers.  Mass numbers are NOT listed on the periodic table.  When you name a specific isotope, you MUST include its mass number, for instance Carbon-14 or Uranium-135.

The number below the symbol of the element on the periodic table is called the atomic mass.  It is the weighted average mass number of all the isotopes of a particular element.  Because it is an average, it has significant digits.

We can represent an isotope in an abbreviated form.  This is called the isotopic notation.

Here is the isotopic notation for carbon 14.

From looking at the isotopic notation, you can determine the number of protons, neutrons and electrons for a given atom.

In a neutral carbon 14 atom, determine the number of protons, neutrons and electrons?
Protons- The atomic number is 6, therefore there are 6 protons
Electrons- If the atoms is neutral the number of protons equals the number of electrons, therefore there are also 6 electrons.
Neutrons- The mass number equals the number of protons + neutrons, therefore 14-6 leaves 8 neutrons.

Chemical Reactions: The Basics

Chemical Reactions describe chemical changes.  Chemical Equations are shorthand descriptions of chemical reactions that use coefficients, symbols and subscripts to describe the ratios of a reaction.  We call the substances before a reaction the reactants and we call the substances that are formed the products.

Antoine-Laurent de Lavoisier (1743-1794) is considered by many to be the father of chemistry. He was the first to clearly state the Law of Conservation of Mass.  This states that matter is neither created nor destroyed in a chemical change.  We can now add to that the idea that the atoms or building blocks of matter are simply rearranged in a chemical reaction.

John Dalton (1766-1844) took Lavoisier’s ideas further by developing the first basic atomic theory.  He stated that an atom is the smallest unit of an element that can exist either alone or in combination with other atoms of the same or different elements.

His supporting evidence:

1. All matter is made up of very small particles called atoms.

2. Atoms of the same element are all chemically alike: atoms of different elements are chemically different.

3. Individual atoms of the same element may not have the same exact mass (isotopes), but for all practical purposes, they all have a definite average mass.

4. The atoms of different elements have different average masses.

5. Atoms are not subdivided in chemical reactions, they unite in simple ratios to form compounds.

He also developed the Law of Multiple Proportions.  If two elements combine to form more than one compound, they will combine in distinct whole number ratios.

Mixtures

Pure substances are uniform throughout with a definite composition and properties, while mixtures are physical combinations of two or more pure substances. The properties of the substances in a mixture retain their own properties.

We can further divide pure substance in chemistry into elements and compounds.  An element is basically the name of a type of atom defined by the number of protons in the nucleus.  A compound in a chemical combination of two or more elements.  A compound is the name of a type of molecule (chemically bonded atoms).

Note:  Physical combination means that substances are just dispersed or close together. A chemical combination means that the atoms are chemical attached creating new molecules with new properties.  Chemical bonds cannot be separated by physical means.

Mixtures can be divided into homogeneous mixtures that appear to be a single substance and heterogeneous mixtures that are obviously two or more substances.

All mixtures are composed of a solute and a solvent. The solute is the substance that is dissolved or dispersed, while the solvent is the substance that does the dissolving.  The solvent separates and keeps the solute particles apart.

Mixture can be divided into three categories:

Solutions are homogeneous and clear.  The particles are so small they cannot be seen and do not reflect light.

     Ex:  windex, tap water, air

Colloids are homogeneous but appear cloudy.  Some of the particles are large enough to reflect light even though they can’t be seen with the naked eye.

     Ex:  milk, fog, mayonnaise

Suspensions are heterogeneous.  Given time gravity will separate a suspension with the most dense particles on the bottom, and the least dense (lightest) particles rising to the top.

     Ex:  Italian salad dressing, oil and water

Describing Matter

In lab we make both qualitative and quantitative measurements.  We can further describe matter in terms of extensive and intensive properties.  Extensive properties are quantity specific, such as mass and volume. The mass of a sample of water depends on how much water you have in a sample. Intensive properties are dependent only on the type of matter, not the quantity.  The density of water is 1.0 g/mL whether you have a drop or a swimming pool full.

We can further divide observations into physical and chemical properties.  Physical properties describe matter without changing its composition.  Chemical properties describe how matter interacts or changes with other matter.

Physical Properties                                Chemical Properties

Color                                                                Rusting

Odor                                                                 Burning

Density                                                            Tarnishing

Boiling Point

Malleability

The state of matter, and its boiling and melting points are all physical properties.  The state of matter is determined by the arrangement of its particles.

The change of state of a substance is a physical change.

                                    Boiling                                                Evaporating

                                    Melting                                               Condensing

                                    Freezing                                              Sublimation

                                    Solidifying

If the identity of a substance is changed, or a new substance is formed, it is a chemical change.