Required reading: Introduction Atomic theory
Review units, dimensional analysis and conversion factors
The method of keeping the associated units throughout the calculation is called dimensional analysis. Consider you want to determine the volume of a cube, given L, the length of its side. You know that volume is equal to length cubed, L3, if L = 10 cm, then V = (10cm)3 = 1000 cm3. Our unit of volume in this case is cm3. Keeping the units throughout the problem, takes the guess work out of what the unit of measurement is in our answer.
If we were asked to report our unit of measure in liters, we know that 1 L = 1000 cm3 so converting from cm3to liters isn’t difficult.
cm3 cancel and our answer 1liter. The ratio 1L/103cm3 is called a conversion factor. Conversion factors allow you to convert between different units of the same measured quantity.
We can use this method to convert between U.S. and metric units. Say you wanted to convert 2.6 miles to kilometers. The conversion factor for miles to kilometers is 1.609 kilometer/mile. Therefore,
Rounded to two sig figs is 4.2 Km.
Dalton’s Atomic Theory
A British chemist by the name of John Dalton, provided the basis for the theory that all matter whether it’s compounds, elements or mixtures, is composed of small particles known as atoms. Dalton’s atomic theory consists of 4 postulates, which explain the structure of matter in terms of different combinations of small particles:
First, all matter is composed of indivisible atoms. Dalton defines an atom as a small particle of matter that retains its identity during chemical reactions.
Second, an element is a type of matter that is composed of only one atom and each atom has a given set of properties. Dalton postulated that mass was one such property. He predicted that atoms of a given element all have a characteristic mass.
Third, compounds are a type of matter composed of atoms of two or more elements that are combined together in fixed proportions. A good example is the compound water. Water exits as hydrogen and oxygen in a 2 to 1 ratio.
Fourth, a chemical reaction is the rearrangement of atoms present in the reacting substances. This rearrangement brings about new chemical combinations present in the substances formed by the reaction.
Structure of the Atom
Dalton postulated that atoms were indivisible particles, but experiments beginning around the last century began to prove otherwise. These experiments showed that atoms where made of two kinds of particles:
First, a nucleus which is at the core of the atom, positively charged and contains nearly the entire atom’s mass. The positive charge associated with the nucleus is due to the protons it contains.
Second, electrons which are very light negatively charged particles that exist in the region around the atom’s positively charged nucleus.
The individual responsible for the discovery of the electron was a British physicist by the name of JJ Thompson. Through cathode ray experiments conducted in 1897, Thompson observed the migration of cathode rays towards and an anode, where some rays pass through a hole to form a beam. Thompson observed that the beam bends away from a negatively charged plate. Thompson was able to show that cathode rays showed that characteristic regardless of material making up the cathode. Thus, he was able to conclude that the rays consist of a beam of negatively charged particles known as electrons. This discovery disproved Dalton’s first postulate that atoms are indivisible particles.
Thompson’s experiments allowed him to calculate the ratio of the electrons mass relative to its charge. In 1909 an American physicist Robert Millikan, devised an experiment that allowed him to determine the charge of the electron by observing how a charged drop of oil falls in the presence and in the absence of an electric field. From his experiments the charge of an electron was determined to be 1.602*10^-19 coulombs. Then using the mass to charge ratio, the mass of the electron was determined to be 9.109*10^-31 kg. This value is more than 1800 times smaller than the mass of the lightest atom, hydrogen.
Another British physicist Ernest Rutherford proposed the existence of the nucleus in 1911 after observing experiments performed in his laboratory by Hans Geiger and Ernest Marsden. They were observing the effect of bombarding thin gold foil with alpha radiation from radioactive substances such as uranium. The scientists found most of the alpha particles passed through the metal foil as though nothing were there; however a few, about 1 in 8000 particles were scattered at large angles and sometimes backwards. They were observing alpha particles being deflected by a massive, positively nuclei in the gold foil.
Rutherford’s model concluded that nearly all of the mass associated with the atom is concentrated in the positively charged nucleus with the negatively charged electrons traveling around it. Nuclei have a diameter of about 10^-3 PM, whereas atomic diameters are about 100 PM. To put this in perspective if you were to use a golf ball to represent the nucleus, the atom would be about 3 miles in diameter. It’s a bizarre concept to think about, but atoms, the building blocks of everything around us, are nearly empty space. Eventually, we’ll learn what they do with that space, but for now just consider the fact that atoms, the building blocks of matter, are merely empty space.
Nuclear Structure
The atomic number is the number of protons in the nucleus of the atom. Protons are nuclear particles with a charge equal to that of an electron; however, their mass is nearly 1800 times greater. Elements are groups of atoms that all of the same atomic number.
Neutrons are nuclear particles with a mass number nearly identical to that of a proton, but contain no charge. Neutrons serve the important role of holding the nucleus of the atom together, but that’s for another course entirely. Chemists, for the most part don’t concern themselves with the nucleus of the atom.
The mass number is the total number of protons and neutrons together in the nucleus. This accounts for nearly all of the associated mass of an element. The mass associated with electrons are considered negligible since they are nearly 1800 times smaller than protons and neutrons.
Isotopes are elements who have the same atomic number, but different mass numbers. Since elements are defined by the number of protons this means the elements have a different number neutrons. One of the central features of Dalton’s atomic theory was the idea that atoms of a given element have a characteristic mass. We now know that elements are a mixture of isotopes with each isotope having its own characteristic mass. Thus what Dalton was actually calculating in his experiments were average masses.
Periodic Table
A Russian chemist Dimitiri Mendeleev and a German chemist Lothar Meyer working independently made similar discoveries. They found that if they arranged the elements in order of atomic weight, the elements could be placed in horizontal rows, one row under each other so that elements in vertical columns have similar properties. This tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of elements, became known as the periodic table.
Periods are the elements in any one horizontal row of the periodic table
Groups are the elements in any one column of the periodic table. As noted earlier elements that are arranged in any one group have similar properties. For instance the group of elements in column one are known as alkali metals. These metals are characterized by their softness and react easily with water.
The group of elements in column 17 are known as the halogens. These elements are the most reactive elements on the periodic table. The halogen elements actually increase in reactivity as you move up the column. The element fluorine is at the top of the column and is the most reactive element on the periodic table.
Elements on the periodic table are divided by a heavy stair case on the right of the table that separates the metals on the left from the nonmetals on the right. A metal is a substance that is characterized by its luster and general ability to conduct heat and electricity. With the exception of the mercury, all metallic elements are solids at room temperature which we define to be about 20 degree Celsius or about 68 degrees Fahrenheit. Metals are more or less malleable, which means they can be hammered into sheets for applications like car bodies, air plane wings, etc. Metals are also ductile which means they can be drawn into wire for applications. The elements in group 11 of the periodic table are copper, silver and gold and are characterized by high ductility and electrical conductivity. This is why copper wire is used throughout our homes to transport electricity.
We define nonmetals as anything that doesn’t exhibit properties of metals. Many nonmetals are gas, for example chlorine and oxygen. Solid nonmetals tend to be hard, brittle substances with the exception of bromine which is a liquid.
The elements that are bordering the stair case of the periodic table are called metalloids or semimetals. These are elements that have both metallic and nonmetallic properties. These elements such as silicone or germanium usually make good semiconductors. Semiconductors are materials that exhibit both conductive properties like copper and have the ability to act as an insulator such as glass. The conductivity of semiconductors increases with increasing temperature; this is the opposite to that of a metal. You have probably heard a lot about semiconductors because of their wide range of uses in electronics.
Conclusion
Since Dalton’s day atomic theory has developed steadily and has been the cornerstone of chemistry ever since. The results of which have been enormous when you consider atoms can be rearranged through chemical reactions to give us all of the millions of compounds we encounter on a daily basis. In further lessons we’ll look closer at how to describe composition and structure for chemical compounds using their building blocks, which are of course, atoms.
This third video picks up were the second video left off about measurement and concludes with dimensional and unit analysis