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  [Home] [Chemistry 30 ] [Chemistry 20] [Science 10] [Science 9Last Updated: Wednesday  Jan. 31, 2001

Matter and Its Changes

What exactly is matter? Matter is anything that has mass and takes up space. There are two main ways of classifying matter. Matter is classified first by its physical state as a solid, liquid, or gas. Secondly, we classify matter by it chemical constitution as an element, a compound, or a mixture. We are going to learn about the differences between atoms, ions, and molecules, the differences between compounds, elements, and mixtures, and the difference between physical and chemical changes.

Solids, Liquids, and Gases

To understand what matter is, we first must comprehend the three different states that it can exist in. Those three physical states are solids, liquids, and gases. A good example to illustrate this is water. Water, in its solid state is ice, in its liquid state is liquid water, and in its gaseous state is steam.

Solids usually have a definite shape and a definite volume. However, when a solid is broken into smaller pieces it is not changed chemically. For example if you crush an aspirin into a power it is still a solid just in smaller pieces.

Now we have the problem of distinguishing between a liquid and a gas. What makes a liquid different from a gas is the characteristic of compressibility. A gas is easily compressible, where a liquid is not. Say for instance that you have a piston within an enclosed tube. If the tube is filled with steam, and then the piston is compressed, it is easy to compress the steam with the piston. As a result, the piston travels far into the tube. Now we put water into the enclosed tube. It is not nearly as easy to push the piston down into the tube now. Why? Well, a liquid is a lot harder to compress than a gas. This is because the molecules in the gas are farther apart than the molecules in the liquid.

commpression of a gas

These two characteristics that we have discussed, how rigid an object is, and an object's ability to be compressed, are used to determine the three basic states of matter

A solid is a form of matter which is made distinct by it rigidity. That is, a solid has a fairly fixed volume and shape, and is harder to compress than a gas or a liquid.

A liquid is a form of matter that is a fairly incompressible. This means that a liqiud basically has a fixed volume, but not a fixed shape. It takes the shape of its container.

A gas is an easily compressible fluid. This means that a given quantity of gas will fit into a container of any size and shape. A gas has neither a definite volume nor a definite shape.


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Atoms, Ions, and Molecules

An atom is an extremely small particle of matter that retains its identity during chemical reactions. During the latter nineteenth century a series of experiments showed that atoms are comprised of smaller particles. An atom consists of a nucleus and one or more electrons surrounding the nucleus. The nucleus, the core of the atom, has the majority of the mass of the atom, and a positive charge. An electron is a very light particle which circles the nucleus. It has a negative charge. In an electrically neutral atom, the number of electrons equals the positive charge on the nucleus. The nucleus of the atom is composed of two smaller particles called neutrons and protons. A proton has a positive charge equal in magnitude to the negative charge of an electron. This means that in an electrically neutral atom (one with an equal number of protons and electrons), the postive charge of the protons, combined with the negative charge of the electrons, would result in no charge because they would cancel each other out. A proton's mass, however, is a whopping 1836 times that of the electron. A neutron has a mass almost identical to a proton's, but it has no electrical charge associated with it.

A molecule is a definite group of atoms that are chemically bonded together. They are tightly connected by attractive forces. A molecular formula is a chemical formula that gives the exact number of different types of atoms in a molecule. Some simple molecular substances are carbon dioxide, CO2; ammonia, NH3; and water, H2O. The atoms that are in a molecule are not just stuffed together without any order. The atoms are chemically bonded to one another in order to form a definite arrangement. A structural formula is a chemical formula which shows how the atoms are bonded to one another to form a molecule. A good example is the structural formula for water, H-O-H. Those two horizontal lines connecting the H with the O (hydrogen and oxygen) represent the chemical bonds joining the atoms.

Structure of Water

An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons. Now what this means is that an ion is the result of taking away, or adding, electrons to an atom or a chemically bonded group of atoms. By taking away, or adding, these electrons, the particle takes on an electrical charge. Atoms are electrically neutral as they contain an equal number of positive and negative charges. An atom that adds an extra electron to it becomes a negatively charged ion. This type of ion is called an anion. An atom which loses one or more of its electrons now has a positive charge, and is called a cation. For example, a sodium atom can lose one of its electrons and form a sodium cation. Now, instead of being Na, it would be Na+1. This means that the sodium atom has an overall positive charge of +1. Another example would be a neutral atom of Sulfur, S. If this atom of S were to gain two electrons it would become S-2. The sulfur atom would now have a total negative charge of -2.

It has 16 protons and 18 electrons.


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Compounds/Elements/Mixtures

John Dalton (1766-1844) provided us with the Atomic Theory. His theory says that all matter is composed of small particles called atoms.
We now know that atoms are not indivisible , since they contain protons, neutrons, electrons and other subatomic particles. However, other than this mistake Dalton's ideas are essentially correct.

An element is a substance that is composed of only one kind of atom like aluminum, iron, or neon. Today, 109 elements are known and listed on the periodic table.

A compound is a substance of more than one element, chemically combined. A more scientific definition is that a compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. An example of a compound would be water. It is a compound that contains the elements hydrogen and oxygen fixed in the ratio 2 to 1. A compound has new properties unlike the elements which make it up. A compound has a chemical formula such as H2O.

A mixture is a material that can be separated by physical means into two or more substances. A classic example of a mixture lab would be one in which you were presented a mixture of sand, iron filings, and salt. You are told to separate these materials. How do you do that? Well, think about the various physical properties of each material. You use a magnet to separate out the iron filings. You then mix water with the sand and salt mixture. You swish the water, salt and sand around for a while and then filter it. The salt dissolved into the water, so the salt water solution passes through the filter while the sand gets left in the filter. Now we slowly heat up the salt water solution, and evaporate the water, and we are left with salt. In a mixture, the compounds are not in a definite proportion. For example, a teaspoon of salt in a liter of water is salt water, but so is a cup of salt in a liter of water.


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Physical & Chemical Changes

A physical change is a change in the form of matter but not in its identity. An example of a physical change would be the dissolving of one thing into another thing. For instance, dissolving sugar into water. The water and the sugar retain their chemical identities and can be separated by physical means. Another example is ice melting to water. Ice and water are both H2O. The identity of the matter is not changed, just the state that it is in.

A chemical change is a change in which one kind of matter is changed into a different type of matter. Some examples of chemical changes: the rusting of your car, setting your shoe on fire, digesting food, and the burning of magnesium metal in oxygen to form magnesium oxide. All of these materials combine chemically with another material , and cannot be separated by any physical means.


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Atomic Structure

 


In chemistry and physics the idea of the atom is a key concept. To understand many of the other concepts in chemistry some knowledge of the atom is necessary. In this section the following topics will be discussed:



 

Subatomic Particles

The basic conception of a subject now known as subatomic particle physics dates back to 500 BC when the Greek philosopher Leucippus and his pupil Democritus suggested that matter consists of small, indivisible particles, which they called atoms. For more than 2000 years after this, the notion of atoms lay in obscurity. For quite a long time, people believed that all matter consisted of four elements: earth, fire, air, and water. We now know that atoms do exist, and that some particles smaller than atoms also exist. These subatomic particles are divided into two main groups, the leptons and the hadrons. The best known lepton ("light" particle) is the electron. In order to account for the emission of electrons from the nucleus, the neutrino, an essentially massless neutral particle was postulated. The muon and the tau, both much more massive than the electron, comprise the rest of the lepton family. The hadrons are divided into two groups, the mesons and the baryons. Protons and neutrons are baryons. Mesons and baryons are made of smaller particles called quarks. There are six different quarks: up, down, charmed, strange, top, and bottom. While these are cool names, they convey nothing about the distinct properties of the quark. Each quark comes in three different colors: red blue and green. Again, the color label has nothing to do with the quark's appearance. Baryons are composed of three quarks, mesons are composed of a quark and an antiquark. Now that you have probably been thoroughly confused, move on, and hopefully that confusion will go away.




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Basic structure of an atom

The picture below is an example of the arrangement of the particles in an atom. Most of the atom is just empty space. The rest of the atom consists of a positively charged nucleus of protons and neutrons that are surrounded by a cloud of negatively charged electrons. The nucleus is the center of the atom. An atom is an extremely small particle of matter that retains its identity during chemical reactions.

During the latter nineteenth century a series of experiments showed that atoms are comprised of smaller particles. An atom consists of a nucleus and one or more electrons surrounding the nucleus. The nucleus, the core of the atom, has the majority of the mass of the atom and a positive charge. An electron is a very light particle which circles the nucleus. It has a negative charge.

In an electrically neutral atom, the number of electrons equals the positive charge on the nucleus. The nucleus of the atom is composed of smaller particles called neutrons and protons. A proton has a positive charge equal in magnitude to the negative charge of an electron. This means that in an electrically neutral atom, the postive of charge the protons, combined with the negative charge of the electrons, would result in no charge because they would cancel each other out. A proton's mass, however, is a whopping 1836 times that of the electron. A neutron, however, has a mass almost identical to a proton's, but it has no electrical charge associated with it.

Spinning Atom Gifexplination of Atom

Particle
Location
Weight
Charge
Proton Nucleus 1.0073 amu Positive
Neutron Nucleus 1.0087 amu Neutral
Electrons Electron Cloud 0.000549 amu Negative

 


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Atomic Number and Mass Number

The atomic number of an element is what distinguishes it from all other elements. An atom's atomic number is the number of protons there are in the nucleus. Hydrogen's atomic number is 1. Helium's atomic number is 2. Any atom that has an atomic number of 1 is a hydrogen atom no matter how many electrons or neutrons the atom has.

The mass number is the number of neutrons added to the number of protons. The mass number of the most common isotope can be obtained from the periodic table. If you take the decimal number on the periodic table and round it to the nearest whole number, you have the mass number. For example the atomic weight of Iron(Fe) is 55.847. When rounded it gives a mass number of 56.

The atomic number of Fe is 26. so most Fe atoms have 30 (56-26) neutrons. In addition, all neutral Fe atoms have 26 protons and 26 electrons. Atoms of the same element with a different number of neutrons are called isotopes. The most common isotope of an element is the one that is on the periodic table.

hydrogen isotopesexplain gif

The above graphic shows two isotopes of Hydrogen. The picture on the left is the most common isotope of hydrogen with one electron and one proton. The picture on the right is another isotope of hydrogen with one proton, one electron, and a neutron. The most common isotope of uranium is uranium-238 which has 92 protons, 92 electrons, and 146 neutrons. Another isotope is uranium-235 with 92 protons, 92 electrons, and 143 neutrons.


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Avagodro's number

Avogadro's number and the mole are very important to the understanding of atomic structure. The Mole is like a dozen. You can have a dozen guitars, a dozen roosters, or a dozen rocks. If you have 12 of anything then you would have what we call a dozen. The concept of the mole is just like the concept of a dozen. You can have a mole of anything. The number associated with a mole is Avogadro's number. Avogadro's number is 602,000,000,000,000,000,000,000 (6.02 x 1023). A mole of marbles would spread over the surface of the earth, and produce a layer about 50 miles thick. A mole of sand, spread over the United States, would produce a layer 3 inches deep. A mole of dollars could not be spent at the rate of a billion dollars a day over a trillion years. This shows you just how big a mole is. This number is so large that it is usually only represented in scientific notation.

Probably the only thing you will ever have a mole of is atoms or molecules. One mole of magnesium atoms (6.02 x 1023 magnesium atoms) weigh 24.3 grams. 6.02 x 1023 carbon atoms weigh a total of 12.0 grams. 6.02 x 1023 molecules of CO2 gas only weigh a total of 44.0 grams. The decimal number on the periodic table is the atomic mass, the mass of one atom measured in atomic mass units(amu). Amu's are defined to be 1/12 the weight of the most common isotope of Carbon. This number in grams is the mass of 1 mole of that element. For example, 6.02 x 1023 iron atoms weigh only 55.847 grams.(This is equivalent to saying one mole of iron atoms weighs 55.847 grams.) One mole of sulfur weighs 32.066 grams. (This is the same as saying 6.02 x 1023 Sulfur atoms weigh 32.066 grams)

When not measured in grams, the decimal number on the periodic table is called the atomic mass and is in atomic mass units(amu). As mentioned earlier, one proton weighs 1.0073 amu and 1 neutron weighs 1.0087 amu. So the atomic mass is the mass in amus of one atom of an element, but you rarely use the mass of one atom. Even if you have a tiny speck of a metal or a microgram of an element, you have billions and billions of atoms. Thus, the mass in grams of one mole of an element (the gram atomic weight) is more useful.


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Electron Configuration & Peroiodicity


Electronic Structure of Atoms

Each electron in an atom is described by four different quantum numbers. Three of these quantum numbers (n, l, and m) represent the three dimensions to space in which an electron could be found. A wave function for an electron gives the probability of finding the electron at various points in space. A wave function for an electron in an atom is called an atomic orbital. The fourth quantum number (ms) refers to a certain magnetic quality called spin.

n-The Principal Quantam Number

The n quantam number relates to the size of the atomic orbital. n can have any positive integer value from 1 to 7. The smaller the n, the lower the energy, the higher the value of n, the higher the energy. In the case of any single-electron atom, or hydrogen atom, n is the only quantum number which determines the energy. The size of an orbital depends on n. The larger the orbital, the larger the value of n. Orbitals of the same quantum state belong the the same shell. To use an analogy for n, why not relate it to the size of a computer, where larger values would represent larger houses.

Computer

l-The angular momentum quantum number

l can have any integer value from 0 to 3. This quantum number distinguishes orbitals of a given n value which have different states. Or, the secondary quantum number gives the shape of the orbital so the analogy can be made to the shape of the computer with larger values associated with computers with more components.

Computer

M-magnetic quantum number

The third quantum number has to do with the orientation of an orbital in a magnetic field. Because of this, we can relate its values to different directions the computer might be facing.

Computer

The final quantum number is the spin quantum number, it describes the spin orientation of an electron.

The electron configuration of an atom is the particular distribution of electrons among available shells. It is described by a notation that lists the subshell symbols, one after another. Each symbol has a subscript on the right giving the number of electrons in that subshell. For example, a configuration of the lithium atom (atomic number 3) with two electrons in the 1s subshell and one electron in the 2s subshell is written 1s22s1.

sublevel
orbital
maximum # of electrons
s
1
2
p
3
6
d
5
10
f
7
14

The notation for electron configuration gives the number of electrons in each subshell. The number of electrons in an atom of an element is given by the atomic number of that element.

On the left we have a diagram to show how the orbitals of a subshell are occupied by electrons. On the right there is a diagram for the filling order of electrons in a subshell.
 
 

fillingorder



Here are some examples that show how to use the filling order diagram to complete the electron configuration for a certain substance.

Element
# of Electrons in Element 
Electron Configuration
He
1s2
Li
1s22s1
Be
1s22s2
O
1s22s22p4
Cl
17 
1s22s22p63s23p5
K
19 
1s22s22p63s23p64s1

 

explanation

Often times you will be asked to find the electron configuration for something that looks like this:

53I

The 53 denotes the number of electrons in an atom of iodine. You would now proceed to do the electron configuration by looking at the filling order chart.

1s22s22p63s23p64s2 3d104p65s24d105p5


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Periodicity

With increasing atomic number, the electron configuration of the atoms display a periodic variation. Because of this the elements show periodic variations of both physical and chemical behavior. The periodic law is a law stating that when the elements are arranged by atomic number, their physical and chemical properties vary periodically. We are going to be looking at three physical properties of an atom: atomic radius, ionization energy, and electron affinity.

Atomic Radius

The size of the electron cloud increases as the principal quantum number increases. Therefore, as you look down the periodic table, the size of atoms in each group is going to increase. When you look across the periodic table, you see that all the atoms in each group have the same principal quantum number. However, for each element, the positive charge on the nucleus increases by one proton. This means that the outer electron cloud is pulled in a little tighter. One periodic property of atoms is that they tend to decrease in size from left to right across a period of the table. So finally we have a good definition for how the atomic radii increases: the atomic radii increases top to bottom and right to left in the periodic table.


Ionization Energy

The energy needed to remove the most loosely held electron from an atom is known as ionization energy. Ionization energies are periodic. The ionization energy tends to increase as atomic number increases in any horizontal row or period. In any column or group, there is a gradual decrease in ionization energy as the atomic number increases. Metals typically have a low ionization energy. Nonmetals typically have a high ionization energy.


Electron Affinity

The attraction of an atom for an electron is called electron affinity. Metals have low electron affinities while nonmetals have high electron affinities. The general trend as you go down a column is a decreasing tendancy to gain electrons. As you go across a row there is also a trend for a greater attraction for electrons.


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