Chemistry and Matter
Section 1
Matter is anything that has mass (material with-in) and volume (takes up space). Atoms are known as the “building blocks” of matter. All matter is made of atoms. Atoms combine to form molecules and compounds (these will be discussed later) to make the things around us.
An atom is the smallest piece of a substance. 7 million atoms joined together in a straight line would be about 1mm long.Themiddle of the atom is called the nucleus. Itcontains protons and neutrons. The protons have a positive electrical charge while the neutrons have no electrical charge. All atoms have electronsorbiting around the nucleus. Electrons have a negative electrical charge. The number of protons is the same as the number of electrons. If an atom loses or gains electrons, it is called an ion. Ions are positively or negatively charged atoms.
Each proton has an electrical charge of +1. Each electron has a charge of -1. The neutron has no charge so it is considered neutral. An atoms charge is neutral when it has the same number of protons and electrons.
Particle / Relative Mass / Relative ChargeProton / 1 / +1
Neutron / 1 / 0
Electron / .054 % / -1
The masses of neutrons and protons are about the same. An electron is about .054 % the size of a proton. The electrons, although tiny, take up most of the space in an atom. This means most of the space of an atom contains very little mass. Nearly all the mass is centered at the nucleus.
An element is a substance made from only one type of atom.For example, Carbon is made entirely from Carbon atoms and Sodium is made entirely from Sodium atoms.An element can not be broken down (chemically) into simpler substance.The Periodic Table shows all known the elements.
The Periodic Table
The Periodic Table came about through attempts by people to group elements according to their chemical properties. John Newlands (1863) noticed that every eighth element seemed to have similar properties when arranged in order of increasing atomic mass. He proposed a similarity with music, where the eighth note is an octave above the first. This became known as Newlands Octaves. But it did not work for the fourth period with the transition metals. It works for the lighter elements because eight electrons complete their outer shells.
Dmitri Mendeleev is credited as being the Father of the modern Periodic Table. In 1869 he arranged the 50 or so known elements in order of atomic mass, putting elements with similar properties in the same vertical group and leaving gaps for unknown elements that were yet to be discovered. When the elements were later discovered, they were found to have the properties predicted by Mendeleev's table. Knowing nothing of protons, nuclei or atomic numbers, Dmitri Mendeleev's Periodic Table was mostly correct. In the modern Periodic Table, the elements are
Group 1 / Alkali MetalsGroup 2 / Alkaline Earth Metals
Group 3-12 / Transition Metals
Group 13 / Boron Family
Group 14 / Carbon Family
Group 15 / Nitrogen Family
Group 16 / Oxygen Family
Group 17 / Halogens
Group 18 / Noble Gases
arranged in order of increasing atomic number.
The columns in the Periodic Table are called groups or families. The rows are called periods (hence the name Periodic Table). The groups or families contain elements with similar chemical properties. Some of the groups have names, here is a list of those names.
The groups may also show the number of valence electrons that an element has. These groups are sometimes labeled with Roman Numerals. GroupIA has one valence electron, group IIA has two valence electrons and the number of valence electrons would continue to go up by one. Group O would have eight valence electrons.
The metal elements are on the left and in the center of the Periodic Table. The nonmetal elements are on the right. Metals and nonmetals are separated by metalloids. Metalloids have properties of both metals and nonmetals. They border the “staircase” that separates the metals and nonmetals. All elements in the same group have the same number of electrons in their outer shells. This is what gives the element its chemical properties. Chemical properties determine how the element will react with another element or substance. Chemists use the electron structure of elements to classify them as metals or nonmetals. As a general rule, elements with three or fewer electrons in the outer level are considered to be metals. Elements with five or more electrons in the outer level are considered to be nonmetals. Transition elements have one or two electrons in the outer level so they show metallic properties.
The number of electron shells an element has, shows the period (row) in which it is found. The first period contains Hydrogen and Helium. The second period is from Lithium to Neon. The third period is from Sodium to Argon etc.Potassium is in GroupIA showing it has 1 electron in its outer shell and since it is in period 4, it has 4 shells. Fluorine in group VIIA showing it has 7 electrons in its outer shell and since it is in period 2 it has 2 shells.
The Transition Metals: The transition metals occupy the central block of the Periodic Table. They are not a group (there is no group number) but are a collection of metals with common properties. They are sometimes called a family of metals. They do not all have the same number of electrons in their outer shell.
Atomic Number
The number of protons in the nucleus is called the atomic number. Sodium (Na) has 11 protons so it is atomic number 11. The atomic number helps you locate the elementon the Periodic Table. An element is matter that contains only one type of atom. For example, Carbon is made entirely from Carbon atoms and Sodium is made entirely from Sodium atoms.An element cannot be broken down (chemically) into a more simple substance.
Mass Number
The number of protons plus the number of neutrons is called the mass number. Sodium has 11 protons and 12 neutrons so the mass number is 23. The mass number is the total number of particles in the nucleus. The mass number and the atomic number are written above and below the chemical symbol for the element, as shown. These numbers may be reversed on some Periodic Tables. The mass number will be the number with the decimal point on many Periodic Tables.. The chemical symbol is a shorthand way of writing the element name on the Periodic Table and in chemical formulas.
Element / Symbol / Atomic Number / Protons / Electrons / Neutrons / Mass NumberSodium / Na / 11 / 11 / 11 / 12 / 23
Iron / Fe / 26 / 26 / 26 / 30 / 56
Aluminum / Al / 13 / 13 / 13 / 14 / 27
Isotopes
Atoms of the same element that have different numbers of neutrons are called isotopes. For example, Chlorine (atomic number 17) may have either 18 or 20 neutrons. The mass number will be either 35 or 37. Isotopes of the same element will have the same chemical properties, because the number of protons and electrons will be the same.
Section 2
Electron Shells (energy levels)
Electrons are arranged in shells (shown as circles around the nucleus). The shells are also called energy levels or orbitals. We will use the term shell. Chemists use letters to name the shells around a nucleus. They use the letters "k,l,m,n,o,p, and q". The "k" shell is the one closest to the nucleus and "q" is the farthest away. Not all shells hold the same number of electrons. The general rule for calculating the number of electrons for the first four shells is 2n2 where n represents the shell number. For the first eighteen elements, there are some easy rules. The “k” shell only holds two electrons. The “l” shell only holds eight electrons. The “m” shell only holds eight electrons (for the first eighteen elements). The “m” shell can actually hold up to 18 electrons as you move farther along the Periodic Table. The maximum number of electrons you will find in any shell is 32. An atom that has a full outer shell will be stable. Being stablemeans the atom will not react with other atoms. The Noble Gases in the right hand column of the Periodic Table are stable because they have full outer shells. The two inner shells of an atom must be full before the outer shells get filled. If the outer shell of an atom has less than the maximum number of electrons it will not be stable so it will react with other atoms forming chemical bonds.
Chemical Reactions and Bonding
All chemical reactionsinvolve atomstrying to get a full outer shellof electrons.When an atom reacts with another atom, it will either: 1. Lose electrons to form a stable positive ion, 2. Gain electrons to form a stable negative ion or 3. Share electrons to form a stable molecule.
Ionic Bonding
An unstable Sodium atom may lose its outer electronto become stable. Sodium has 1 electron in its outer shell. Because of this, it is in Group IA of the Periodic Table. When sodium reacts, it will lose its outer electron. Its outer shell will then have no electrons. It is as though the outer shell has vanished. The next shell in is full. This full shell becomes the new outer shell so the Sodium is now stable. The Sodium ion still has 11 protons (11 positive charges), but now only 10 electrons (10 negative charges). So the Sodium ion has an extra positive charge, shown by a + sign.
The reaction between Sodium and Chlorine is different from that of Sodium and Sodium. Chlorine has 7 electrons in its outer shell so it is in Group VIIA of the Periodic Table.When an atom of Chlorine reacts with Sodium, it will gain one electron from the Sodium. The outer shell of Chlorine will then have 8 electrons and be stable. The stable Chloride ion will have an extra electron so it will have an extra negative charge shown as a - sign.
The force of attraction between the oppositely charged ions is called an ionic bond. Sometimes they are shown as dots and crosses.The chemical equation for the above reaction is: Sodium (solid) + Chlorine (gas) = Sodium Chloride (solid).
Na(s) +Cl(g) =NaCl(s)
Metals and Nonmetals
Metals are malleable meaning they can be pressed or beaten into thin sheets. Metals can conduct heat and electricity.Metals have high melting and boiling points. They are all solid at room temperature except for Mercury which is liquid. Nonmetals have properties different than metals. Nonmetals are generally gases or brittle solids. Their surfaces are dull and they are insulators.
When a metal reacts with a non-metal, the metal will lose electrons to form a positive ion while the non-metal will gain electrons forming a negative ion. Together they form an ionic compound.
This is the reaction between Magnesium and Oxygen.Magnesium is in Group IIA. A Magnesium atom will lose 2 electrons to form a stable ion. Oxygen is in Group VIA. An Oxygen atom will gain 2 electrons to form a stable ion. In this example, the electrons are shown as dots and crosses.
The ionic bond between Magnesium and Oxygen is stronger than the ionic bond between Sodium and Chlorine because of the greater charge on the ions. Magnesium Oxide has a higher melting point because of the stronger bond.
The Noble Gases are in Group 18 or Group O of the Periodic Table. They have full outer shells of electrons. They are stable and will not react with other atoms.Atoms that have lost or gained electrons to form ions will have a full outer shell. Stable ions are said to have achieved a Noble Gas electron structure.
Properties of Ionic Compounds
When metals react with non-metals they form an ionic compound. A compound is a type of matter that has properties different from the properties of each of the elements in it.Ions have a charge because electrons are lost or gained in forming an ionic bond. Ionic bonds are strong. Ionic bonds can only be separated by a chemical change. All ionic compounds are solid at room temperature.Solid ionic compounds do not conduct electricity because the ions are not free to move.If the solid is heated until it melts, the liquid will conduct electricity because the ions can move.
Covalent Bonding
When non-metals react with non-metals, they share electrons to form a covalent bond. Covalentmeans sharing. (If you do not know whether an element is a metal or a non-metal, check the Periodic Table). Covalently bonded compounds containing a small number of atoms are called simple molecules. A molecule is the smallest particle of a compound that still retains all of the properties of the compound. Two Chlorine atoms will share one electron each to form a stable Cl2 molecule. Each Chlorine atom in the molecule has 8 electrons in its outer shell by sharing the two electrons between them. The outer shell is now stable and the Cl2 molecule will not react further with chlorine. There are no ions present (no + or - charges) because the electrons are shared, not transferred from one atom to another.
These pictures are simple covalent molecules. Count the electrons in the outer shell of each atom, to see that they add up to 8 for the Nitrogen and Chlorine or 2 for Hydrogen.
Note the 3 pairs (6 electrons) shared between the atoms. Nitrogen has a triple bond. Each electron pair is one bond. This is what makes nitrogen so stable and unwilling to react with other atoms.
Note the 2 pairs (4 electrons) shared between the atoms. Oxygen has a double bond. Each electron pair is one bond.
Note the shape of the water molecule, with both Hydrogen atoms on the same side of the oxygen atom. This “polarizes” the molecule making it seem as though it has a negative end (Oxygen) and a positive end (Hydrogen).
Properties of Molecules
When a non-metal reacts with a non-metal, they form a covalently bonded molecule.Covalent bonds are strong but only exist between the atoms of the molecule. The force of attraction between molecules (called the intermolecular force) is very weak. The weak force between molecules means that very little energy is required to separate them. Many molecular compounds are liquid or gas at room temperature. They have low melting and boiling points.
When a molecular compound melts or boils molecules separate from each other but the covalent bond between the atoms of the molecule does not break. The molecule is the same molecule in the solid, liquid or gas state. The molecule is said to decompose (break-up) if the covalent bond breaks between its atoms.
Molecules do not have a charge (no ions) because the electrons are shared between atoms to form a covalent bond. Molecular compounds will therefore not conduct electricity (with the exception of Graphite).
Chemical Formulas
Name / Formula / Number of Elements / Names of Elements / Number of Atoms for Each Element / Total Number of Atoms in One MoleculeWater / H2O / 2 / Hydrogen
Oxygen / 2
1 / 3
Ammonia / NH3 / 2 / Nitrogen
Hydrogen / 1
3 / 4
Methane / 2CH4 / 2 /
Carbon
Hydrogen
/ 28 / 5
Chemicals are represented by formulas. Each chemical has its own unique chemical formula. The formula is shown by element symbols with small numbers, (subscripts), written after the symbols. KClrepresents one Potassium atom bonded to one Chlorine atom. The formula is not written, K1Cl1. Na2O means two Sodium atoms are bonded with one Oxygen atom. Al2O3 means two Aluminum atoms are bonded to three Oxygen atoms. A big number in front of a formula multiplies the number of elements that follow it in the formula. 2K means two separate potassium atoms. 2Cl2 represents 2 Cl2 molecules equaling four chlorine atoms total. 2KCl represents 2 KCl molecules equaling two Potassium atoms and two Chlorine atoms total.
Elements, Compounds and Mixtures
Elements: An element is a substance made from only one type of atom.For example, Carbon is made entirely from Carbon atoms and Sodium is made entirely from Sodium atoms.An element can not be broken down (chemically) into simpler substance.The Periodic Table shows all known the elements.
A compound is a substance made from two or more elements that have reacted chemically with each other. Molecules may make-up compounds. A molecule is the smallest particle of a compound that still retains all of the properties of the compound. A compound is a completely new material that will often have totally different properties from the elements that made it.For example, the element Sodium is a highly reactive metal. The element Chlorine is a yellow-green poisonous gas (non-metal). When the two react together, they form a compound called Sodium Chloride. Sodium Chloride is common salt, which you eat with food. (You wouldn't want to eat either element separately). You cannot separate the elements of a compound by physical methods. It can only be done by using chemical reactions or by passing electricity through it (if it conducts electricity).