Atomic Structure and Chemical Bonds Chapter 16
Sec7on 1 Why do Atoms Combine? Let s remember the basics: Ma@er is made up of Atoms The center of an atom contains a small nucleus that has a posi7ve charge. The rest of the atom is mostly empty space except for the 7ny electrons that have a nega7ve charge.
Electrons Where are electrons in an atom located? How do they travel? Electron cloud Around the nucleus in predictable areas (orbitals), but the exact posi7on of any par7cular electron cannot be calculated
Each element has it s own, unique atomic structure with a specific number of protons, neutrons, and electrons. In a neutral atom, the number of protons will always equal the number of electrons
Electrons! Although electrons are incredibly small, they are responsible for many of the physical and chemical proper7es of the element Energy levels- The different loca0ons for an electron in an atom The electrons can be located closer to or farther from the nucleus, and this changes the energy of the electron
Each energy level can hold a maximum number of electrons The farther an energy level is from the nucleus, the more electrons it can hold The first level can hold up to 2 electrons The second level can hold up to 8 electrons The third level can hold up to 18 electrons The fourth level can hold up to 32 electrons
Electrons nearest the nucleus have the least energy (they are in energy level 1) Electrons farthest away from the nucleus have the most energy (they are in energy level 4) You can calculate the maximum number of electrons that can occupy an energy level by using the equa7on 2n 2 where n is the number of the energy level
Why do the electrons that are the farthest from the nucleus have the most energy? Electrons are nega7vely charged and are therefore a@racted to the posi7vely charged nucleus (remember: opposites a@ract). The closer the electron to the nucleus the more strongly a@racted they are. It is easier to remove electrons that are farther away from the nucleus because they have a weaker a@rac7on to the nucleus.
How the Periodic Table Helps Us The periodic table is arranged in periods according to increasing atomic number What is atomic number? The number of protons in the nucleus of an atom The atoms are neutral, therefore the number of protons equals the number of electrons, so the atomic number also tells you the number of electrons in in atom of the element
Atomic Stability An atom is considered stable when it s outer energy level is full For example: Helium has an atomic # of 2. It has 2 electrons in energy level 1 (remember, energy level 1 can only hold 2 electrons). This energy level is full, so helium is stable.
Let s try another one: Lithium Group1, Period 2, atomic # 3 Lithium has 3 protons and 3 electrons 2 electrons will fill energy level 1 and 1 electron will be in energy level 2 (this is why the alkali metals are so reac7ve, there is only 1 electron in the outermost energy level!)
What about Carbon? Where is it located (group/ period)? Group 14, period 2 What is its atomic number? 6 How many electrons does it have? 6 Where are the electrons located? 2 in the first energy level, 4 in the second
Stable Octet Any element that has 8 electrons in its outermost energy level is stable This is why the noble gases are so stable Neon: Group 18, period 2, atomic #10 10 electrons total 2 electrons in the 1 st energy level, 8 in the second (stable octet)
Argon: Group 18, period 2, atomic #18 18 electrons total 2 electrons in the 1 st energy level, 8 electrons in the 2 nd energy level, 8 electrons in the 3 rd energy level (stable octet) Even though the 3 rd energy level is not full (the 3 rd level can hold 18 electrons), it is s7ll stable because it contains the magic number of 8 electrons in its outermost energy level
Element Families Element groups (families) have similar chemical proper7es because they have the same number of electrons in their outermost energy levels Alkali metals- 1 outer electron Alkaline earth metals- 2 outer electrons Halogens- 7 outer electrons Noble gases- 8 outer electrons
Halogens All the halogens need one electron to achieve a stable octet (they are in group 17 and have 7 electrons in their outermost energy level) The more reac7ve halogens form bonds more easily Reac7vity of halogens decreases down the group (fluorine is the most reac7ve because its outermost energy level is closer to the nucleus making its ability to a@ract another electron the greatest)
Alkali Metals The alkali metals all have one electron in their outer energy level All alkali metals form compounds that are similar to each other because of this one outer electron This outer electron is easier to remove from more reac7ve atoms Alkali metals increase in reac7vity DOWN the group (the opposite of the halogens) Why? Going down the group, the atoms become larger and the outer energy levels are farther away from the nucleus. There is less aarac0on between outer electrons and nucleus so they are easier to remove.
Electron Dot Diagrams Electron dot diagrams show only the electrons in the outer energy level This makes it easier to show reac7ons between atoms because the outer electrons determine how the element will react To make an electron dot diagram you surround the element symbol with as many dots as there are electrons in its outer energy level
For example: For the representa7ve elements (groups 1-2, 13-18) you can use the periodic table: Group 1 has 1 outer electron Group 2 has 2 outer electrons Group 13 has 3 outer electrons Group 14 has 4 outer electrons Etc to Group 18, which has 8 outer electrons (except for Helium which has 2)
The dots are wri@en in pairs on four sides of the element You start off with one dot and follow around the element un7l you run out of outer electrons or the energy level is full (remember how many electrons each level can hold) Let s use Oxygen as an example: It is in Group 16 so it has 6 outer electrons O
Chemical Bonds Once you know how to write electron dot diagrams you can use them to show how atoms bond with each other Chemical bond The force that holds two atoms together Atoms bond so that they can be more stable (they want their outer energy levels to be like noble gases- a stable octet)
How Elements Bond Sec7on 2
Atoms form bonds with other atoms using the electrons in their outermost energy levels These bonds do not easily fall apart The bonds can be formed by: - Losing electrons - Gaining electrons - Pooling electrons - Sharing electrons
Ionic Bonds The alkali metals all have 1 electron in their outermost energy level When this electron is lost, they end up with 8 outer electrons- a stable octet The alkali metals tend to react with the halogens, why? Because the halogens have 7 electrons in their outermost energy level, adding one more will make them have a stable octet
Na loses its lone outermost electron Cl accepts this lone electron from Na Now both Na and Cl are stable, crea7ng NaCl (table salt)
Ions Can an atom lose or gain an electron and remain the same? No. When an electron is lost or gained the balance of charges changes. When an electron is lost, the atom becomes posi7vely charged (there is now one less electron than there are protons in the nucleus), and is referred to as a posi7vely charged ion When an electron is gained, the atom becomes nega7vely charged (there is now one more electron than there are protons in the nucleus), and is referred to as a nega7vely charged ion Ions are atoms that are no longer neutral because they have either lost or gained an electron
Let s go back to Na and Cl Na has become a posi7vely charged ion by losing an electron and is now wri@en Na + Cl has become a nega7vely charged ion by gaining an electron and is now wri@en Cl -
Ionic Bonds The posi7ve Na ion and the nega7ve Cl ion are strongly a@racted to each other (remember, opposites a@ract) ionic bond The bond that holds ions (atoms of opposite charges) together Compound a pure substance containing 2 or more elements that are chemically bonded
Ionic bonds form between metals and nonmetals Ionic bonds can form between more than 2 atoms Alkaline earth metals have 2 electrons in their outermost energy levels. They donate these electrons to other atoms to become more stable
For example: Magnesium Chloride It would take 2 chlorine atoms to accept Magnesium s 2 outer Electron Mg gave up 2 electrons, it is now wri@en as Mg 2+ The 2 nega7vely charged Cl ions are a@racted to the posi7vely charged Mg ion and form ionic bonds crea7ng MgCl 2, magnesium chloride
Elements from Group 16 (the Oxygen Family) also combine with alkaline earth metals as they need 2 electrons to make them stable
Metallic Bonding- Pooling Metals can form bonds with other metals via metallic bonds The outer electrons of metals are not held 7ghtly to the individual atoms, they move freely among all the ions in the metal forming a shared pool of electrons Metallic bonds form when metal atoms share their pooled electrons This is why metals are malleable and duc7le- the metal does not break, the atoms slide over one another and the pooled electrons s7ll hold the atoms together Because the pooled electrons can move freely, they also conduct electricity well (the electrons can move from one atom to another and carry the electrical current)
Covalent Bonds Some atoms will share electrons with others because their outer energy levels are at least half full Gaining or losing 4 or more electrons would be much more difficult than sharing them Covalent bond Forms between nonmetal atoms through sharing of electrons
The shared electrons are a@racted to the nuclei of both atoms The shared electrons move between the outer energy levels of the atoms sharing the bond This lets each atom be stable for part of the 7me Form a neutral par7cle (has the same number of posi7ve and nega7ve charges)- a molecule Molecules are formed by sharing electrons Ions are not formed because no electrons are gained or lost
Atoms can share more than one electron with another atom This would create a double or triple bond, depending on how many electrons were shared If 2 pairs of electrons are shared, a double bond is made If 3 pairs of electrons are shared, a triple bond is made
Single bond The atoms share 1 electron Double bond The atoms share 2 electrons Triple bond The atoms share 3 electrons
Polar and Nonpolar Molecules Atoms do not always share electrons equally Some atoms a@ract electrons more strongly than others This makes the electrons want to be closer to one of the atoms Because the electrons are closer to one atom, that atom becomes more nega7ve than the other
Polar bond Bond where electrons are shared unevenly Let s use water as an example: 2 hydrogen + 1 oxygen The oxygen atom has more electrons giving it a slight nega7ve charge and the hydrogen a slight posi7ve charge
This polarity makes water act kind of like a magnet Water molecules are a@racted to each other, the posi7ve end of one molecule is a@racted to the nega7ve end of another- this accounts for many of the physical proper7es of water
Nonpolar molecules do not have these uneven charges. The only true nonpolar molecules are those formed between atoms of the same element
Intro to Chemical Formulas To put chemical formulas together you join the symbols with numbers indica7ng how many atoms there are For example: One water molecule is made up of 2 hydrogen atoms and 1 oxygen atom The formula is H 2 O The 2 tells us that there are 2 hydrogen atoms per oxygen atom What do the formulas MgCl 2 and Al 2 S 3 tell you?