2.7 Lewis Diagrams

Question 1

What are valence electrons, and why do we care

Answer 1

Valence electrons are defined as those electrons in the outer most,unfilled,s and p orbitals of an atom

The number of valence electrons often has an effect on the chemical properties of the atom. And chemical reactions often involve losing or gaining valence electrons

Question 2

How can we find the number of valence electrons in an element

Answer 2

You can count the number of spaces the element is from the left edge of the table (including its place) to determine the number of valence electrons. However the D orbital electrons do not count for valence electrons, so the transition metals (groups 3-12) do not count and they (for the most part) all have 2 valence electrons

The noble gases have zero valence electrons since their outer shell is filled

Atoms have a maximum of 8 valence electrons

Question 3

How do you drawn a lewis dot diagram

Answer 3

To draw an electron dot diagram for an atom:

1. Determine the number of valence or outer shell electrons by using the group number for Groups 1 and 2, and the group number minus 10 for groups 13 to 18 elements.
2. Distribute the valence electrons on the sides of the “imaginary square.” Make no pairs of electrons until there are four single electrons surrounding the atom.

Question 4

What is the combining capacity of an element

Answer 4

Combining capacity is a number used to represent how many valence electrons the atom has to gain or loss to complete the out shell (usually whichever is closer, so losing electrons for sodium or magnesium, but gaining them for chlorine or sulphur)

Question 5

What is an isoelectric ion

Answer 5

An atom becomes and isoelectronic ion when it reaches the stable configuration of the closest noble gas (which has a complete outer shell)

Elements from groups 1-2 tend to loss electrons, while those from groups 16-17 tend to gain them. Meanwhile other groups tend to share electrons to fill their outer shells

Question 6

What is the octet rule part 1

Answer 6

One atom will react with another atom by GAINING electrons, LOSING electrons, or SHARING electrons.

Question 7

What is the octet rule part 2

Answer 7

The number of electrons gained, lost, or shared, is such that the atom will end up with eight electrons in its outermost orbitals. More precisely, the atom will end up with the same number of electrons as the noble gas that is nearest to it in the Periodic Table. The number of electrons is usually 8, except for those atoms that become isoelectronic with helium.

Question 8

What is the octet rule part 3

Answer 8

The number of electrons gained, lost, or shared, is referred to as the combining capacity of the element.

Question 9

How do lewis dot diagrams show molecules

Answer 9

The Lewis structure of water shows each atom and its position in the structure of the molecule using its chemical symbol. Lines are drawn between atoms that are bonded to one another. Excess electrons that form lone pairs are represented as pairs of dots, and are placed next to the atom.

Question 10

What is electronegativity

Answer 10

Electronegativity is the ability of an atom to pull electrons towards itself.

Question 11

What happens when electronegativity is similar between two atoms

Answer 11

When the electronegativity is similar (in atoms with similar combining capacities), they will often share electrons in the outer shell and form covalent bonds

Question 12

What happens when electronegativity is opposite

Answer 12

When the electronegativity differs (in atoms with opposite combining capacities), one atom will lose electrons, while the other will gain electrons. This will result in the formation of two or more ions with electric charge (the one that lost electrons is positive, the one that gained electrons is negative) and they will attract each other. This is called an ionic bond

Question 13

How do ionic bonds work

Answer 13

If a metal atom, such as sodium, collides with a nonmetal ion, such as fluorine, it will give up its valence electron to the fluorine atom. The sodium is now a positive ion and the fluorine is a negative ion which is then called fluoride. The opposite charges attract and the result is an “ionic formula

Question 14

How do we drawn the lewis dot diagram for molecules

Answer 14

1) Count total number of valence electrons

2) Write the skeleton structure, with the element that needs the most electrons in the middle

3) Use two valence electrons to show each covalent bond in the molecule (you can also use dashes)

4) place the remaining electrons in octets (pairs) around the middle element

Question 15

Can each atom only have one bond with another

Answer 15

Sometimes molecules between 2 atoms need to share multiple bonds, such as diatomic nitrogen which shares 3 octets, it is triple bonded. double bonded and others are possible too

Question 16

What’s up with covalent bonds

Answer 16

Covalent bonds typically form between two nonmetal, but a metal can be involved. All bonds that are not ionic, are by default covalent. So, remember that ionic bonds only form between metals from groups 1-2 and nonmetals from groups 16-17 (or they may include polyatomic ions).

Question 17

What is the first clue to how the atoms are arranged

Answer 17

1. When there is only one atom of one element and several atoms of another, the single atom is usually the central atom. Thus, oxygen is the central atom in H-O-H.

Question 18

What is the second clue to how the atoms are arranged

Answer 18

2. The combining capacities of an element often indicates how many bonds it will form. In HO, each hydrogen atom has a combining capacity of +1 and so forms one bond. The oxygen atom has a combining capacity of -2 and forms two bonds. If the H atoms bonded with each other they would have no remaining bonds for the oxygen. Therefore, the O atom must form one bond to each H.

Question 19

How do we determine lone pairs and bonding electrons

Answer 19

To determine lone pairs and bonding electrons

we need to consider the octet rule. Any electron that is paired up is considered a lone pair and a single electron needing to bond to make a full octet is a bonding pair. Eg. O has 6 electrons in its outer shell so it has 2 lone pairs and 2 bonding electrons as it needs 2 electrons to make a full octet.

Question 20

What are structural diagrams

Answer 20

Structural diagrams represent bonding electrons only. Each pair of bonding electrons is represented by a line:

The peripheral valence electrons are not shown.

Question 21

How do we drawn polyatomic ions

Answer 21

As this structure has a net charge of -1 (since there is 1 more electron than there are protons), it is bracketed and the charge is indicated outside the bracket. In this example, iodine contributes two electrons to each bond and oxygen contributes none. The situation where one atom does not contribute electrons to a chemical bond is called a co-ordinate bond or “dative bond”. The x indicates the extra electron which causes the -1 charge.

Question 22

what are the steps of the first part of finding the lewis structure of neutral molecules

Answer 22

1) find total number of valence electrons

2) calculate total number of electrons for each atom to satisfy octet rule

3) subtract number of valence electrons from total needed number to find number of bonding electrons. Divide that number by 2 to find total number of bonds

4) subtract number of bonding electrons from number of available electrons to find number of non bonding electrons

5) Find all possible arrangements that give the total number of bonds

6) place in all non bonding electrons in octets

Question 23

How do we determine which arrangement is the most logical (in neutral molecules)

Answer 23

7) find the formal charge for each atom ( FC = # of valence electrons - (# of non bonding electrons + # of bonds)

8) ensure adding the formal charge of each atom equals the molecule charge

9) use these two rules to find the best:

• Choose the Lewis structure in which the atoms have formal charge closest to zero
• choose the lewis structure in which the negative charges are on the most electronegative atoms

Question 24

What are the steps of the first part of finding the lewis structure of a polyatomic ion

Answer 24

1) find total number of valence electrons

2) calculate total number of electrons for each atom to satisfy octet rule plus or minus electrons to equal charge

3) subtract number of valence electrons from total needed number to find number of bonding electrons. Divide that number by 2 to find total number of bonds

4) subtract number of bonding electrons from number of available electrons to find number of non bonding electrons

5) arrange the ions so that the most electron negative atom is in the middle

6) show the charge outside of brackets

7) Find all possible arrangements that give the total number of bonds

8) place in all non bonding electrons in octets

Question 25

How do you find the most logical arrangement (polyatomic ions)

Answer 25

9) find the formal charge for each atom ( FC = # of valence electrons - (# of non bonding electrons + # of bonds)

10) ensure adding the formal charge of each atom equals the molecule charge

11) use these two rules to find the best:

• Choose the Lewis structure in which the atoms have formal charge closest to zero
• choose the lewis structure in which the negative charges are on the most electronegative atoms

12) if two structure have arrangement of atoms but differ only in the arrangement of electron pairs they are called resonance structures. Both are similarly reasonable and in reality the actual ion is a blend of the resonance structures.