2.2 - electrons, bonding and structure Flashcards
An electron shell is made up of atomic orbitals with the same principal quantum number, n. Within each shell, orbitals of the same type are grouped together as a sub-shell. Each sub-shell is made up of one type of atomic orbital only. So, there are s, p, d and f sub-shells. What does figure 1 show? (see page 78)
Figure 1 shows the orbitals available in the first three shells:
- each orbital is shown as a box that can hold a maximum of two electrons
- each shell gains a new type of sub-shell
The sub-shells within a shell have different energy levels. Within a shell, the sub-shell energies increase in the order s, p, d and f. Draw a diagram showing the order of these energy levels. (See figure 2 on page 78)
–diagram–
What is the set of rules for working out the election configuration called?
the Aufbau principle
The electrons in the shells of an atom are arranged as follows (include information about sub-shells):
- electrons are added, one at a time, to ‘build up’ the atom
- the lowest available energy level is filled first. You can consider this level as being closest to the nucleus
- each energy level must be full before the next, higher energy levels start to fill
Sub-shells are made up of several orbitals, each with the same energy level:
- when a sub-shell is built up with electrons, each orbital is filled singly before pairing starts
- the 4s orbital is at a slightly lower energy level than the 3d orbital. This means 4s will fill before 3d.
Turn to page 79. Figure three shows how the electron configuration is built up for the elements B, C, N and O. What three things do you notice?
1) orbitals fill from the lowest energy level upwards
2) the 2p-orbitals are filled singly before pairing starts at oxygen
3) paired electrons have opposite spins
Turn to page 79. Table 1 shows the electron configurations for atoms of B, C, N and O. Notice that each occupied sub-shell is written in the form nx to the y, where:
n is the shell number
x is the type of orbital
y is the number of electrons in the orbitals making up the sub-shell
When are ions formed?
When atoms lose or gain electrons.
What will the electron configuration show when atoms are ionised to become positive ions?
If atoms are ionised to become positive ions, then the electrons found in the highest energy levels are lost first. The electron configuration will therefore show fewer electrons at the highest energy levels.
What happens to electron configuration when atoms gain electrons and become negative ions?
When atoms gain electrons and become negative ions, the extra electrons will continue to fill the sub-shells in the same way shown in table 1.
Why do elements react and bond together? (brace yourself)
Chemical reactions are often accompanied by electron transfers. Elements involved in chemical reactions will often become more stable by combining with other elements or transferring electrons. The most stable and unreactive elements are the noble gases. These elements are stable because they contain a full outer shell of electrons. Other elements react in ways that allow them to end up with the electron configuration of a noble gas. The eight electrons in the outer shell of a noble gas (except helium) are made up of two in the s-orbital and two each in the three p-orbitals. During reactions, other elements will bond with a tendency to acquire this noble gas configuration. Although other orbitals can be used for bonding, the s- and p-orbitals are perhaps the most important, especially for the formation of compounds involving the first 18 elements.
How do elements have a tendency to bond?
Elements have a tendency to bond so that they acquire a noble gas configuration. A noble gas configuration is energetically stable.
What three main types of chemical bond are there?
Ionic, covalent and metallic.
When is a compound formed?
A compound is formed when atoms of different elements are chemically bonded together. In a compound, the atoms of the different elements are always in the same proportions. For example, H2O always has two hydrogen atoms to one oxygen atom.
