Electron Shells and Valency: Quick Revision

A brief note on Electron Shells and Valency. Read it before a test. Try each question at the end before you open its answer.

The rules

• Shell $n$ holds at most $2n^2$ electrons: K $2$, L $8$, M $18$, N $32$.
• The outermost shell holds at most $8$. Fill the shells from the inside.
• Valency: with $1$ to $4$ outer electrons, the valency is that number; with $5$ to $7$, it is $8$ minus it; a full shell gives $0$.
• Atomic number $Z$ = protons. Mass number $A$ = protons + neutrons. Neutrons $= A - Z$.
• In a neutral atom, electrons $= Z$. An ion has the same nucleus, with the electrons changed by the charge.

Some atoms at a glance

A worked example

An atom has the mass number $19$ and $10$ neutrons. Find its atomic number, its arrangement, its valency and its ion.

1. Atomic number. $Z = 19 - 10 = 9$. It is fluorine.
2. Arrangement. $9$ electrons: $2, 7$.
3. Valency. $8 - 7 = 1$.
4. Ion. It gains $1$ electron: $\text{F}^-$, with $10$ electrons.

Practice with new numbers

A program to try

The program finds the arrangement and the valency for six atomic numbers.

Mistakes to avoid

• Putting more than $8$ in the outer shell, as in $2, 8, 9$ for potassium.
• Taking the outer electrons as the valency when there are more than $4$.
• Using $A$ as the number of protons. Protons are $Z$.
• Changing the protons of an ion. Only the electrons change.

Check yourself

1. Write the electron arrangement of phosphorus, $Z = 15$.

2. What is the valency of an atom with the arrangement $2, 3$?

3. How many neutrons are there in ${}^{56}_{26}\text{Fe}$?

4. An ion $\text{X}^{2+}$ has $18$ electrons. What is the atomic number of X?