Electronic configurations and the blocks

The table is built by electrons

The rows and columns of the table are not arbitrary. They come from the way electrons fill the shells and subshells, in the Aufbau order: $1s$, $2s$, $2p$, $3s$, $3p$, $4s$, $3d$, $4p$, $5s$, $4d$, $5p$, and so on.

Periods

The period number is the principal quantum number $n$ of the outermost shell.

Each period begins when a new shell starts to fill and ends when its p subshell is full.

Groups and blocks

Elements in a group have the same outer configuration, so they behave alike. The table splits into four blocks, named after the subshell that the last electron enters.

• s-block, groups 1 and 2: outer configuration $ns^1$ or $ns^2$. The group number is the number of s electrons. These are reactive metals: the alkali and alkaline earth metals. Helium, $1s^2$, also belongs here, though it sits in group 18 with the noble gases.
• p-block, groups 13 to 18: $ns^2\,np^1$ to $ns^2\,np^6$. The group number is $10$ plus the outer s and p electrons. The block holds metals, non-metals and metalloids, and ends with the noble gases.
• d-block, groups 3 to 12: $(n - 1)d^{1 \text{ to } 10}\,ns^{0 \text{ to } 2}$. The group number is the number of d electrons plus the outer s electrons. These are the transition elements: metals with coloured ions and several valencies.
• f-block: $(n - 2)f^{1 \text{ to } 14}\,(n - 1)d^{0 \text{ or } 1}\,ns^2$. These are the inner transition elements, the lanthanoids and the actinoids. Most actinoids are radioactive, and those after uranium are made in reactors.

Placing an element from its atomic number

Take $Z = 35$. The configuration is $[\text{Ar}]\,3d^{10}\,4s^2\,4p^5$.

1. Period. The outermost shell is $n = 4$, so period 4.
2. Block. The last electron enters a $4p$ subshell, so the p-block.
3. Group. Outer electrons $2 + 5 = 7$, so group $10 + 7 = 17$. It is bromine, a halogen.

Move $Z$ through period 4, from 19 to 36. Watch the dot cross the s-block, then the ten d-block groups, then the p-block.

Metals, non-metals and metalloids

• Metals make up about four fifths of the elements and fill the left side and the middle. They are usually solids that conduct heat and electricity.
• Non-metals sit at the top right. They are often gases or brittle solids, and poor conductors.
• Metalloids, or semi-metals, lie along a zigzag line between them: boron, silicon, germanium, arsenic, antimony and tellurium. Their properties are in between.

Across a period, the character changes from metallic to non-metallic. Down a group, the metallic character increases.

Practise with new numbers

Try it in Python

The program fills the subshells in the Aufbau order, then reads the period, the group and the block for several elements.

Common mistakes

• Putting helium in the p-block. Its last electron enters $1s$, so it is s-block, though it sits in group 18.
• Taking the group from the outer electrons alone in the p-block. In groups 1 to 18, add 10: $ns^2\,np^3$ is group 15.
• Forgetting the 4s electrons in a d-block group. Iron, $3d^6\,4s^2$, is in group $6 + 2 = 8$.
• Thinking all f-block elements are in period 7. The lanthanoids are in period 6.

Remember these

• Period = the principal quantum number of the outermost shell.
• s-block: groups 1 and 2. p-block: groups 13 to 18. d-block: groups 3 to 12. f-block: lanthanoids and actinoids.
• Group of a p-block element = $10$ + outer s and p electrons. Group of a d-block element = d + outer s electrons.
• Metals on the left and middle, non-metals at the top right, metalloids on the zigzag between.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. An element has the configuration $1s^2\,2s^2\,2p^6\,3s^2\,3p^4$. Give its period, group and block.

2. Why do period 2 and period 3 hold only 8 elements each?

3. In which group is an element with $3d^3\,4s^2$?

4. Name two metalloids and say where they sit in the table.

5. Which block holds the transition elements, and what is special about their ions?