Grade 9

Introduction to the periodic table

Read periods and groups to connect element positions with electron patterns and broad chemical trends.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionA map of elements

The periodic table is a map: a position gives clues about an atom’s electrons and the kinds of substances it tends to form.

Rows run left to right; columns gather elements with related outer-electron patterns. The stair-step boundary loosely separates metals from non-metals, with metalloids nearby.

Choose period 2 or 3 to see metallic character across a row, or group 1 or 17 to compare elements down a column.

Select an element to inspect it; compare blocks, atomic radius, ionisation energy or electronegativity.

SchoolPeriods, groups and atomic number

Definition: Atomic number

The number of protons in the nucleus, denoted Z. In a neutral atom, it also equals the number of electrons. The modern table is ordered by increasing Z.

Definition: Period and group

A period is a horizontal row; a group is a vertical column. For main-group elements, a group often signals the number of valence electrons and helps predict similar chemistry.

Parts of the table
Position	Meaning	Example
Period	row; same highest occupied principal shell in the simple model	Na, Mg, Cl are period 3
Group	column; often similar valence patterns among main-group elements	Li, Na, K are group 1
Block	subshell filled last: s, p, d or f	transition metals occupy d block

Most elements are metals. Metals are concentrated on the left and centre, non-metals on the upper right; hydrogen is a non-metal placed above group 1 because its single electron does not make it an alkali metal.

Z=\text{number of protons}=\text{number of electrons (neutral atom)}

Example: Locate chlorine

Chlorine has atomic number 17. State its period and group, and its number of valence electrons.

Solution

Its shell distribution is 2, 8, 7: three occupied shells place it in period 3; seven outer electrons place it in group 17 (halogens).

Example: Compare sodium and chlorine

Which is more likely to form a positive ion, Na or Cl?

Solution

Na has one valence electron and commonly loses it to form Na⁺. Chlorine more readily gains one electron to form Cl⁻. Periodic position helps predict, but compounds and conditions matter.

UndergraduateWhy the table repeats

The periodic law states that the properties of elements are periodic functions of atomic number. Repeating valence-shell patterns explain recurring chemistry, while subshell filling, shielding and effective nuclear charge create the trends and their exceptions.

Broad trends for main-group elements
Property	Across a period →	Down a group ↓
Atomic radius	generally decreases	generally increases
First ionisation energy	generally increases, with exceptions	generally decreases
Metallic character	generally decreases	generally increases

\mathrm{Na:}\ 1s^2 2s^2 2p^6 3s^1\qquad \mathrm{Cl:}\ 1s^2 2s^2 2p^6 3s^2 3p^5

Mendeleev’s 1869 table used atomic weights and left gaps where he predicted undiscovered elements. The modern ordering by Z, established after nuclear charge was understood, resolves anomalies that an atomic-mass ordering could not.

Example: Why do sodium and potassium resemble one another?

Use the periodic table to compare their outer electrons.

Solution

Both are group 1 elements with one outer s electron, which they readily lose to form +1 ions. Potassium has an additional occupied shell, so it is larger and its outer electron is less tightly held.

References

The Periodic Table: Its Story and Its Significance · Eric R. Scerri, 2007