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.

The elements of the modern periodic table are arranged in order of increasing…

Which group of the periodic table contains the noble gases, chemically very inert?

References

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