Start with the recap, study the fully worked examples, then use the practice problems to
check your understanding of Periodic Table.
This page combines explanation, solved examples, and follow-up practice so you can move
from recognition to confident problem-solving in Chemistry.
Concept Recap
A systematic arrangement of all known chemical elements organized by increasing atomic number into rows (periods) and columns (groups), where elements in the same group share similar chemical properties because they have the same number of valence electrons.
A map of all elements organized so similar ones are in the same column.
Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Periodic Table asks which group, period, and trend justify the prediction.
Common stuck point:Students often know a formula related to periodic table but skip the recognition step: Am I using an element position, group, period, or trend to predict a chemical property or behavior? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I using an element position, group, period, or trend to predict a chemical property or behavior?
Worked Examples
Example 1
easy
Using the periodic table, predict whether silicon (Si) is a metal, nonmetal, or metalloid. Justify your answer.
Answer
Silicon is a metalloid.
First step
1
Silicon is in Group 14, Period 3, located along the staircase line (zigzag) dividing metals from nonmetals.
Full solution
2
Elements along this boundary are typically metalloids.
3
Silicon exhibits properties of both metals and nonmetals: it has a metallic luster but is a poor conductor (semiconductor).
The periodic table's organization allows us to predict properties based on an element's position. Metalloids lie along the metal-nonmetal boundary and have intermediate properties.
Example 2
medium
Arrange the following elements in order of increasing atomic radius: Na, Mg, K, Al.Al < Mg < Na < K in order of increasing atomic radius
Example 3
medium
How does atomic radius change as you move left to right across a period? Explain.
Example 4
hard
Arrange F, Cl, Br by increasing atomic radius.
Example 5
challenge
Element A (period 2, group 1) reacts with element B (period 3, group 17). Identify A, B, the compound, and the ion charges involved.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
Which group of the periodic table contains the noble gases? Why are they chemically inert?
Example 2
medium
An element is in Group 17 and Period 3 of the periodic table. What do the group and period tell you about its electrons?Period 3, Group 17 — how many valence electrons and electron shells?
Example 3
easy
On the periodic table, what do the horizontal rows represent?
Example 4
easy
On the periodic table, what do the vertical columns represent?
Example 5
easy
The periodic table is arranged in order of increasing what property?
Example 6
easy
Which side of the periodic table holds the metals?
Example 7
easy
Elements in group 18 are called what, and what is their key trait?
Example 8
easy
What does the period number of an element tell you about its atom?
Example 9
easy
Which group does an element with the valence configuration matching 1 outer electron belong to?
Example 10
easy
The block of elements in the center of the periodic table (groups 3-12) is called what?
Example 11
medium
Element A is in period 3, group 17. Name it and state its number of shells and valence electrons.Period 3, Group 17 — name this element and state its shells and valence electrons
Example 12
medium
As you move left to right across period 2, how does the number of valence electrons change?Valence electrons increase by 1 each step across Period 2
Example 13
medium
Why are sodium, potassium, and lithium grouped together in column 1?
Example 14
medium
Mendeleev left gaps in his table and predicted unknown elements. What organizing idea let him do this?
Example 15
medium
An element has 2 electrons in shell 1 and 5 in shell 2. Give its period and group.
Example 16
medium
Which has more electron shells: an element in period 2 or an element in period 4? By how many?
Example 17
medium
Why do elements in the same group tend to form ions of the same charge?
Example 18
medium
Element J is in period 3, group 2. Name it and give its number of shells and valence electrons.Element J is in Period 3, Group 2 — identify it and state its shells and valence electrons
Example 19
medium
Down group 1 from lithium to sodium to potassium, how does atomic size change and why?
Example 20
challenge
Element Z is a metalloid in period 3 lying on the staircase, with 4 valence electrons. Identify Z and explain its position.
Example 21
challenge
Two elements, one in group 1 period 3 and one in group 17 period 3, react. Identify both and predict the compound formula.Group 1 Period 3 + Group 17 Period 3 react — identify both and predict the compound
Example 22
challenge
Argon (period 3, group 18) is inert while chlorine (period 3, group 17) is reactive. Explain using valence electrons.
Example 23
easy
Which group on the periodic table is called the alkali metals?
Example 24
easy
Which group is called the halogens?
Example 25
easy
What is the atomic number of carbon (C)?
Example 26
easy
Are alkaline earth metals in group 1 or group 2?
Example 27
medium
An element is in period 2, group 14. Name it and give its valence electron count.Period 2, Group 14 — name this element and state its valence electron count
Example 28
medium
How does atomic radius change as you move down a group? Explain.
Example 29
medium
Which has a larger atomic radius: lithium or fluorine? Justify.
Example 30
medium
Which has a larger atomic radius: sodium or potassium? Justify.
Example 31
medium
An element X is in period 4, group 1. Name X and predict the formula of its chloride.Period 4, Group 1 — identify element X and predict the formula of its chloride
Example 32
medium
An element Y is in period 3, group 16. Name Y and predict its ion charge.Period 3, Group 16 — identify element Y and predict its ion charge
Example 33
medium
Why do all noble gases sit in the same column?
Example 34
medium
How does ionization energy generally change across a period (left to right)?
Example 35
medium
How does ionization energy change down a group?
Example 36
hard
Predict the formula of the compound formed between magnesium (group 2) and chlorine (group 17).Mg and Cl ion charges determine the MgCl₂ formula
Example 37
hard
Element Q is a metalloid in period 4 sitting on the staircase with 4 valence electrons. Identify Q.
Example 38
hard
Which has greater electronegativity: oxygen or sulfur? Explain.
Example 39
challenge
Atom W has 17 protons and 18 electrons. Locate W on the periodic table and identify the charge state.W has 17 protons and 18 electrons — locate on periodic table and identify the charge state