- Define and explain Arranging the Elements in your own words
- Use key terms such as group accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
The periodic table is chemistry's greatest piece of organisation: every known element in one pattern where position predicts behaviour. Mendeleev arranged it before some elements were even discovered — and left gaps for them. In this chapter you will learn to read the table like a map.
This lesson focuses on Arranging the Elements: see how Mendeleev ordered elements by atomic mass and properties.
See how Mendeleev ordered elements by atomic mass and properties.
Key ideas
Position predicts electron arrangement
Elements are arranged in order of increasing atomic number, and this order creates the pattern of groups and periods. The group number tells you the number of outer-shell electrons (for the main groups), and the period number tells you the number of shells. That is why all Group 1 elements have one outer electron and react in similar ways.
Metals and non-metals sit on opposite sides
Metals occupy the left and centre of the table, while non-metals sit to the right, separated by a zig-zag line of metalloids. This split mirrors their properties: metals lose electrons to form positive ions, while non-metals tend to gain or share electrons. Knowing which side an element is on lets you predict how it will react.
Key term — group: A vertical column of the periodic table; elements in the same group have similar properties.
Why did Mendeleev leave gaps in his periodic table?
The pattern of properties showed that undiscovered elements must exist to fill the gaps, so he left spaces and even predicted their properties.
Answer: The pattern of properties showed that undiscovered elements must exist to fill the gaps, so he left spaces and even predicted their properties.
- Saying Mendeleev ordered by atomic number Mendeleev ordered by atomic mass and left gaps for undiscovered elements; the modern table is ordered by atomic (proton) number. Getting this backwards loses easy marks.
- Reading the group number as the total electrons The group number gives the number of outer-shell electrons, not the total. Chlorine in Group 7 has 7 outer electrons, but 17 electrons in total.
Practice
Both are in Group 1, so both have one electron in their outer shell, which they lose in similar reactions.
A non-metal; non-metals sit to the right of the table.
2,7.
Lithium, then sodium, then potassium — reactivity increases down Group 1.
Quick check
Which of these best defines "group"?
Astatine sits below iodine in Group 7. Predict whether it is more or less reactive than iodine, with a reason.
- Arranging the Elements: see how Mendeleev ordered elements by atomic mass and properties.
- Position predicts electron arrangement: Elements are arranged in order of increasing atomic number, and this order creates the pattern of groups and periods.
- metal: An element that is typically shiny, conducts heat and electricity, and is malleable.
- Watch out for: saying Mendeleev ordered by atomic number