- Define and explain Groups and Periods 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
This lesson focuses on Groups and Periods: read the table's columns and rows and what they tell you.
Read the table's columns and rows and what they tell you.
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.
Element X sits in Group 2, Period 3 of the periodic table. Predict its electron arrangement, whether it is a metal, and how it will react with dilute acid.
Period 3 means 3 electron shells; Group 2 means 2 electrons in the outer shell, giving arrangement 2,8,2. Group 2 is on the metals side of the table, so X is a metal (it is magnesium). Metals react with dilute acids to form a salt plus hydrogen, so X will fizz, giving off hydrogen gas. The outer electrons are easily lost, so X forms 2+ ions.
Answer: Electron arrangement 2,8,2; X is a metal (magnesium); it reacts with dilute acid to form a salt and hydrogen.
- 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.
- 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.
Practice
The pattern of properties showed that undiscovered elements must exist to fill the gaps, so he left spaces and even predicted their properties.
A non-metal; non-metals sit to the right of the table.
2,7.
Both are in Group 1, so both have one electron in their outer shell, which they lose in similar reactions.
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.
Lithium, sodium and potassium are dropped into water. Put them in order of vigour of reaction, least vigorous first.
- Groups and Periods: read the table's columns and rows and what they tell you.
- Position predicts electron arrangement: Elements are arranged in order of increasing atomic number, and this order creates the pattern of groups and periods.
- period: A horizontal row of the periodic table; the period number equals the number of electron shells.
- Watch out for: reading the group number as the total electrons