Unit 6 Arrangement of elements in the periodic table
Key unit competence: Classify elements in the periodic table using their atomic number, electronic structure and observable trends to explain their properties and relationships.
Key ideas
- Early attempts to classify elements: Döbereiner's triads (groups of three similar elements, e.g. Li, Na, K), Newlands' law of octaves (every eighth element similar) and Mendeleev's table (by atomic mass, leaving gaps for undiscovered elements).
- The modern periodic table arranges elements in order of increasing atomic number. Vertical columns are groups; horizontal rows are periods.
- The group number equals the number of electrons in the outer shell (for groups 1, 2 and 13–18 using 3–8 for groups 13–17 and 8 for group 18). The period number equals the number of occupied shells. Example: 2,8,3 is period 3, group 13 (III).
- Group names: group 1 alkali metals, group 2 alkaline earth metals, group 17 halogens, group 18 noble gases. The central block contains the transition metals.
- Metals are on the left and centre, non-metals on the right; metalloids (B, Si, Ge, As) lie along the staircase line. Metallic character increases down a group and decreases across a period.
- Elements in the same group have similar chemical properties because they have the same number of outer electrons.
Common misconceptions
- "The modern table is arranged by atomic mass." No: It is arranged by atomic number (protons).
- "Noble gases are in group 0 because they have no outer electrons." No: They have full outer shells (2 for He, 8 for the others).
- "Elements in the same period have similar properties." No: Elements in the same GROUP have similar properties.
Worked examples
Example 1. Element E has atomic number 12. State its electronic configuration, group, period and whether it is a metal.
Solution
Configuration: 2,8,2.
Period 3 (three shells).
Group 2 (two outer electrons).
It is a metal (magnesium).
Example 2. Explain why sodium and potassium have similar chemical properties.
Solution
Both have one electron in their outer shell (Na 2,8,1; K 2,8,8,1).
Chemical properties depend on outer electrons, so they react in similar ways (e.g. both react with water to give hydrogen and an alkali).
Practice questions
What is a period in the periodic table?
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A horizontal row of elements.
Common mistake: Confusing it with a group.
State the basis of arrangement of the modern periodic table.
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Increasing atomic number.
Common mistake: Writing 'atomic mass'.
Name the group of fluorine and chlorine.
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Halogens (group 17).
Common mistake: Calling them noble gases.
Give the group and period of 2,8,8,1.
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Group 1, period 4 (potassium).
Common mistake: Giving period 1 or group 4.
Name one metalloid and state one of its uses.
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Silicon: used in computer chips and solar cells.
Common mistake: Naming a metal such as aluminium.
State two contributions of Mendeleev.
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Arranged elements by atomic mass and properties; left gaps for and predicted properties of undiscovered elements.
Common mistake: Saying he arranged by atomic number.
Why are noble gases unreactive?
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They have full outer electron shells.
Common mistake: Saying they have no electrons.
Element W has 18 electrons. Name its group and suggest its reactivity.
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Group 18 (noble gas, argon); very unreactive.
Common mistake: Calling it a halogen.
Explain why metallic character increases down group 1.
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Atoms get bigger, the outer electron is further from the nucleus and is lost more easily.
Common mistake: Saying the nucleus becomes weaker.
Two elements: A in group 2 period 2 and B in group 16 period 2. Write the formula of their compound.
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A is Be (Be2+), B is O (O2−): formula AB (BeO).
Common mistake: Writing A2B.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which element has the greatest metallic character? a. three elements with similar properties 2. Which element is in group 2, period 3? b. Sodium 3. Döbereiner's triads were groups of: c. Magnesium 4. Which element is an alkali metal? d. Potassium Show answer
1 → d (Potassium) · 2 → c (Magnesium) · 3 → a (three elements with similar properties) · 4 → b (Sodium)
- Metallic character increases down group 1, so K > Na > Li; and decreases across, so Na > Mg.
- Period 3 group 2 has 2,8,2: magnesium.
- In a triad such as Ca, Sr, Ba, the middle mass was close to the average of the other two.
- Group 1 metals (Li, Na, K) are alkali metals.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
every eighth element has similar properties · metals to non-metals · 2,3 · elements are arranged by atomic number- Across period 3 from sodium to argon, elements change from .
- An element X is in group 13 and period 2. Its electronic configuration is .
- Newlands' law of octaves stated that .
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1: metals to non-metals · 2: 2,3 · 3: every eighth element has similar properties
- Na, Mg, Al are metals; Si a metalloid; P, S, Cl, Ar non-metals.
- Period 2: two shells; group 13: three outer electrons.
- Like musical notes, the 8th element resembled the 1st.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. An element with configuration 2,8,6 is in group: a. 16 (VI) 2. Which group contains the halogens? b. Lithium and potassium 3. Which pair would react in a similar way with water? c. Group 17 (VII) 4. Element Z (Z = 20) is in which group and period? d. Group 2, period 4 Show answer
1 → a (16 (VI)) · 2 → c (Group 17 (VII)) · 3 → b (Lithium and potassium) · 4 → d (Group 2, period 4)
- Six outer electrons place it in group 16 (VI): sulfur.
- Fluorine, chlorine, bromine and iodine are halogens.
- Both are group 1 metals with one outer electron.
- Configuration 2,8,8,2: 4 shells, 2 outer electrons.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
3 · noble gases · the same number of outer electrons · halogens- An element with configuration 2,8,1 is in period .
- Elements in the same group have similar chemical properties because they have .
- Group 18 elements are called .
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1: 3 · 2: the same number of outer electrons · 3: noble gases
- Three occupied shells means period 3.
- Chemical reactions involve outer electrons.
- They are very unreactive gases with full outer shells.