C. Chemistry: The Nature of Matter · Atoms, Elements and CompoundsThe Periodic Table and Atomic Structure
C2.4 C2.5 A2.5About 70 minutes
Learning goals- I can read an element's box on the periodic table.
- I can describe how Mendeleev organized the elements and how the modern periodic table is arranged.
- I can explain how an element's period tells the number of occupied energy levels and its group tells the number of valence electrons.
- I can predict an element's position from its electron arrangement, and its arrangement from its position.
Words to know
periodic table
A chart of all the elements arranged in order of atomic number, in rows and columns that group elements with similar properties.
period
A horizontal row of the periodic table. Elements in the same period have the same number of occupied energy levels.
group (family)
A vertical column of the periodic table. Elements in the same group have the same number of valence electrons and similar properties.
periodic law
When elements are arranged by atomic number, their properties repeat in a regular pattern.
atomic mass unit (u)
A unit for the mass of atoms, about the mass of one proton.
A chemist's card game
In 1869, the Russian chemist Dmitri Mendeleev was writing a chemistry textbook. About 63 elements were known, and he wanted a sensible order to teach them in. He wrote each element and its properties on a card and arranged and rearranged the cards. When he put them in order of atomic mass, he noticed that similar properties repeated at regular intervals, like the days of the week. That repeating pattern gave the table its name: periodic.
Remember from Grade 7: you met Mendeleev and the idea that he left gaps for undiscovered elements. Today you will see why his pattern works: it comes from the electrons.
Mendeleev's bold predictions
Mendeleev's table had gaps. Instead of forcing known elements into the wrong places, he said the gaps belonged to elements nobody had discovered yet, and he predicted their properties. For example, he predicted an element he called "eka-silicon", just below silicon. When germanium was discovered in 1886, its properties closely matched his predictions. A scientific model that makes correct predictions is very convincing.
In 1913, the English physicist Henry Moseley used X-rays to measure the positive charge of each nucleus, which is its atomic number. Arranging the elements by atomic number instead of atomic mass fixed the few places where Mendeleev's order didn't quite work (for example, argon, atomic mass 39.95, comes before potassium, atomic mass 39.10). Today's periodic law: when elements are arranged by atomic number, their properties repeat in a regular pattern.
Reading an element's box
Each box on the periodic table gives key information. Periodic tables differ a bit in what they show, but nearly all include these four things.
Rows and columns tell you about the electrons
The modern table has 7 periods (rows) and 18 groups (columns). The pattern comes straight from the Bohr-Rutherford diagrams you drew last lesson.
- Period number = number of occupied energy levels. Sodium (2, 8, 1) has 3 occupied levels, so it is in period 3.
- Group number tells the number of valence electrons for the main groups: groups 1 and 2 have 1 and 2 valence electrons; groups 13 to 18 have 3 to 8 valence electrons (subtract 10 from the group number). Helium is the exception: it has 2 valence electrons but sits in group 18 because its shell is full, like the other members of that group.
- Groups 3 to 12 (the transition metals) follow more complex rules that you will study in Grade 11.
Worked exampleExample 1: Find the element from its arrangement
An element has the electron arrangement 2, 8, 5. Find its period, group and name.
- Count the occupied energy levels: 3 → period 3.
- Count the valence electrons: 5. For groups 13–18, group = valence + 10, so group 15.
- Period 3, group 15 on the periodic table: phosphorus (P). Check: 2 + 8 + 5 = 15 = atomic number of phosphorus. ✔
Phosphorus, period 3, group 15.
Worked exampleExample 2: From position to atom
An element is in period 2, group 16. Give its electron arrangement, name and Lewis dot diagram.
- Period 2 → 2 occupied energy levels.
- Group 16 → 16 − 10 = 6 valence electrons.
- The first level is full (2), and the second has 6: 2, 6. Total 8 electrons → atomic number 8 → oxygen.
- Lewis diagram: O with 6 dots (two pairs and two singles).
Oxygen, arrangement 2, 6, with 6 valence electrons.
Why this matters
Because elements in the same group have the same number of valence electrons, they react in similar ways. That is why chemists can predict how an element will behave just from where it sits on the table. The next two lessons use this idea.
Hands-on activityBe Mendeleev: arrange the cards
You need: 20 index cards or paper squares, a pencil, and a periodic table to check your work at the end.
- On each card, write one element's symbol, atomic mass and property from this list: H 1.01, reactive gas; He 4.00, unreactive gas; Li 6.94, soft reactive metal; Be 9.01, metal; B 10.81, metalloid; C 12.01, solid nonmetal; N 14.01, gas; O 16.00, gas; F 19.00, very reactive gas; Ne 20.18, unreactive gas; Na 22.99, soft very reactive metal; Mg 24.31, metal; Al 26.98, metal; Si 28.09, metalloid; P 30.97, solid nonmetal; S 32.06, yellow solid nonmetal; Cl 35.45, very reactive gas; Ar 39.95, unreactive gas; K 39.10, soft very reactive metal; Ca 40.08, metal.
- Shuffle the cards. Without looking at a periodic table, arrange them in order of increasing atomic mass. Start a new row after each unreactive gas, so that H and He make row 1.
- Look down each column. What do the cards have in common?
- Notice argon (39.95) and potassium (39.10). Where must they go so the columns make sense? This is the problem Moseley solved with atomic numbers.
- Check your layout against a real periodic table.
What to look for: the reactive soft metals (Li, Na, K) line up in one column, the unreactive gases (He, Ne, Ar) in another, and the very reactive gases (F, Cl) together. To keep the columns consistent, argon must come before potassium even though its atomic mass is larger.
Just for funMendeleev is often said to have dreamed the arrangement of the periodic table. Whether or not that story is true, he spent many hours shuffling cards first. Moral: even dream-inspired ideas need a lot of homework.
Your turn
1What does the period number of an element tell you about its atoms?
The number of occupied energy levels (shells).
2How many valence electrons does an element in group 17 have?
17 − 10 = 7 valence electrons.
3Which element is in period 3, group 2? Give its electron arrangement.
4An element has the arrangement 2, 8, 8, 1. What are its period and group?
4 shells → period 4; 1 valence electron → group 1. It is potassium.
5Why are lithium, sodium and potassium in the same group?
They each have 1 valence electron, so they have similar properties.
6Why is helium placed in group 18 even though it has only 2 valence electrons?
Its only shell is full, so it is very stable and unreactive, just like the other group 18 elements.
7What did Moseley change about the periodic table?
He showed the elements should be ordered by atomic number (number of protons) instead of atomic mass.
More practice: warm-up
1What is a horizontal row of the periodic table called?
2What is a vertical column called?
3How many valence electrons do group 2 elements have?
4In what year did Mendeleev publish his periodic table?
5In the sodium box, what does 22.99 stand for?
The atomic mass (average mass of sodium atoms, in u).
More practice: on your own
1Find the period, group and name of the element with arrangement 2, 3.
2 shells → period 2; 3 valence → group 13; total 5 electrons → boron.
2Give the arrangement and name of the element in period 3, group 18.
Three full shells: 2, 8, 8 → argon.
3Chlorine and fluorine are both very reactive. Use their electron arrangements to explain why they behave alike.
Fluorine is 2, 7 and chlorine is 2, 8, 7. Both have 7 valence electrons, so they react in similar ways (both are in group 17).
4Why was Mendeleev's prediction of germanium important evidence that his table was right?
A model is strongest when it makes predictions that later turn out to be true. Germanium's discovered properties matched what Mendeleev predicted for the gap, showing that his pattern was real.
5Which two elements among the first 20 have 4 valence electrons? Which periods are they in?
Carbon (2, 4) in period 2 and silicon (2, 8, 4) in period 3.
Challenge
1Element X is in the same group as oxygen but has one more energy level. Give its arrangement, name and number of protons.
Oxygen is 2, 6 (group 16). One more level: 2, 8, 6, which is sulfur, with 16 protons.
2Tellurium (atomic mass 127.6) comes before iodine (126.9) on the modern table. Explain why, and why Mendeleev also put them this way even before atomic numbers were known.
The modern table uses atomic number: tellurium is 52 and iodine is 53, so tellurium comes first. Mendeleev placed them by properties: iodine behaves like chlorine and bromine, so it had to go in their group, even though that broke the atomic-mass order. Later, atomic numbers showed his choice was correct.
3Explain why the period number equals the number of occupied energy levels, using the way electrons fill shells.
A new period starts each time a new energy level begins to fill. Period 1 fills the first level (H, He); period 2 starts the second level at Li and fills it at Ne; period 3 starts the third level at Na. So every element in a period has the same number of occupied levels, equal to the period number.
Quiz
1. What does an element's group number (for groups 1, 2 and 13–18) tell you?
2. An element has 3 occupied energy levels. Which period is it in?
3. Which element is in period 2, group 17?
4. What is the modern periodic table arranged by?
5. Which pair of elements would have the most similar properties?
6. Mendeleev left gaps in his table because...
Big idea: C2.4 asks students to explain the relationship between an element's position on the periodic table and the structure of its atoms, using models. The key links: period = number of occupied energy levels; group = number of valence electrons (main groups). C2.5 begins here with the idea that similar elements share a column.
Common difficulties:
- Mixing up periods (rows) and groups (columns). A trick: you read a "period" across, like a sentence ending in a period.
- Forgetting to subtract 10 for groups 13–18.
- Helium: it is in group 18 because its shell is full, not because it has 8 electrons.
How to help: keep a periodic table on the fridge for this unit. Play "Where am I?": you give an electron arrangement, your teen finds the element; then switch roles.