A periodic table lesson where each student predicts a missing element

Why this matters

Many students learn the periodic table as a list in order of atomic mass. A list can't predict anything, and strict mass order puts iodine under selenium, where it doesn't belong. On a worksheet a memorized table looks the same as one the student worked out, and the teacher has to wait for quiz day to find out.

Leo's chat: 15 element cards and one empty square

Leo is a 10th grader who collects trading cards and sorts them by their stats. He has memorized the first 20 elements and thinks the periodic table is a list in order of mass.

The whole class gets the same activity: the same greeting, the same 15 element cards with the facts Mendeleev had, and the same goal of predicting the missing element. Each student's chat is their own. Sparky's next message depends on how the student placed the cards and what they wrote.

Sparky uses the student's interestSparky sends the student back to the cards he already placedA right answer gets a harder question

L

Leo

Interest: Collects trading cardsStarting point: Thinks the table is ordered by mass

Sparky

Hi! I'm Sparky. Today you'll rebuild part of the periodic table from element cards, the way Mendeleev did in 1869, and predict an element nobody had found yet. What's something you collect or like to sort, and how do you think the periodic table is put in order?

Leo

I collect trading cards and sort them by stats. I memorized the first 20 elements. I think the table is just a list in order of mass

Sparky

Sorting by stats is the right instinct. Mendeleev did something similar. Here's your card table: each card has the mass, density and formula with hydrogen he had to work with. Drag the cards into the grid so patterns show up, then type your bottom two rows.

Leo

Row 3 is K, empty, Se, Br. I left the empty square because Se is H2Se and that doesn't match SiH4. Row 4 is Rb, Sn, I, Te because iodine is lighter than tellurium

Sparky

Leaving the empty square because H₂Se doesn't match SiH₄ is the right reasoning. Now look at column 4 in your grid: F, Cl, Br, and then Te at the bottom. What formula with hydrogen do F, Cl and Br all have, and does Te's formula match?

Leo

F, Cl and Br are HF, HCl, HBr. Te is H2Te so it doesn't fit. Iodine is HI, so I goes under Br and Te goes under Se, even though Te is heavier

Understanding check

Exactly right: the formula pattern wins over the mass order. That was Mendeleev's key idea. So if mass doesn't set the order, what does?

Leo

something inside the atom? maybe the number of protons

Sparky

Yes. In 1913 Henry Moseley used X-rays to show that each element has an atomic number, the number of protons in its nucleus. By atomic number, tellurium (52) comes before iodine (53), so no swap is needed. Mendeleev didn't know that in 1869; he trusted the properties over the masses. Now the missing element under silicon: using this graph, what's your best guess for its atomic mass and its density?

Leo

it's between Si and Sn so I'll take halfway. mass (28.09 + 118.71)/2 = 73.4 and density (2.33 + 7.27)/2 = 4.8

Sparky

Both calculations are correct. Your prediction: 73.4 and 4.80 g/cm³. Mendeleev's in 1871: 72 and 5.5 g/cm³. The real element, found in 1886: 72.63 and 5.32 g/cm³. Your atomic mass is only 0.77 off. Your density is lower than the real one because silicon and tin were the only neighbors you used.

Leo

what is the element called?

Sparky

Germanium, discovered by Clemens Winkler in 1886. Mendeleev called it eka-silicon, "one below silicon", and predicted it 15 years before anyone found it. Last question: in one or two sentences, how did an empty square let Mendeleev predict an element nobody had seen?

Fixed his own iodine mistake, then predicted the missing element's mass within 0.8

Leo put iodine before tellurium because that is what mass order says. Sparky didn't tell him to swap the two cards. It pointed him back to column 4, where the formulas with hydrogen he had already read didn't match, and Leo moved the cards himself. His harder question asked what really sets the order, which mass couldn't answer.

Leo and his chat are an example we wrote for this page. His messages and Sparky's replies are shortened from a test run of the activity in which a student sent these messages.

What Flint did for Leo

• Sparky built a card table Leo could drag cards around, with the facts Mendeleev had and nothing that gave away which elements belong together.
• When Leo put iodine before tellurium, Sparky asked him to compare the formulas with hydrogen in that column instead of naming the cards to swap.
• After Leo fixed the order, Sparky asked what sets it if mass doesn't, then told him about Moseley's atomic numbers.
• Sparky drew a graph of density against atomic mass for carbon, silicon and tin, with the range for the missing element shaded.
• Sparky checked Leo's averages with its calculator before putting his prediction next to Mendeleev's and germanium's.

A teacher can't do this by hand for 30 students, and Sparky does it in every student's chat.

Who ordered the table by mass alone

When students finish, Sparky reads every chat and sums up the class on the activity page. The teacher sees who is stuck right now, which mistake came up most, and what to practice next.

Set up the activity for free

The Mendeleev activity is in Flint's public library, with the 15 element cards, Sparky's guidelines, greeting and greeting suggestions already written. Open it, select Try this activity to copy it into a free Flint account, and move it to a class. In the activity settings, select Try session myself to test it as a student before you assign it.

For more science activities, see Flint for science.