A table that made predictions
In 1869, the Russian chemist Dmitri Mendeleev arranged the 60-some elements known at the time in order of atomic mass and noticed that elements with similar properties showed up again and again, at regular intervals. Boldly, he left gaps in his table, claimed they belonged to elements nobody had found yet, and predicted their properties. Within a few years, gallium and germanium were discovered, and they matched his predictions remarkably well. His table grew into today’s periodic table元素周期表.
How the table is arranged
Today’s periodic table holds 118 elements, arranged in order of atomic number. For an atom:
Each box gives the element’s basic information. Here is iron:
| Where | What it shows | Iron’s box |
|---|---|---|
| Top | Atomic number | 26 |
| Middle | Element symbol | Fe |
| Below the symbol | Name (on many tables) | iron |
| Bottom | Atomic mass | 55.85 |
The atomic mass is the relative atomic mass from lesson 1.3.5. In calculations it’s usually rounded, so iron becomes 56.
Periods and groups
Rows: periods. The table has 7 rows, and each row is a period周期. The periods aren’t the same length: period 1 has only 2 elements, periods 2 and 3 have 8 each, periods 4 and 5 have 18 each, and periods 6 and 7 have 32 each.
Columns: groups. The table has 18 columns, numbered 1 to 18, and each column is a group族 (sometimes called a family). Elements in the same group have similar properties.
- Groups 1, 2, and 13–18 hold the main-group elements.
- Groups 3–12 hold the transition metals.
- Several groups have family names: group 1 (except hydrogen) is the alkali metals, group 2 the alkaline earth metals, group 17 the halogens, and group 18 the noble gases.
To keep the table from getting too wide, two rows of 15 elements each, the lanthanides and actinides, are pulled out of periods 6 and 7 and printed underneath.
Older American textbooks label the main groups 1A–8A. Chinese textbooks use a similar system (ⅠA–ⅦA, then 0 for the noble gases) and count only 16 groups, because they treat columns 8, 9, and 10 as a single group.
Click any element to see its period and group. Then use the “Find it” game to locate elements from their positions:
The table is wide: swipe sideways to see all of it.
Click any element in the table.
Example 1: From atomic number to position
Where is element 16 on the periodic table?
- Find the period. Period 1 holds elements 1–2, period 2 holds 3–10, and period 3 holds 11–18. 16 falls in 11–18, so it’s in period 3.
- Find the group. Periods 2 and 3 have no transition metals, so group 2 is followed directly by group 13. Counting from element 11 (sodium, group 1): 12 is group 2, 13 is group 13, 14 is group 14, 15 is group 15, and 16 is group 16.
- Answer. Element 16 is sulfur (S), in period 3, group 16.
Example 2: Reading a box
Calcium’s box shows 20, Ca, calcium, and 40.08. What does each item tell you, and what else can you work out?
- 20 is the atomic number. A calcium atom has 20 protons, a nuclear charge of +20, and 20 electrons.
- Ca and calcium are the symbol and name. Calcium is in group 2, so it’s an alkaline earth metal.
- 40.08 is the atomic mass. In calculations it’s usually rounded to 40.
- You can estimate the neutrons. The most common calcium atom has a mass number of about 40, so it has about 40 − 20 = 20 neutrons.
- Hydrogen (element 1)
Period 1 has only hydrogen and helium.
- Oxygen (element 8)
Period 2 holds elements 3–10.
- Sodium (element 11)
Period 3 starts with sodium, element 11.
- Chlorine (element 17)
Period 3 holds elements 11–18.
- Potassium (element 19)
Period 4 starts with potassium, element 19.
- Neon (element 10)
Neon is the last element of period 2, in group 18.
- Helium (element 2)
Helium is the last element of period 1, in group 18.
- Calcium (element 20)
Potassium and calcium, elements 19 and 20, are both in period 4.
- Aluminum (element 13)
13 falls in 11–18, so aluminum is in period 3, group 13.
Common mistakes
Mistake 1: “Periods are columns and groups are rows.” It’s the other way around: periods run across (rows), and groups run down (columns).
Mistake 2: “The periodic table is arranged by atomic mass.” It’s arranged by atomic number. Mendeleev used atomic mass, so in a few places the two orders disagree: argon (39.95) comes before potassium (39.10).
Mistake 3: “Every period has 8 elements.” Only periods 2 and 3 do. Period 1 has 2, periods 4 and 5 have 18, and periods 6 and 7 have 32.
Quick check
Which element is in period 3, group 2?
Period 3 starts with sodium (element 11) in group 1, so group 2 holds the next element: magnesium, element 12.
Which statement about the periodic table is correct?
The 7 rows are periods. Groups are the columns, the table is arranged by atomic number, and the periods have different lengths.
Summary
- The periodic table is arranged by atomic number. Atomic number = nuclear charge = protons = electrons. Each box shows the atomic number, the symbol, and the atomic mass.
- The 7 rows are periods; the 18 columns are groups 1–18. Groups 1, 2, and 13–18 are main groups; groups 3–12 are the transition metals.
- Periods for the first 20 elements: 1–2 period 1, 3–10 period 2, 11–18 period 3, and 19–20 period 4.
On to AP (for later)
AP Chemistry uses the group numbers 1–18 from today’s lesson and expects you to know the family names: alkali metals (group 1), alkaline earth metals (group 2), halogens (group 17), and noble gases (group 18). Topics 1.7 and 1.8 use an element’s period and group to predict its properties and the charge on its ion.