Course 9.1Lesson 3 of 3

The modern periodic table

The modern periodic law says that the properties of elements repeat in order of atomic number. The modern table has 18 groups and 7 periods. The period is the number of shells, and elements in a group have the same number of outer electrons.

about 25 min1 simulation2 practice questions

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A better number

In $1913$ , the English physicist Henry Moseley measured the X-rays of the elements. He found that each element has its own whole number, the atomic number, which is the number of protons in its nucleus. The atomic number is the true order of the elements.

The modern periodic law

The properties of elements are a periodic function of their atomic numbers.

This fixes the problems of Mendeleev's table.

Argon (Z $= 18$ ) comes before potassium (Z $= 19$ ). The order is right, though the masses are not.
Isotopes have the same atomic number, so they have the same place.
The number of elements between two places is known: one for each atomic number. There are no unknown gaps.

The modern table

The modern periodic table for the first 36 elements, from hydrogen to krypton, with the groups 1 to 18 across the top and the periods 1 to 4 down the side. The alkali metals Li, Na and K are red. The alkaline earth metals Be, Mg and Ca are orange. The transition metals from Sc to Zn are yellow. Al and Ga are other metals. B, Si, Ge and As are metalloids. The halogens F, Cl and Br are pink. The noble gases He, Ne, Ar and Kr are in group 18.

Periods are the $7$ horizontal rows. The number of elements in them is $2$ , $8$ , $8$ , $18$ , $18$ , $32$ and $32$ .
Groups are the $18$ vertical columns, numbered $1$ to $18$ .
Metals are on the left and in the middle. Non-metals are on the right. Metalloids are between them, near a stair-step line.

Why the table works: electrons

The place of an element depends on the way its electrons are arranged in shells.

The period is the number of shells that hold electrons.
The group depends on the outer electrons. In groups $1$ and $2$ , the group number is the number of outer electrons. In groups $13$ to $18$ , the group number is $10$ plus the number of outer electrons.
Elements in one group have the same number of outer electrons, so they have similar chemical properties.

Example. Chlorine, Z $= 17$ . The arrangement is $2, 8, 7$ .

Three shells: period 3.
Seven outer electrons: $10 + 7 = 17$ , so group 17.

Example. Calcium, Z $= 20$ . The arrangement is $2, 8, 8, 2$ .

Four shells: period 4.
Two outer electrons: group 2.

Helium has only $2$ electrons, and its one shell is full. So it goes in group $18$ , with the noble gases, and not in group $2$ .

You should find that each new period starts again at group 1, when a new shell begins.

Practise with new numbers

Try it in Python

This program finds the period and the group from the electron arrangement.

Remember these

Modern periodic law: the properties of elements repeat in order of atomic number.
$18$ groups and $7$ periods. The period is the number of shells.
Groups $1$ and $2$ : group $=$ outer electrons. Groups $13$ to $18$ : group $= 10 +$ outer electrons.
Elements in one group have the same number of outer electrons.