How elements are arranged by atomic number, and how position predicts properties.
↔ Swipe the diagram sideways to see all of it
Mendeleev (1869) arranged the known elements by increasing atomic mass and left gaps for undiscovered elements, correctly predicting their properties. Moseley later showed that atomic number is the real basis. The modern periodic law states that the properties of elements are a periodic function of their atomic numbers. The modern table has 7 horizontal periods and 18 vertical groups. The first three periods hold 2, 8 and 8 elements.
How many elements are there in periods 1, 2 and 3 together?
2 + 8 + 8 = 18.
Why was Mendeleev's table successful?
It left gaps for undiscovered elements and predicted their properties, which later proved correct.
State the modern periodic law.
The properties of elements are a periodic function of their atomic numbers.
Elements in a period have the same number of shells, equal to the period number. Elements in a group have the same number of valence electrons and so similar properties. For the main groups: group number = valence electrons for groups 1 and 2, and 10 + valence electrons for groups 13 to 18. Sodium (2,8,1) is in period 3, group 1. Chlorine (2,8,7) is in period 3, group 17. Argon (2,8,8) is in period 3, group 18.
Find the period and group of the element with Z = 17.
Configuration 2,8,7: 3 shells, so period 3; 7 valence electrons, so group 17 (halogen).
Find the period and group of Z = 20.
Configuration 2,8,8,2: 4 shells, period 4; 2 valence electrons, group 2 (alkaline earth metal).
Find the period and group of oxygen (Z = 8).
Configuration 2,6: period 2, group 16.
Across a period, atomic size decreases (more protons pull the same shells closer), metallic character decreases and non-metallic character increases. Down a group, atomic size increases (more shells) and metallic character increases. Group 1 are the alkali metals (Li, Na, K): soft, very reactive, valency 1. Group 17 are the halogens (F, Cl, Br, I): reactive non-metals, valency 1. Group 18 are the noble gases (He, Ne, Ar): unreactive with full outer shells.
Which is larger, a sodium atom or a chlorine atom? Explain.
Sodium: both have three shells but chlorine has 17 protons pulling the electrons closer than sodium's 11.
Which is more reactive, lithium or potassium?
Potassium; its outer electron is further from the nucleus and is lost more easily.
Why are noble gases unreactive?
They have full outer shells (2 for helium, 8 for the others).
1. Who proposed the first useful periodic table?
2. What is the basis of classification in the modern table?
3. How many groups and periods are there in the modern table?
4. What do elements in the same group have in common?
5. Name two alkali metals and two halogens.
6. Element X has configuration 2,8,3. Give its period and group.
7. How does atomic size change down a group?
8. How does metallic character change across a period?
9. Why do the noble gases form few compounds?
10. How many elements are in period 4 up to calcium (Z = 20)?