Why do sodium and potassium behave so alike? The answer is hidden in how the periodic table is arranged.
The modern periodic law states that the properties of elements are a periodic function of their atomic numbers. The modern table has 18 groups and 7 periods. The period number equals the number of electron shells, and elements in a group have the same number of valence electrons.
An element has the electronic configuration 2, 8, 7. State its period and group.
Period 3 (three shells) and Group 17 (seven valence electrons); it is chlorine, a halogen
Why do sodium (2, 8, 1) and potassium (2, 8, 8, 1) have similar chemical properties?
Both have one valence electron and belong to Group 1, so they react in a similar way
Across a period from left to right, atomic size decreases while ionisation potential, electron affinity and electronegativity increase. Down a group, atomic size increases while ionisation potential and electronegativity decrease. Metallic character decreases across a period and increases down a group.
Which has the larger atomic size, Na or Cl?
Na is larger. Both are in Period 3, and size decreases from left to right as the nuclear charge increases
Which has the larger atomic size, Li or K?
K is larger, because it has more electron shells (four against two)
Define ionisation potential.
The energy required to remove an electron from an isolated gaseous atom
1. State the modern periodic law.
2. How many groups and how many periods are there in the modern periodic table?
3. An element has the electronic configuration 2, 8, 3. State its period and group.
4. What decides the period number of an element?
5. How does atomic size change across a period from left to right?
6. How does ionisation potential change down a group?
7. Which has the larger atomic size, Li or K?
8. Which element of a period is the most electronegative, ignoring the noble gases?
9. What name is given to the elements of Group 1?
10. Why do elements in the same group show similar chemical properties?