– The Periodic Law states that when elements are arranged according to their atomic numbers, their physical and chemical properties recur in a systematic and predictable way.
– With more electrons (and electron shells), the atomic size of the elements increase down the group.
– Elements in the same group:
- have similar properties (both physical and chemical)
- have equal number of valence electrons and thus,
- usually form the same type and number of bonds
For example, all group II elements lose 2 valence electrons to form +2 ions (cations) while all group VII elements gain 1 electron to form -1 ions (anions).
In summary, the group number= no. of valence electrons= +/- charge of the ion formed.
– Across the period, elements change from metals to metalloids to non-metals, with metalloids having some properties of both metals and non-metals.
– Elements want to obtain the stable octet state, whereby their valency becomes zero. Hence, they would either lose their valence electrons or gain electrons such that their outermost electron shells are fully filled.
– Cations are positive ions, which are formed by mainly metals with the lost of electrons.
As metals lose electrons to form cations, they also lose their outermost electron shells. Thus, cations are smaller in size than the original atom.
– Anions are negative ions which are formed by non-metals when they gain electrons.
As electrons are gained, there are corresponding lesser protons to attract the increased number of electrons, resulting in a reduced attractive force. With increased number of electrons, the electron-electron repulsion is greater, pushing them further apart from each other. These two reasons account for anions being larger than its parent atom.
– Isotopes are different forms of an element in that they have the same number of protons but different number of neutrons.