It decreases across the period
I am supposing that you know about the elements present in each period of periodic table. The periodic table has been divided into four types.i.e. S, P, D and F blocks., based on electronic configuration .For further convenience we divide the periodic table into four groups. 1) Noble gases: These are placed in-group zero. Their electronic configuration is ns2, np6 (except Helium). 2) Representative Elements: These elements include metals and non-metals. Some are diamagnetic and some are paramagnetic. They include S and P blocks. S-block: The elements in which last electron enter ns orbital are called as S block elements The elements of group IA and IIA are included in this block. Their electronic configuration varies from ns1 to ns2. P-block: The elements in which p-orbital begin to fill np1 to np6 are called P-block elements. These include elements of groups IIIA to VIIA and zero groups. 3) D-block or outer-transition elements: The elements in their atomic or in their common oxidation state, in which last electron enters to (n-1) d orbital, are called as d-block elements. 4) F-block elements: Progressive fills the elements in which inner f-orbital addition of electrons are called as f-block elements. They have outer configuration ns2 (n-1) d1, (n-2) f1-14. When we talk about metallic character, we consider Ionization Potential and low electron population of outer most shell. Ionization Potential of S-block elements is generally low. The two groups of S-block show variation in their Ionization Potential as Group IA, the Alkali metals have low Ionization Potential as compared to Group IIA, the Alkaline Earth Metals. The Ionization Potential decreases from Li to Cs. In Group IA. The Ionization Potential decreases down the group in all P-block elements except IIIA because as we know that ionization potential is inversely proportional to atomic size and shielding effect of an element. In a group, the metallic character increases down the group in all P-block elements. As Ionization potential and electron population of the outer-most shell decreases from top to bottom in a group, the metallic character increases down the group. For e.g. B (Boron) has atomic no: 5 and its electronic configuration is 1s2, 2s2 2p1. The outermost shell is L. In a period, the metallic character decreases along a period of P block elements. As we move from left to right, along a period of P-Block element, the valence electrons are attracted more and more towards nucleus due to regular increase in nuclear charge. Thus Ionization Potential increases and metallic character decreases. The transition element lie in between S and P block element, so the value of Ionization Potential is also in between those of S and P block elements The Ionization Potential decreases as the atomic size increases in these elements.