The elements which show the properties of both metals and non-metals are known as metalloids.
e.g. Si, Ge, As.

The vertical columns in the periodic table are known as groups.

The horizontal rows in the periodic table are known as periods.

The number of electrons accepted, donated or shared by an atom so as to complete the octet (or duplet) in its outermost shell is called its valency.
OR
Valency is the combining power of an element.

(For an isolated atom) Atomic radius is the distance between the centre of the atom and its outermost shell.

Different atoms of the same element having same atomic number but different atomic mass number are called isotopes. e.g. ${}_{17}{}^{35}\mathrm{Cl}$ and ${}_{17}{}^{37}\mathrm{Cl}$ are the isotopes of chlorine.

Properties of elements are a periodic function of their atomic masses.

Properties of elements are a periodic function of their atomic numbers.

The elements in the atoms of which, all the shells are completely filled, are known as inert gases or noble gases.
e.g. He, Ne, Ar.

The elements in the atoms of which, only the last shell is incompletely filled, are known as normal elements.
e.g. Na, K, Ca.

The elements in the atoms of which last two shells are incompletely filled are known as transition elements.
e.g. Fe, Co, Ni.

The elements in the atoms of which last three shells are incompletely filled are known as transition elements.
e.g. La, Ce, Pr.

Elements from groups 1 and 2 are known as s-block elements.
e.g. Li, Be, Na.

Elements from groups 13 to 18 are known as p-block elements.
e.g. B, C, N.

Elements from groups 3 to 12 are known as d-block elements.
e.g. Cr, Mn, Fe.

Elements in the lanthanide and actinide series are known as f-block elements.
e.g. La, Ce, Pr.

The fourteen elements from Lanthanum (z = 57) to Lutetium (z = 71) are known as lanthanides.

The fourteen elements from Actinium (z = 89) to Lawrencium (z = 103) are known as actinides.