Answer:
1) The vertical and horizontal trends found in the main-group elements of the periodic table with respect to electronegativity (EN), are that EN increases the further you go from the bottom to the top, and from left to right.
2) According to Pauling's scale, the two most electronegative elements are Fluorine (F) and Oxygen (O), while the two least electronegative elements are Francium (Fr) and Caesium (Cs).
Step-by-step explanation:
1) When we are talking about the main-group elements of the periodic table (main-group elements are the ones positioned in Groups 1, 2, and 13 to 18 of the periodic table), there exists a trend in which the further you go from left to right, the higher the electronegativity (EN) the elements will have.
We could think of this like as the elements are getting closer and closer to having the "ideal" electron composition of a noble gas (by having 8 electrons in its outer shell), they become more needy of electrons, prone to get them from other atoms, and as a result have a higher EN.
On the other hand, the EN increases the further you go from the bottom to the top in the main-group elements of the periodic table. This happens because, as atoms get smaller, they come with fewer shells of electrons, and, as a result, said electrons (negative charge) are much closer to the nucleus (positive charge) than if there were a dozen more electrons: this lets the positive charge of the nucleus "grab" electrons from other atoms way more easily. So, the less amount of shells an atom has, the higher its EN.
2) By applying these general trends in EN according to Pauling scale, we can find the most electronegative elements by going up and right in the periodic table. This way, we find that the two elements with the highest EN are Fluorine (F) and Oxygen (O), with an EN of 3.98 and 3.44, respectively. (Please refer to the image attached).
Finally, we can find the least electronegative elements by going down and left in the periodic table. We find this way that the two elements with the lowest EN are Francium (Fr) and Caesium (Cs), with an EN of 0.7 and 0.79, respectively.