Account for the fact that the line that separates the metals from the nonmetals on the periodic table is diagonal downward to the right instead of horizontal or vertical.

To begin, let's review the layout of the periodic table. The elements are arranged in columns called groups and rows called periods. Going across a period, the elements exhibit a pattern in terms of their atomic structure and properties.

Elements in the same group have similar electron configurations and properties, while elements in the same period show a pattern of increasing atomic number (number of protons). As we move from left to right across a period, the number of protons and electrons increases, and the atomic radius generally decreases. The overall effect is an increase in the effective nuclear charge - the net positive charge experienced by the electrons. This strengthens the attraction between the nucleus and the electrons, leading to changes in the properties of the elements.

Elements can be broadly classified as metals or nonmetals based on their properties. Metals tend to be shiny, malleable, ductile, and good conductors of heat and electricity. They are generally electropositive - they tend to lose electrons in chemical reactions. Nonmetals, on the other hand, are usually brittle, poor conductors of heat and electricity, and more electronegative - they tend to gain electrons in chemical reactions.

As we move across the periodic table from left to right, the properties change gradually from metallic to non-metallic. This transition is not sharp and instead follows a diagonal pattern, with the elements along this line (metalloids) showing intermediate properties between metals and nonmetals. The diagonal line separating metals and nonmetals is due to the overlapping trends of atomic structure and electronegativity.

When looking at electronegativity, metals tend to have lower values whereas nonmetals have higher values. Electronegativity increases across a period from left to right, and decreases down a group. The diagonal boundary stems from the way that electronegativity values change across and down the periodic table. As one moves down and to the right, the elements exhibit a gradual change from metallic to non-metallic properties, with no sudden or abrupt transition. In conclusion, the diagonal line that separates metals from nonmetals on the periodic table is a result of the gradual change in atomic structure and electronegativity of elements as we move across periods and down groups. Both the properties of elements and the electronegativity values exhibit a smooth change, leading to a diagonal boundary between metals and nonmetals.