Why? The periodic table has been organized in such a way that it has elements with strong similarities being grouped together. For example, going down the periodic table, we see elements organized as groups. Each group of elements has been grouped because they have the same number of electrons in their valence orbital. Going horizontally along the periodic table, the elements are organized into periods, according to the outermost electron orbital. As you move along the period, each additional group would have another electron added to it orbital. Group 1 elements all have one electron in their valence orbital, while all group 17 elements have seven electrons in their valence orbital.
As a result of the organization of the periodic table, other trends can be determined. As you move from left to right, you move from metals to non-metals. Period 3 starts with sodium (Na), which is a metal and ends with Argon (Ar), which is a noble gas. Also, going from left to right, there is an increase in the number of protons and electrons. This results in greater attraction between the protons and electrons and a slight decrease in atomic radius as a result of the increased attraction. This means moving from left to right there is a decrease in the radius of the elements.
Answering the Question:
To answer this question an understanding of the organization of the periodic table is very important. Answer (1) states nonmetallic properties and atomic radius will decrease from left to right across the periodic table. Nonmetallic properties actually increase when moving from left to right across the periodic table. Group 1 and 2 are metals while group 17 and 18 are gases, meaning that as you go from left to right there is an increase in nonmetallic behavior of the elements or a decrease in metallic properties.
Answer (2) has a decrease in nonmetallic properties, which as stated before is incorrect, immediately eliminating answer (2) a possible right answer.
Answer (3) states a decrease in metallic properties and atomic radius. Both these properties are in agreement with trends observed in the periodic table. There is a decrease in metallic properties when moving from the group 1 metals towards the group 18 gases. Also there is a decrease in atomic radius due to the increase in nuclear attraction for electrons as the number of protons increase. Answer (3) is correct and is a possible answer to the question.
Answer (4) suggests that metallic properties and ionization energy decrease. While it is true that metallic properties will decrease the ionization energy will not. Ionization energy is a measure of the energy required to remove an electron from an element. Group 1 and 2 will readily give up electrons to form positive ions, however group 17 elements will readily accept an electron. Therefore, ionization energy increases from left to right.
This will allow for the analysis of trends horizontally and vertically along the periodic table. Therefore, it can be concluded that (3) is the best possible answer as it has two of the trends present in the period.