Remembering which elements have a low atomic number and high valency can be aided by familiarizing yourself with certain patterns and trends on the periodic table. Here are a few tips:
Valency trends: Elements in the same group (vertical column) on the periodic table tend to have similar valency. For example, Group 1 elements (alkali metals) have a valency of +1, Group 2 elements (alkaline earth metals) have a valency of +2, and Group 17 elements (halogens) have a valency of -1.
Group 1 and Group 2 elements: The elements in Group 1 (such as hydrogen, lithium, sodium, etc.) and Group 2 (such as beryllium, magnesium, calcium, etc.) generally have low atomic numbers and exhibit high valency due to the relatively few valence electrons they possess.
Transition metals: Transition metals, located in the d-block of the periodic table, often exhibit variable valency. The valency of transition metals can vary depending on the specific compound or reaction they are involved in.
Nonmetals: Many nonmetals, particularly in Group 15 (such as nitrogen, phosphorus, etc.) and Group 16 (such as oxygen, sulfur, etc.), can have higher valencies due to their ability to gain electrons to complete their valence shells.
To remember these patterns, it can be helpful to review and practice with the periodic table, learn about common groups and their valencies, and engage in activities that involve naming and writing chemical formulas for compounds. With time and practice, you will become more familiar with the elements and their characteristics.