In chemistry, the charge, also known as the oxidation state or valency, varies across a period and down a group in different ways.

Across a Period:

As you move from left to right across a period, the atomic number increases, and the number of valence electrons also increases. This results in:

1. Increase in electronegativity: Atoms become more electronegative, meaning they have a greater tendency to attract electrons.

2. Increase in oxidation state: The oxidation state of elements generally increases across a period, with some exceptions.

3. Change in charge: The charge on ions formed by elements in a period can vary. For example, in Period 3, the charge on ions formed by elements like Na (sodium) and Mg (magnesium) is +1 and +2, respectively.

Down a Group:

As you move down a group, the atomic number increases, and the number of valence electrons remains the same. This results in:

1. Decrease in electronegativity: Atoms become less electronegative, meaning they have a lower tendency to attract electrons.

2. Similar oxidation state: The oxidation state of elements in a group remains relatively constant, with some exceptions.

3. Similar charge: The charge on ions formed by elements in a group is often similar. For example, in Group 1, the charge on ions formed by elements like Li (lithium), Na (sodium), and K (potassium) is +1.

In summary, the charge on ions formed by elements varies across a period due to changes in electronegativity and oxidation state, while down a group, the charge remains relatively constant due to similar electronegativity and oxidation states.