Because the nuclear charge increases across a period and so it has a stronger pull on the outer electrons and will pull in the radius
When you are asked a question like this, you can always ask yourself this question. Can I change it back after this change? For example, if you are burning wood, you cannot bring it back to wood after you burn it, therefore, it is a chemical change. However, if you boil and evaporate water, you can make the water condense again back into its liquid form. In this case, you cannot bring the tomato back to its raw state. Therefore, cooking raw tomatoes is a chemical change.
