Final answer:
Elements in a horizontal row of the periodic table do not have an equal number of electrons in their outermost shell; instead, this number increases from one element to the next within the row.
Step-by-step explanation:
The statement 'Elements in a given horizontal row all have an __________ number of electrons in their outermost shell' is meant to be completed with the word 'equal.' However, it's important to understand that while elements in a given horizontal row (also known as a period) of the periodic table all have the same number of electron shells, the number of electrons in the outermost shell actually increases from left to right. This increasing number corresponds with the group number in the periodic table.
For example, every element in the first group has one electron in their outermost shell, while each element in the second group has two, and so on. Elements like helium (He), neon (Ne), and argon (Ar) have completely filled outermost shells, making them inert or noble gases and highly stable. Conversely, elements such as hydrogen (H), lithium (Li), and sodium (Na) have just one electron in their outermost shell and easily form ions by donating that electron. The group 17 elements like fluorine and chlorine have seven electrons in their outermost shell and often gain an electron to achi