Final answer:
In the periodic table, groups consist of elements with similar chemical properties and are numbered 1-18. Periods are rows of elements with increasing atomic numbers, showing a change in properties across the table. The difference between periods and groups lies in their direction (vertical vs. horizontal) and in the elements' properties and behaviors.
Step-by-step explanation:
Compare and Contrast Periods and Groups on the Periodic Table
In the modern periodic table, elements are ordered by increasing atomic number and are arranged in rows and columns that highlight periodic trends and chemical properties. The vertical columns on the periodic table are known as groups, and they are numbered from 1 to 18 from left to right. Elements within the same group exhibit similar chemical properties because they possess the same number of electrons in their outermost shell. For instance, the alkali metals (Group 1) and the alkaline earth metals (Group 2) on the far left are known for being highly reactive, particularly with water. On the far right, the halogens (Group 17) are known for their reactivity with metals to form salts, while the noble gases (Group 18) are known for their lack of reactivity due to having a full valence shell.
The horizontal rows are called periods, and there are seven periods in the periodic table. Each period starts with the least reactive elements with a given number of electron shells, which then increase in reactivity and atomic number across the period. The length of each period is determined by the filling of the quantum levels for electrons; therefore, the first period has only two elements (hydrogen and helium), while subsequent periods have more. As you move from left to right across a period, the elements change from metallic, to metalloids, to nonmetals.
In summary, groups classify elements with similar chemical behaviors, while periods organize elements with increasing atomic numbers and show the gradual changes in properties across a single row.