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The elements are arranged in the form of a table, known as periodic table. There are 18 columns (known as groups) and 7 rows (known as periods, including actinides and lanthanides). The question refers to elements in a particular group ad relates to the differences in properties as one goes from top to bottom. This difference in properties is attributed to differences in atomic size and ease of removal (or acceptance) of electrons. As we move from top to bottom within a group, the atomic size increases. Due to this increase, the force with which nucleus attracts the valence electrons decreases. This causes easier loss of electrons. That is to say, electronegativity (ability to attract electrons) and electron affinity decrease as we move down a group. For example, chlorine will readily accept an electron, while lower down the group, other halides will not be able to attract electrons with same force. This results in differences in properties of top elements, as compared to elements down the group.
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