In general for main-group elements, atomic size _____ from left to right across a period and _____ down a group.


Question: In general for main-group elements, atomic size _____ from left to right across a period and _____ down a group.

In general, for main-group elements, atomic size decreases from left to right across a period and increases down a group.

Across a period: As we move from left to right across a period, the number of protons in the nucleus increases. This increases the effective nuclear charge, which is the pull that the nucleus exerts on the electrons. The stronger the pull, the closer the electrons are held to the nucleus, and the smaller the atom is.

Down a group: As we move down a group, the number of energy levels increases. This means that the electrons are further away from the nucleus, and the atom is larger.

Here is a table that shows the atomic radii of some main-group elements:

ElementSymbolAtomic radius (pm)
LithiumLi152
SodiumNa186
PotassiumK227
RubidiumRb244
CesiumCs265
FranciumFr270

As you can see, the atomic radius of the elements increases as we move down the group.

There are a few exceptions to this general trend. For example, the atomic radius of gallium (Ga) is smaller than that of aluminum (Al), even though gallium is below aluminum in the group. This is because gallium has a higher effective nuclear charge than aluminum due to its more compact electron configuration.

Overall, the trend of decreasing atomic size from left to right across a period and increasing atomic size down a group is a useful rule of thumb for understanding the atomic structure of main-group elements.

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