Question #23973

The first thing to do when trying to write an atom's electron configuration is to figure you exactly how many electrons must be accounted for.

To do that, grab a periodic table and look for that atom's atomic number.

In your case, beryllium, #"Be"#, is located in period 2, group 2 of the periodic table, and has an atomic number equal to #4#. This means that a neutral beryllium atom will contain #4# protons in its nucleus and #4# electrons surrounding its nucleus.

Therefore, your electron configuration must account for #4# electrons. The complete electron configuration for beryllium will be

#"Be: " 1s^2 2s^2#

Now, in order to use the noble gas shorthand notation, you must first identify which noble gas comes immediately before beryllium in the periodic table.

In this case, the only option available is helium, #"He"#. Helium has #2# electrons surrounding its nucleus, so its electron configuration will look like this

#"He: " color(blue)(1s^2)#

Notice that the configuration that accounts for the first two electrons is identical for both atoms. This means that you can replace it in beryllium's configuration to get

#"Be: " color(blue)(1s^2) 2s^2#

The electron configuration of helium is written like this, #["He"]#, which means that the noble gas shorthand for beryllium will be

#"Be: " ["He"] 2s^2#