The radii of the atoms become smaller from sodium to chlorine across period 3. This decrease is primarily a result of

Nuclear Charge

Explanation:

As we move from sodium to chlorine across, there's increase in atomic number (protons). This also means there is increase in the valence electrons (The valence electrons is denoted by the group).

As the number of protons increases, the nuclear energy also increases. Due to this, the electrons are attracted more strongly and pulled closely to the nucleus.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.  

metric time was an attempt to create a standard unit of time based off of 10, but it never took off an is not included in the metric system (si)