What two features must a sample have if it is to accurately represent a population

It must be large

Random

Explanation:

For a sample to represent a population accurately, it must be large and contain random sample spaces.

A population is a group of organism of the same species living in a particular place where they can reproduce.

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.