Question

unit 3 chem l test 2526
figure 6 - 8 represents the nucleus of an atom.
what are the atomic number and mass number of this atom?
the atomic number is 9 and the mass number is 20.
the atomic number is 9 and the mass number is 11.
the atomic number is 11 and the mass number is 19.
the atomic number is 11 and the mass number is 20.

Explanation:

Step1: Count protons

The atomic number is equal to the number of protons. Counting the black - colored (proton) spheres in the nucleus, we find there are 9 protons. So the atomic number is 9.

Step2: Count neutrons

The mass number is the sum of protons and neutrons. Counting the white - colored (neutron) spheres, we find there are 11 neutrons. Mass number = number of protons+number of neutrons = 9 + 11=20.

Answer:

The atomic number is 9 and the mass number is 20.