A certain atom has 11 electrons 8 neutrons and a charge of - 1. What is it's atomic mass

20 amu

Explanation:

An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.

All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons. The sum of neutrons and protons is the mass number of an atom while the number of protons are number of electrons is the atomic number of an atom.

Atomic mass = Number of protons + number of neutrons

Atomic number = Number of electrons or number of protons.

In given question it is stated that atom has 11 electrons and - 1 charge it means this atom has 12 electrons in neutral state.

Thus it has 12 protons because number of electrons and protons are always equal.

Atomic mass of given atom:

Atomic mass = Number of protons + number of neutrons

Atomic mass = 12 + 8 = 20 amu