Unit 4 Pre-Assessment

An electron has a ______ charge

(Positive, negative, or neutral)

The ​______ in the nucleus of an atom give it a ​______ charge

• protons, neutral

• electrons, negative

• protons, positive

• neutrons, positive

Answer explanation

The nucleus of the atom is positively charged because it contains protons, which are positively-charged!

Sodium is an important element in the human body. If you put solid sodium metal in your mouth, it would explode. But if you eat a sodium ion, your body can use it!

Which subatomic particle is responsible for the way that sodium reacts with the moisture in your mouth and body?

• proton

• electron

• neutron

Answer explanation

Protons give the identity of an atom, neutrons make the nucleus more stable, and electrons determine the properties and reactivity of an atom.

The example describes how sodium reacts, which is a property of sodium. Since electrons determine the properties of an element, we can say that electrons are responsible for the change in reactivity.

What is the mass of a neutron?

• 1 AMU

• 9.1 x 10^-31 kg

• 0 AMU

• 1 mole

What is the atomic mass of a Strontium (Sr) atom that has 51 neutrons?

• 89 AMU

• 87.62 AMU

• 100 AMU

• 118 AMU

Answer explanation

Sr has the atomic number 38, so it has 38 protons. To find mass, add the number of protons and neutrons (each proton and neutron has a mass of 1 AMU).

protons + neutrons = atomic mass

38 protons + 51 neutrons = 89 AMU

Name the element that contains 31 protons and 50 neutrons. A single drop of this metalloid on an aluminum can will cause it to crumble!

• Gallium (Ga)

• Tin (Sn)

• Mercury (Hg)

• Thallium (Tl)

Answer explanation

The number of protons tells you what element you have. An atom with 31 protons has the atomic number 31, which is Gallium (Ga).

The isotope of Phosphorous (P) with an atomic mass of 33 AMU contains ___ neutrons.

Answer explanation

Phosphorous has the atomic number 15, so it has 15 protons. The question says that this atom of phosphorous has a mass of 33 AMU. To find the number of neutrons, subtract the number of protons from the atomic mass.

atomic mass - protons = neutrons

33 AMU - 15 protons = 18 neutrons

What is the atomic mass of the isotope Uranium-238?

• 238 AMU

• 92 AMU

• 238.03 AMU

• 146 AMU

The image shows a diagram of magnesium. It has 12 protons, 12 neutrons, and 12 electrons. Is the atom neutral, or does it have a charge?

• Neutral, because it has the same number of protons and electrons.

• Neutral, because it contains neutrons.

• It has a net positive charge because it contains protons.

• It has a negative charge because it contains electrons.

Answer explanation

Neutral atoms contain the same number of protons as electrons.

Protons have a positive charge, so the total number of positive charges is 12+. Electrons have a negative charge, so the total negative charge is 12-. Together, there is 0 total charge on this atom so it is neutral.

Most atoms have at least 1 neutron, but the neutrons do not affect the charge of an atom.

Which element contains 0 neutrons?

• Helium

• Hydrogen

• Deuterium

• Carbon

Answer explanation

Hydrogen is the only element on the perioidic table that does not have neutrons!

The atomic number tells you how many protons an atom has. The mass can tell you how many protons and neutrons an atom has.

Hydrogen has the atomic number 1, and the atomic weight on the periodic table is 1.008, which is rounded to 1 AMU. Since the mass is 1 and hydrogen has 1 proton, it must have 0 neutrons.

atomic mass - number of protons = number of neutrons

1 AMU - 1 proton = 0 neutrons