STAAR review 8.5A

How many protons, neutrons, and electrons are present in an atom of hafnium, Hf, with a mass number of 178?

• 72 protons, 178 neutrons, 106 electrons

• 72 protons, 106 neutrons, 72 electrons

• 178 protons, 250 neutrons, 72 electrons

• 106 protons, 72 neutrons, 106 electrons

An atom has 25 protons, 30 neutrons, and 25 electrons. What is the charge of the atom's nucleus?

• +25

• +30

• -25

• -30

Which of these best describes one of the subatomic particles that could be found at location X in the model of an atom shown?

• It has mass but no charge

• It has no mass and a positive charge

• It has a large mass and a negative charge

• It has no mass and an equal number of positive and negative charges.

In the early 1900s many scientists thought that an atom consisted of a positive with negative charges scattered throughout the substance. Then Ernest Rutherford completed an experiment that changed the concept of an atom. His discovery led to the understanding that an atom consists mostly of empty space with -

• protons orbiting a dense nucleus made of electrons and neutrons

• electrons orbiting a dense nucleus made of protons and neutrons

• neutrons and protons orbiting a cloud of electrons

• electrons and protons orbiting a cloud of neutrons