Science Review 8.5(A)

What is the difference between the number of electrons in an atom of selenium, Se, and the number of electrons in an atom of aluminum, Al?

• 20
• 21
• 22
• 23

How many protons, neutrons, and electrons are present in an atom of hafnium, 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 of certain element has 36 protons, 36 electrons, and a mass number of 84.  At room temperature this element is a very stable gas.  How many neutrons are in this atom?

• 1
• 2
• 3
• 48

In the early 1900s many scientists thought that an atom consisted of a positive substance 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 nucles ade of elctrons and neutrons
• electrons orbiting a dense nucleus made of protons and neutrons
• neutrons and protons orbiting a clous of electrons
• electrons and protons orbiting a cloud of neutrons

A model of beryllium atom is shown above.  What types of particles are found in the cloud surrounding the atom's nucleus? 

• Positively charged particles and negatively charged protons
• Negatively charged particles only
• Neutral particles and positively charged particles
• Positively charged particles only

What is the mass number of potassium (K) atom that has 20 neutrons?

• 18
• 19
• 20
• 39

Which of the following best describes an electron?

• It has no charge and about the same mass as a proton.
• It has a negative charge and much less mass than a proton. 
• It has a positive charge and much more mass than a neutron.
• It has a negative charge and about the same mass as a neutron. 

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

• +25
• +30
• -25
• -30