Question

c. thomson set up the gold foil experiment. most of the alpha particles passed right through the gold foil. this led to the discovery of the nucleus. d. thomson used a cathode - ray tube for this experiment. the deflection of the ray towards the positively charged plate led to the discovery of electrons. 12. the drawing below represents the bohr model of an isotope of helium. what is the mass number of 1 atom of this isotope of helium? a. 1 amu b. 2 amu c. 3 amu d. 4 amu 13. in order for an atom to be neutral, it must contain an equal number of electrons and protons. why is this true? a. they have the same charge, but different masses. b. they have equal but opposite charges. c. they have the same mass, but unequal charges, which stabilizes the atom. d. protons are positive and electrons are neutral, which covered. 14. the neutron was the last subatomic particle to be discovered. what would be a plausible explanation as to why that happened? a. it has no charge, therefore it wouldnt respond to typical experiments using positive and negative charges. b. it was the heaviest subatomic particle and technology wasnt in existence yet to measure. c. being inside the protons in the nucleus masked their existence. d. it is smaller than the other subatomic particles and was more difficult to find. 15. physical and chemical properties can be used to describe substances. which of the follow tests can be used to determine a substances physical properties? a. physical description and reaction to acid b. physical description and reaction to heat

Explanation:

12.

Step1: Recall mass - number formula

Mass number (A)=Number of protons + Number of neutrons.

Step2: Identify protons and neutrons in Bohr model

In the Bohr model of helium, there are 2 protons and 1 neutron in the nucleus.

Step3: Calculate mass number

$A = 2+1=3$.

Protons have a positive charge (+1) and electrons have a negative charge (- 1). For an atom to be electrically neutral, the positive and negative charges must cancel out. So, it must have an equal number of electrons and protons because they have equal but opposite charges.

Neutrons are neutral particles (no charge). Most early - atomic experiments relied on the interaction of charged particles with electric and magnetic fields. Since neutrons have no charge, they do not respond to these typical experiments using positive and negative charges, making them difficult to discover.

Answer:

C. 3 amu

13.