Question

ex) phosphorus, p
atomic # = 15 (top # in h box on the periodic table)
mass # = 31 (rounded atomic mass from the periodic table)

protons = 15 (equal to atomic #)

electrons = 15 (also equal to atomic #)

neutrons = 16 (equal to mass # - atomic #)

1. helium, he

atomic # = 2
mass # = 4

protons = 2

electrons = 2

neutrons = 2

drawing of helium

Explanation:

Step1: Recall the formula for neutrons

The number of neutrons is calculated as \( \text{Mass \#} - \text{Atomic \#} \).

Step2: Substitute the values for Helium

For Helium, Atomic \# = 2 and Mass \# = 4. So, \( \text{\# Neutrons} = 4 - 2 = 2 \). Wait, the original had a mistake (it said 3, but correct is 2). Let's recalculate properly.
Given Atomic number (\( Z \)) = 2, Mass number (\( A \)) = 4. The formula for number of neutrons (\( N \)) is \( N = A - Z \).
Substituting the values: \( N = 4 - 2 = 2 \).

Answer:

The correct number of neutrons for Helium is 2 (the original 3 was incorrect). The steps show that using \( \text{\# Neutrons} = \text{Mass \#} - \text{Atomic \#} \), with Mass \# = 4 and Atomic \# = 2, we get 2 neutrons.