Question

An atom has a mass number of 9 and 5 neutrons. What is its atomic number? A 19 B. 14 O c.4 D. 11 SN​

Answer 1

`\boxed {\boxed {\sf C. \ 4}}`

Step-by-step explanation:

First, define some important terms.

• Mass number: the sum of the nucleons (protons and neutrons)
• Atomic number: the number of protons

We know the mass number is 9. So, the sum of protons and neutrons is 9.

• 
  `mass \ number =9`

• 
  `protons + neutrons = 9`

We also know the atoms has 5 neutrons. Therefore, the rest must be protons.

• 
  `protons + 5 \ neutrons =9`

Subtract the 5 neutrons from the overall mass number of 9.

• 
  `protons = 9- 5 \ neutrons \\`

• 
  `protons=4`

There are 4 protons, so the atomic number is also 4. This atom is beryllium.