Atomic number and Mass number

Almost all the mass of the atom comes from the sub-atomic particles found in the nucleus, which are protons and neutrons.

So, mass number = number of protons + number of neutrons

The number of protons in an atom is called the proton number or atomic number.



All atoms of a particular element have the same number of protons.

Atoms of different elements have different numbers of protons.




E.g. Number of neutrons in lithium atom = 7 - 3 = 4 


Reminder: 

The overall charge of an atom is zero / neutral 

because there is an equal number of electrons (negative) and protons (positive) in an atom.




E.g. Number of protons in lithium atom = 3

So, number of electrons in lithium atom = 3





Atoms are very small, having a radius of about 0.1 nm (1 x 10-10 m).

 

The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10-14 m).

 



Q1. Where is almost all the mass in an atom? 

in the nucleus

Q2. What sub-atomic particles are found in the nucleus? 

protons and neutrons

Q3. What is the mass of a proton? 

1

Q4. What is the mass of a neutron? 

1

Q5. What is the mass of a electron? 

almost zero (0) or 1 / 2000

Q6. What is the atomic number? 

number of protons

Q7. What is the mass number (atomic mass)? 

number of protons + number of neutrons

Q8. How do we work out the neutron number? 

number of neutrons = mass number - atomic number 

Q9. Why do atoms have no overall charge? 

The number of negatively charged electrons is equal to the number of positively charged protons.

Q10. Approximately how large are atoms? 

Radius is about 0.1 nm (1 x 10-10 m)

Q11. How large is the nucleus compared to the whole atom? 

About 1/10 000 the size of the atom (about 1 x 10-14 m)

Q12. The figure below shows information about an atom of boron.

What is the mass number and atomic number of boron? 

mass number = 11

atomic number = 5

Q13. The figure below shows information about an atom of boron.

What is the number of protons, electrons and neutrons in an atom of boron? 

number of protons = 5

number of electrons = 5

number of neutrons = 11 - 5 = 6

Q14. The figure below shows information about an atom of carbon.

What is the mass number and atomic number of carbon? 

mass number = 12

atomic number = 6

Q15. The figure below shows information about an atom of carbon.

What is the number of protons, electrons and neutrons in an atom of carbon? 

number of protons = 6

number of electrons = 6

number of neutrons = 12 - 6 = 6

Q16. The figure below shows information about an atom of aluminium.

What is the mass number and atomic number of aluminium? 

mass number = 27

atomic number = 13

Q17. The figure below shows information about an atom of aluminium.

What is the number of protons, electrons and neutrons in an atom of aluminium? 

number of protons = 13

number of electrons = 13

number of neutrons = 27 - 13 = 14

Q18. An atom of argon is represented as in the figure below. 

Determine the number of protons and the number of neutrons in one atom of argon. 

number of protons = 18

number of neutrons = 40 - 18 = 22

Q19. An atom of sulfur is represented as in the figure below. 

 Complete the table above. 

number of neutrons = 32 - 16 = 16

number of protons = 16

Q20. Complete the sentences. (3)

Atoms of the same element have the same atomic number because they have the same number of _______________.

Atoms of the same element can have different mass numbers because they have different numbers of _______________.

Atoms have no overall charge because they have the same number of _______________ and _______________.

Atoms of the same element have the same atomic number because they have the same number of protons.

Atoms of the same element can have different mass numbers because they have different numbers of neutrons.

Atoms have no overall charge because they have the same number of electrons and protons.

Q21. If an atom has 9 protons and 10 neutrons, what is the mass number and name of this element? 

Mass number = number of protons + number of neutrons

Mass number = 9 + 10 = 19


Number of protons = 9, so atomic number = 9


We look in the periodic table and find the element fluorine.

Q22. An oxygen atom can be represented as the figure below. 

Explain why the mass number of this atom is 16.

You should refer to the numbers of sub-atomic particles in the nucleus of the atom. (2)

mass number is 16, 

because there are 8 protons and 8 neutrons in the nucleus.