C1 Atomic structure: Atomic number and Mass
C1 Atomic structure: Atomic number and Mass
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.
How do we work out the number of neutrons in any atom?
Number of neutrons = mass number - atomic number
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.
How do we work out the number of electrons in any atom?
Number of electrons = number of protons
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).
in the nucleus
protons and neutrons
1
1
almost zero (0) or 1 / 2000
number of protons
number of protons + number of neutrons
number of neutrons = mass number - atomic number
The number of negatively charged electrons is equal to the number of positively charged protons.
Radius is about 0.1 nm (1 x 10-10 m)
About 1/10 000 the size of the atom (about 1 x 10-14 m)
mass number = 11
atomic number = 5
number of protons = 5
number of electrons = 5
number of neutrons = 11 - 5 = 6
mass number = 12
atomic number = 6
number of protons = 6
number of electrons = 6
number of neutrons = 12 - 6 = 6
mass number = 27
atomic number = 13
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.
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 the sum of protons and neutrons in the nucleus,
so, there are 8 protons and 8 neutrons in the nucleus.