ATOMIC STRUCTURE
An atom has a central nucleus. This is surrounded by electrons arranged in shells.
Subatomic particles
The nuclei of all atoms contain subatomic particles called protons. The nuclei of most atoms also contain neutrons.
ATOMIC NUMBER & MASS |
Atomic number
The number of protons in an atom of an element is its atomic number. Remember that:
- all atoms of a given element have the same number of protons
- atoms of different elements have different numbers of protons
Atomic Mass
Key fact
The mass number of an atom is its total number of protons and neutrons.
Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.
Calculating numbers of subatomic particles
The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.
To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number:
- number of protons = atomic number
- number of electrons = atomic number
- number of neutrons = mass number - atomic number
Q & A
1. How can subatomic particles be used to determine the atomic mass of an atom?
Subatomic particles like protons and neutrons inside an atom's nucleus help figure out its atomic mass. Protons have a positive charge and weigh about the same as neutrons, which have no charge. Electrons, which are outside the nucleus, are much lighter and don't count much in determining atomic mass. For instance, let's take carbon, which has 6 protons in its nucleus. By adding up the number of protons and neutrons (which weigh about the same), we get its atomic mass. For carbon, the most common form, carbon-12, has 6 protons and 6 neutrons, making its atomic mass about 12 atomic mass units. So, by knowing how many protons and neutrons an atom has, we can figure out its atomic mass.
2. Using the periodic table record the number of protons, neutrons, electrons, atomic mass and period (horizontal rows) for Sodium.
- number of protons = atomic number.
So protons = 11
- number of electrons = atomic number.
So electrons = 11
- number of neutrons = mass number - atomic number.
mass number is rounded up to 23 - atomic number (11).
23-11 = 12. Neutrons = 12.
Atomic Mass number = 22.99 (the large number listed on the each element in the periodic table).
Period (horizontal rows) = 3 (electron configuration).