Relative charge and mass of subatomic particles
Definition
Relative charge
The charge on a particle compared with the charge on a proton, which is given a relative charge of +1.
Definition
Relative mass
The mass of a particle compared with the mass of a proton, which is given a relative mass of 1.
- A proton has a relative charge of +1+1+1 and a relative mass of 111.
- A neutron has a relative charge of 000 and a relative mass of 111.
- An electron has a relative charge of −1-1−1 and a relative mass of about 12000\frac{1}{2000}20001, or 0.00050.00050.0005.
- Protons and neutrons sit in the nucleus, while electrons are found in shells around it.
- Almost all of an atom's mass comes from its protons and neutrons, because each electron is so light.
Common Mistake
- Do not give the electron a relative mass of 000, because it is very small but not zero.
- Write the electron's relative charge as −1-1−1, since a minus sign on its own is not a charge value.
- Relative charges and relative masses have no units, because each one is a comparison with a proton.
Atomic and mass numbers give protons and neutrons
Definition
Atomic number
The number of protons in the nucleus of an atom.
Definition
Mass number
The total number of protons and neutrons in the nucleus of an atom.
- In the symbol 1123Na{}^{23}_{11}\text{Na}1123Na, the mass number 232323 is written at the top left and the atomic number 111111 at the bottom left.
- Every atom of an element has the same atomic number, so the atomic number alone tells you which element it is.
- The Periodic Table lists the elements in order of atomic number, starting with hydrogen at 111.
- The number of neutrons is the difference between the two numbers: neutrons=mass number−atomic number\text{neutrons} = \text{mass number} - \text{atomic number}neutrons=mass number−atomic number
- In a neutral atom, the number of electrons equals the number of protons, because the charges must balance.
Example
- Find the numbers of protons, neutrons and electrons in an atom of 919F{}^{19}_{9}\text{F}919F.
- The atomic number is 999, so the atom has 999 protons.
- Subtract the atomic number from the mass number: 19−9=1019 - 9 = 1019−9=10 neutrons.
- The atom is neutral, so it has 999 electrons.
Isotopes differ in their number of neutrons
Definition
Isotope
An atom of an element with the same number of protons as other atoms of that element but a different number of neutrons.
- Carbon-12 and carbon-14, written 612C{}^{12}_{6}\text{C}612C and 614C{}^{14}_{6}\text{C}614C, both have 666 protons.
- Carbon-12 has 12−6=612 - 6 = 612−6=6 neutrons, while carbon-14 has 14−6=814 - 6 = 814−6=8 neutrons.
- The number after the name, as in carbon-14, is the mass number of that isotope.
- Chlorine-35 and chlorine-37 both have 171717 protons, but they have 181818 and 202020 neutrons respectively.
- Isotopes of an element have the same number of electrons, so they react in the same way.
Common Mistake
Do not say that isotopes have different numbers of electrons or protons, because only the number of neutrons differs.
Ions form when atoms lose or gain electrons
Definition
Ion
An atom or group of atoms that carries an overall electric charge.
- A positive ion forms when an atom loses electrons, leaving more protons than electrons.
- A negative ion forms when an atom gains electrons, giving more electrons than protons.
- The number of protons never changes when an ion forms, because a different proton number would mean a different element.
- The number of neutrons also stays the same, so an ion has the same mass number as its atom.
- To find the electrons in an ion, start from the number of protons, then subtract the size of the charge for a positive ion or add it for a negative ion.
Example
- Find the numbers of protons, neutrons and electrons in the ion 1327Al3+{}^{27}_{13}\text{Al}^{3+}1327Al3+.
- The atomic number is 131313, so the ion has 131313 protons.
- Subtract the atomic number from the mass number: 27−13=1427 - 13 = 1427−13=14 neutrons.
- The 3+3+3+ charge means three electrons were lost: 13−3=1013 - 3 = 1013−3=10 electrons.
Self review
- What are the relative charge and relative mass of a neutron?
- Why is the relative mass of an electron not written as zero?
- How do the atoms of chlorine-35 and chlorine-37 differ?
- How many protons, neutrons and electrons are in 1632S2−{}^{32}_{16}\text{S}^{2-}1632S2−?
- Why does forming an ion never change the number of protons?
