Atoms are neutral because protons and electrons balance
Definition
Atomic number
The number of protons in the nucleus of an atom; it decides which element the atom is.
- In any atom the number of electrons equals the number of protons.
- The positive charge of the protons exactly balances the negative charge of the electrons, so an atom has no overall charge.
- Every atom of a particular element has the same number of protons, and this number is the atomic number.
Mass number counts the protons and neutrons together
Definition
Mass number
The total number of protons and neutrons in the nucleus of an atom.
- Almost all of an atom's mass is in its protons and neutrons, so counting them gives the mass number.
- The number of neutrons is found by subtracting: number of neutrons === mass number −-− atomic number.
Writing an atom: mass number on top, atomic number below
- An atom is written with its chemical symbol, the mass number as a superscript to the left, and the atomic number as a subscript to the left.
- For sodium, 1123Na^{23}_{11}\text{Na}1123Na has 111111 protons, 111111 electrons and 23−11=1223 - 11 = 1223−11=12 neutrons.
Example
Give the numbers of protons, neutrons and electrons in 1224Mg2+^{24}_{12}\text{Mg}^{2+}1224Mg2+.
- Atomic number =12= 12=12, so there are 121212 protons.
- Neutrons === mass number −-− atomic number =24−12=12= 24 - 12 = 12=24−12=12.
- The 2+2+2+ charge means the atom has lost 222 electrons, so it has 12−2=1012 - 2 = 1012−2=10 electrons.
Isotopes: same element, different number of neutrons
Definition
Isotope
Atoms of the same element, with the same number of protons, that have different numbers of neutrons.
- Because isotopes are the same element, they have the same atomic number but a different mass number.
- Chlorine has two common isotopes, 1735Cl^{35}_{17}\text{Cl}1735Cl and 1737Cl^{37}_{17}\text{Cl}1737Cl, each with 171717 protons but with 181818 and 202020 neutrons.
Common Mistake
- Isotopes differ in the number of neutrons, not protons.
- Changing the number of protons would change the element itself, so it would no longer be the same isotope.
Losing outer electrons turns an atom into a positive ion
Definition
Ion
A charged particle formed when an atom loses or gains electrons.
- If an atom loses one or more of its outer electrons, it is left with more protons than electrons.
- It then has an overall positive charge and is called a positive ion.
Exam technique
When a question gives the symbol notation, read it in a fixed order to avoid slips:
- the bottom number (atomic number) is the number of protons,
- the top number (mass number) minus the bottom number is the number of neutrons,
- for a neutral atom the electrons equal the protons, then adjust for any charge shown.
Self review
- What does the atomic number of an atom tell you?
- How do you work out the number of neutrons from the mass number and atomic number?
- How many protons, neutrons and electrons are in 1737Cl^{37}_{17}\text{Cl}1737Cl?
- What is the same, and what is different, about two isotopes of an element?
- How is a positive ion formed from an atom?
