Working Out The Number of Protons, Electrons and Neutrons From the Atomic Symbol |
 Video tutorial |
An atom is the smallest part of an element that can exist alone or in combination with other atoms. Atoms are themselves made up of a positively charged nucleus and negatively charged electrons which orbit around the nucleus. The nucleus is itself made up of protons and (usually) neutrons.
Elements, atoms and isotopes:
Every atom of an element has the same number of electrons and protons. A neutral atom has equal numbers of electrons and protons.Properties of electrons, protons and neutrons:
The last column gives the mass in atomic mass units (u) where 1 u = 1.66054 × 10-27 kg
Particles Symbol Charge (C) Mass (kg) Mass (u) electron e- -1.6022 × 10-19 9.1094 × 10-31 5.4858 × 10-4 proton p +1.6022 × 10-19 1.6726 × 10-27 1.0073 neutron n 0 1.6749 × 10-27 1.0086
The atomic number Z gives the number of protons in the nucleus of an atom. In a neutral atom this is also equal to the number of electrons. Many atoms can, however, gain electrons to become anions or lose electrons to become cations.
The mass number A is the number of protons plus the number of neutrons in the nucleus. Although each atom of an element has the same number of protons, the number of neutrons can vary. Atoms of an element with different number of neutrons are called isotopes.
The atomic symbol:
A X Z The names of elements have been chosen for a variety of reasons over the course of human and chemical history. It is represented by a unique chemical symbol, X.
The atomic symbol is used to identify the element to which an atom belongs and the number of electrons, protons and neutrons it contains. As noted above, every neutral atom of an element has the same number of electrons and protons:number of protons = number of electrons = atomic number = ZAs the mass number is equal to the number of protons plus the number of neutrons:number of neutrons = mass number - number of protons = A - Z
Because the atomic number gives the number of protons and as this characterizes the element, it also specifies what the chemical symbol is. For this reason, the atomic number is sometimes not given when the atomic symbol is written down. For example both 12C and 13C have Z = 6 - they just differ in the number of neutrons and so have different mass numbers.
Many atoms can gain or lose one or more electrons when involved in chemical reactions. A charged atom is called an ion.
Cations and anions:
Cations have lost one or more electrons. This is represented using a "+" sign on the atomic symbol such as Li+ (a lithium atom with one electron removed) and Mg2+ (a magnesium atom with two electrons removed). Anions have gained one or more electrons. This is represented using a "-" sign on the atomic symbol such as Cl- (a chlorine atom with one extra electron) and O2- (a oxygen atom with two extra electrons).
For an ion,number of electrons = atomic number - chargeNote that (negatively charged) anions have more electrons than the neutral atom and (positively charged) cations have fewer electrons than the neutral atom. The number of protons and the number of neutrons in an ion are exactly the same as they are in the neutral atom.
Work through these examples carefully to see how the rules are applied and then test yourself with the self test quiz.
1. There are 1 electron, 1 proton and 0 neutrons in the species with the atomic symbol
1 H 1 The number of protons = atomic number = Z = 1
The number of neutrons = mass number - atomic number = A - Z = 1 - 1 = 0
As the species is neutral, the number of electrons = atomic number = 1
A H Z
2. There are 1 electron, 1 proton and 1 neutron in the species with the atomic symbol
2 H 1 The number of protons = atomic number = Z = 1
The number of neutrons = mass number - atomic number = A - Z = 2 - 1 = 1
As the species is neutral, the number of electrons = atomic number = 1
A H Z
2. There are 6 electrons, 6 protons and 6 neutrons in the species with the atomic symbol
12 C 6 The number of protons = atomic number = Z = 6
The number of neutrons = mass number - atomic number = A - Z = 12 - 6 = 6
As the species is neutral, the number of electrons = atomic number = 6
A C Z
4. There are 6 electrons, 6 protons and 7 neutrons in the species with the atomic symbol
13 C 6 The number of protons = atomic number = Z = 6
The number of neutrons = mass number - atomic number = A - Z = 13 - 6 = 7
As the species is neutral, the number of electrons = atomic number = 6
A C Z
5. There are 12 electrons, 12 protons and 12 neutrons in the species with the atomic symbol
24 Mg 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 24 - 12 = 12
As the species is neutral, the number of electrons = atomic number = 12
A Mg Z
6. There are 12 electrons, 12 protons and 13 neutrons in the species with the atomic symbol
25 Mg 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 25 - 12 = 13
As the species is neutral, the number of electrons = atomic number = 12
A Mg Z
7. There are 12 electrons, 12 protons and 14 neutrons in the species with the atomic symbol
26 Mg 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 26 - 12 = 14
As the species is neutral, the number of electrons = atomic number = 12
A Mg Z
8. How many electrons, protons and neutrons are there in the species with the atomic symbol
24 Mg2+ 12 ?
8. There are 10 electrons, 12 protons and 12 neutrons in the species with the atomic symbol
24 Mg2+ 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 24 - 12 = 12
As the species is a +2 cation, it has lost 2 electrons
The number of electrons = atomic number - charge = 12 - 2 = 10
A Mg2+ Z
9. How many electrons, protons and neutrons are there in the species with the atomic symbol
25 Mg2+ 12 ?
9. There are 10 electrons, 12 protons and 13 neutrons in the species with the atomic symbol
25 Mg2+ 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 25 - 12 = 13
As the species is a +2 cation, it has lost 2 electrons
The number of electrons = atomic number - charge = 12 - 2 = 10
A Mg2+ Z
10. How many electrons, protons and neutrons are there in the species with the atomic symbol
26 Mg2+ 12 ?
10. There are 10 electrons, 12 protons and 14 neutrons in the species with the atomic symbol
26 Mg2+ 12 The number of protons = atomic number = Z = 12
The number of neutrons = mass number - atomic number = A - Z = 26 - 12 = 14
As the species is a +2 cation, it has lost 2 electrons
The number of electrons = atomic number - charge = 12 - 2 = 10
A Mg2+ Z
11. How many electrons, protons and neutrons are there in the species with the atomic symbol
79 Br 35 ?
11. There are 35 electrons, 35 protons and 44 neutrons in the species with the atomic symbol
26 Br 12 The number of protons = atomic number = Z = 35
The number of neutrons = mass number - atomic number = A - Z = 79 - 35 = 44
As the species is neutral, the number of electrons = atomic number = 35
A Br Z
12. How many electrons, protons and neutrons are there in the species with the atomic symbol
79 Br- 35 ?
12. There are 36 electrons, 35 protons and 44 neutrons in the species with the atomic symbol
26 Br- 12 The number of protons = atomic number = Z = 35
The number of neutrons = mass number - atomic number = A - Z = 79 - 35 = 44
As the species is a -l anion, it has gained 1 electron
The number of electrons = atomic number - charge = 35 - (-1) = 36
A Br- Z
Test your understanding using the quiz below. The quiz is made up of 10 questions randomly drawn from a large set so you can repeat it many times over. The results are not stored!Practice multiple choice questions (not an assessment)
