The structure of an atom
Definition
Nucleus
The very small, dense centre of an atom, which contains the protons and neutrons.
Definition
Electron shell
A region around the nucleus of an atom in which electrons are found, with each shell able to hold only a limited number of electrons.
- Every atom has a tiny central nucleus with electrons moving around it.
- The nucleus contains two kinds of particle, protons and neutrons.
- The electrons are arranged in electron shells, which surround the nucleus at increasing distances from it.
- The nucleus is extremely small compared with the whole atom, so most of the volume of an atom is empty space through which the electrons move.
- If an atom were enlarged to the size of a football stadium, its nucleus would be about the size of a pea on the centre spot.
- Almost all the mass of an atom is concentrated in its nucleus, because protons and neutrons are far heavier than electrons.
- The positively charged nucleus attracts the negatively charged electrons, and this attraction holds the electrons in the atom.
Protons, neutrons and electrons
Definition
Proton
A positively charged particle in the nucleus of an atom, with a relative mass of 111 and a relative charge of +1+1+1.
Definition
Neutron
An uncharged particle in the nucleus of an atom, with a relative mass of 111 and a relative charge of 000.
Definition
Electron
A negatively charged particle found in the shells around the nucleus, with a relative mass of 11840\frac{1}{1840}18401 and a relative charge of −1-1−1.
- The real masses and charges of these particles are extremely small, so chemists compare them using relative mass and relative charge.
- Relative values compare each particle with a proton, which is given a relative mass of 111 and a relative charge of +1+1+1.
- A proton is found in the nucleus and has a relative mass of 111 and a relative charge of +1+1+1.
- A neutron is found in the nucleus and has a relative mass of 111 and a relative charge of 000, so it is neutral.
- An electron is found in the shells and has a relative mass of 11840\frac{1}{1840}18401 and a relative charge of −1-1−1.
- The mass of an electron is so small that it is often described as negligible, meaning it is almost nothing compared with the mass of a proton or a neutron.
- The charge on an electron is the same size as the charge on a proton but opposite in sign, so one electron exactly cancels the charge of one proton.
- An atom has no overall charge, because it contains equal numbers of protons and electrons.
- A carbon atom has 6 protons with a total charge of +6+6+6 and 6 electrons with a total charge of −6-6−6, giving an overall charge of 000.
Proton number and mass number
Definition
Proton number
The number of protons in the nucleus of an atom, also known as the atomic number.
Definition
Mass number
The total number of protons and neutrons in the nucleus of an atom, also known as the nucleon number.
- The proton number is the number of protons in the nucleus of an atom, and it is also called the atomic number.
- The proton number identifies the element, so every atom with 6 protons is a carbon atom and every atom with 11 protons is a sodium atom.
- The Periodic Table lists the elements in order of increasing proton number, starting with hydrogen, which has a proton number of 111.
- Protons and neutrons are together called nucleons, because they are the particles found in the nucleus.
- The mass number is the total number of protons and neutrons in the nucleus of an atom, and it is also called the nucleon number.
- Electrons are not counted in the mass number, because their mass is negligible.
- In a neutral atom, the number of electrons is equal to the proton number.
- The number of neutrons in an atom is found by subtracting the proton number from the mass number, number of neutrons=mass number−proton number\text{number of neutrons}=\text{mass number}-\text{proton number}number of neutrons=mass number−proton number.
Example
Counting the particles in an atom
- An atom of sodium has a proton number of 11 and a mass number of 23.
- The proton number gives the number of protons, so the atom contains 11 protons.
- The atom is neutral, so it also contains 11 electrons.
- The neutrons are the nucleons that are not protons.
- An atom of argon has a proton number of 18 and a mass number of 40, so it contains 18 protons and 18 electrons.
Common Mistake
- Do not give an electron a relative mass of 000, because its relative mass is 11840\frac{1}{1840}18401, which is very small but not zero.
- Do not include electrons in the mass number, because the mass number counts only the protons and neutrons in the nucleus.
- Do not say that the number of neutrons identifies an element, because only the proton number does this.
Exam technique
Writing the definitions
- Learn the definitions of proton number and mass number word for word, because each one is usually worth a mark on its own.
- Always include the words nucleus and atom, because an answer such as the number of protons on its own is incomplete.
- When you give a relative charge, include the sign as well as the number, such as +1+1+1 for a proton and −1-1−1 for an electron.
- When completing a table of particles, check that the number of protons plus the number of neutrons equals the mass number.
Self review
- Where in an atom are protons, neutrons and electrons found?
- State the relative mass and relative charge of an electron.
- Define mass number.
- Why does an atom have no overall charge?
- How many neutrons are in an atom with a proton number of 13 and a mass number of 27?
