Radioactivity » Atomic Structure and Isotopes

Inside the atom

Everything around you is made of atoms. An atom is made of three smaller particles. In the middle is a tiny nucleus. It is made of protons and neutrons. The nucleus has a positive charge, because protons are positive and neutrons have no charge. Around the nucleus are the electrons, which are negative.

The nucleus is very small compared with the whole atom. Almost all of the mass of an atom is in its nucleus, because electrons are so light. The rest of the atom is mostly empty space where the electrons are found.

Key terms:

• Nucleus: the tiny, positively charged centre of an atom, made of protons and neutrons.
• Proton: a positively charged particle in the nucleus.
• Neutron: a particle in the nucleus with no charge.
• Electron: a negatively charged particle that moves around the nucleus.

Charges and masses

Physicists compare particles using relative values. The proton is given a relative charge of +1 and a relative mass of 1. The other particles are compared with it.

Particle	Where it is	Relative charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Around the nucleus	−1	about 1/1840 (almost nothing)

A normal atom has no overall charge. It is neutral. This is because it has the same number of electrons as protons. The +1 charges cancel the −1 charges exactly.

Reading the symbol

Scientists use a short symbol to describe a particular nucleus. Carbon-14 is written like this, with the mass number 14 above the atomic number 6, in front of the symbol C:

¹⁴₆C

The symbol has three parts:

• The letter(s), here C, are the chemical symbol for the element.
• The number at the bottom (6) is the atomic number, also called the proton number. It is the number of protons.
• The number at the top (14) is the mass number, also called the nucleon number. It is the total number of protons and neutrons.

Protons and neutrons are both called nucleons, because they are found in the nucleus. That is why the mass number is also called the nucleon number.

Key terms:

• Atomic (proton) number: the number of protons in the nucleus of an atom.
• Mass (nucleon) number: the total number of protons and neutrons in the nucleus.
• Nucleon: a particle in the nucleus, either a proton or a neutron.

Isotopes

All atoms of one element have the same number of protons. That number decides which element it is. But the number of neutrons can be different. Atoms with the same number of protons but different numbers of neutrons are called isotopes.

Isotopes of an element have the same atomic number but different mass numbers.

Isotope	Symbol	Protons	Neutrons	Electrons (neutral atom)
Carbon-12	126C	6	6	6
Carbon-14	146C	6	8	6
Uranium-235	23592U	92	143	92
Uranium-238	23892U	92	146	92

The number after the name is the mass number. Carbon-12 and carbon-14 are both carbon, because both have 6 protons. Carbon-14 has two more neutrons. Uranium-235 and uranium-238 are both uranium, with 92 protons. They differ by three neutrons.

Key terms:

• Isotopes: atoms of the same element (the same number of protons) with different numbers of neutrons.

Exam-style question

The nucleus of a carbon atom can be written as ¹⁴₆C.

(a) Give the number of protons, neutrons and electrons in a neutral atom of carbon-14. [3 marks]

(b) State the relative charge of an electron. [1 mark]

(c) Carbon-12 is another form of carbon. Explain why carbon-12 and carbon-14 are isotopes of the same element. [2 marks]