1. Structure of the Nucleus – Outcomes
Describe the nucleus as consisting of protons and neutrons.
Describe nuclei by their mass number and atomic number.
Discuss isotopes.

2. Protons
The atom consists of mostly empty space with a small positively charged core, called the nucleus.
The positive charge is due particles called protons. Protons have:
a mass of 1.67× 10 −27 𝑘𝑔, about 2000 times as massive as electrons. This is also known as 1 amu (atomic mass unit).
a charge of +1.6× 10 −19 𝐶, the same as an electron, but positive.

3. Neutrons
The mass of the nucleus does not come solely from protons. It also comes from neutrons. Neutrons have:
a mass of 1.68× 10 −27 𝑘𝑔, very slightly more than the proton.
no charge – they are electrically neutral, hence the name neutron.

4. Atomic Number
Elements are defined by the number of protons in their nucleus, called their atomic number, Z.
e.g. iron has atomic number 26, so it has 26 protons in its nucleus.
Every iron atom in the universe has 26 protons in its nucleus, because that is how we define iron.
If an atom has 27 protons in its nucleus, we call it cobalt.
If an atom has 28 protons in its nucleus, we call it nickel.
etc.

5. Mass Number Atoms also have a mass number, A.
The mass number of an atom is the sum of the number of protons and neutrons in the nucleus in the atom.
Atoms are usually written 𝑍 𝐴 𝑋 , where X is the element symbol.
e.g. a carbon atom has 6 protons and 6 neutrons in its nucleus, what is its mass number? What is its symbol?
e.g. an yttrium atom has mass number 89. How many neutrons are in its nucleus? What is its symbol?

6. Isotopes
While a particular element must have a particular atomic number (otherwise it is no longer that element), they often exist with varying numbers of neutrons.
Isotopes are atoms with the same number of protons, but a different number of neutrons.
e.g. hydrogen exists in three forms:
hydrogen-1, 1 1 𝐻 (1 proton, 0 neutrons)
hydrogen-2, 1 2 𝐻 (1 proton, 1 neutron)
hydrogen-3, 1 3 𝐻 (1 proton, 2 neutrons).
On the periodic table, mass numbers are often given as a weighted average of the isotopes, taking abundance into account. e.g. hydrogen is listed with A = amu.