Atoms, Elements, and the Periodic Table

Matter is anything that has mass and occupies space. Solids, liquids, and gases are all matter.

An atom is the smallest unit of an element that still has that element’s chemical identity. Atoms contain three main subatomic particles:

• Protons have a charge of $+1$ and are found in the nucleus.
• Neutrons have no charge and are found in the nucleus.
• Electrons have a charge of $-1$ and move in regions around the nucleus.

The nucleus is extremely small compared with the whole atom, but it contains nearly all of the atom’s mass.

An element is a pure substance made of atoms with the same number of protons. This proton count is called the atomic number, written as $Z$.

$Z = \text{number of protons}$

Atomic number determines the element’s identity:

• Hydrogen has $Z=1$, so every hydrogen atom has 1 proton.
• Carbon has $Z=6$, so every carbon atom has 6 protons.
• Oxygen has $Z=8$, so every oxygen atom has 8 protons.

Changing the number of neutrons can make a different version of the same element, called an isotope. Changing the number of protons creates an entirely different element.

The periodic table organizes elements by increasing atomic number. A typical element square includes its atomic number, symbol, name, and average atomic mass.

For oxygen, a square may look like this:

8\\ \mathrm{O}\\ \text{Oxygen}\\ 16.00 \end{array}$$ Here, $8$ is the atomic number and $\mathrm{O}$ is the chemical symbol. Symbols use a capital first letter; if there is a second letter, it is lowercase. For example, sodium is $\mathrm{Na}$ and gold is $\mathrm{Au}$. In a **neutral atom**, positive and negative charges balance, so: $$\text{number of electrons} = \text{number of protons}$$

Magnesium, $\mathrm{Mg}$, has atomic number $12$.

1. Atomic number gives the proton count: magnesium has 12 protons.
2. A neutral atom has equal numbers of protons and electrons: it has 12 electrons.
3. If a particular magnesium atom has mass number $24$, then:

$\text{mass number} = \text{protons} + \text{neutrons}$

$24 = 12 + \text{neutrons}$

So it has 12 neutrons.

The average atomic mass shown on the periodic table is often a decimal because naturally occurring samples contain a mixture of isotopes.