2 Atoms, Elements, and the Periodic Table

Atomic structure and particles

An atom has a tiny central nucleus surrounded by electrons. The nucleus holds nearly all the atom’s mass, while electrons occupy the space around it.

The three main subatomic particles differ in charge, location, and approximate relative mass:

• Proton: charge $+1$, located in the nucleus, with approximate relative mass $1$.

• Neutron: charge $0$, located in the nucleus, with approximate relative mass $1$.

• Electron: charge $-1$, located outside the nucleus, with a mass much less than $1$ on this relative scale.

Atomic number and mass number

The number of protons defines an atom’s element and is its atomic number, represented by $Z$. The mass number, represented by $A$, is the total number of protons and neutrons in a particular atom. These relationships let you determine particle counts:

• Protons = atomic number.

• Neutrons = mass number $-$ atomic number.

• In a neutral atom, electrons = protons.

For example, a neutral carbon-12 atom has $6$ protons, $12 - 6 = 6$ neutrons, and $6$ electrons. In general, changing the number of protons changes the atom’s element.

Elements, isotopes, and atomic mass

Atoms of the same element can have different numbers of neutrons; these forms are isotopes. Carbon-12 and carbon-14 are both carbon atoms with $6$ protons. Carbon-12 has $6$ neutrons, while carbon-14 has $8$. The number after an element name gives the isotope’s mass number. Some isotopes are stable, while others are radioactive.

The decimal value commonly listed for an element on the periodic table is its average atomic mass, not the mass number of one particular atom. It reflects the masses and natural abundances of that element’s isotopes. For example, chlorine’s listed atomic mass is about $35.45$ because natural chlorine is a mixture of isotopes.

Ions and electrical charge

An ion is an atom or group of atoms with an electrical charge because it has gained or lost electrons. The number of protons does not change when an atom becomes an ion.

• Losing electrons produces a positively charged cation. A sodium atom with $11$ protons and $10$ electrons is $\mathrm{Na}^{+}$.

• Gaining electrons produces a negatively charged anion. A chlorine atom with $17$ protons and $18$ electrons is $\mathrm{Cl}^{-}$.

Compare proton and electron counts to find an ion’s charge:

For instance, an ion with $12$ protons and $10$ electrons has a $2+$ charge.

How the periodic table is organized

The periodic table arranges elements by increasing atomic number. Its repeating pattern places elements with related properties together. An element’s box commonly shows its atomic number, chemical symbol, name, and average atomic mass; layouts can vary, so check the table’s key.

Rows are called periods, and the table has $7$ of them. Moving across a row from left to right, atomic number increases. Columns are called groups or families, and there are $18$. Elements in a group often have similar chemical behavior. For example, Group $18$ contains the noble gases, which are generally unreactive.

Element regions and table placement

Metals are mostly on the left and in the center of the periodic table, while nonmetals are mostly on the right. Metalloids lie along the zigzag boundary and have a mix of metallic and nonmetallic properties. Hydrogen is a nonmetal even though it appears at the top of Group $1$.

The two rows commonly placed beneath the main table are the lanthanides and actinides. They belong within periods $6$ and $7$, respectively.

Applying the ideas to oxygen

Oxygen’s atomic number is $8$, so every oxygen atom has $8$ protons. A neutral oxygen atom also has $8$ electrons. Its table atomic mass is close to $16$, but that value is an isotope-weighted average, not a count of particles in every oxygen atom.

The key distinctions are that protons determine an element’s identity, neutrons distinguish its isotopes, and gaining or losing electrons makes an ion. The periodic table orders elements by atomic number, with periods as rows and groups as columns; a listed atomic mass generally represents a weighted average of isotope masses rather than one atom’s mass number.

• period