Atoms, elements, and the periodic table

Why this matters

Every material an engineer specifies, every reagent a lab technician measures and every blood test a medical laboratory scientist runs comes down to atoms. The periodic table is the reference sheet that packs the most useful facts about every element into one page. If you can read it fluently, the rest of this course (bonding, reactions, moles, pH) becomes arithmetic instead of memorisation.

What an atom is made of

An atom has a tiny, dense nucleus surrounded by electrons.

u is the atomic massThe average mass of an element's atoms printed on the periodic table, weighted by how common each natural isotope is. unit (also called a dalton, Da). Electrons are so light that an atom's mass is almost entirely its protons and neutrons.

Two numbers describe an atom:

• Atomic numberThe number of protons in an atom's nucleus (Z); it decides which element the atom is. (Z): the number of protons. It defines the element. Every carbon atom has 6 protons; an atom with 7 protons is nitrogen, never carbon.
• Mass numberThe number of protons plus neutrons in one atom (A), so neutrons = A − Z. (A): protons + neutrons. So neutrons = A − Z.

A neutral atom has as many electrons as protons, so its charges cancel.

Isotopes and atomic mass

Atoms of the same element can have different numbers of neutrons. These are isotopesAtoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14.. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons. Same element, same chemistry, different mass.

The atomic mass printed on the table (for example 35.45 for chlorine) is a weighted average over the isotopes found in nature. Chlorine is about 75.8% chlorine-35 (mass 34.969 u) and 24.2% chlorine-37 (mass 36.966 u):

0.7576 × 34.969 + 0.2424 × 36.966 ≈ 26.49 + 8.96 = 35.45 u

That is why atomic masses are rarely whole numbers, and why you never see a "35.45" mass number on a single atom.

Reading an element tile

A typical tile for carbon shows:

   6        <- atomic number Z
   C        <- symbol
 Carbon     <- name
 12.011     <- average atomic mass (u, or g/mol)

Worked example: an atom of sodium-23.

• The table gives Na, Z = 11, so 11 protons.
• Mass number 23, so 23 − 11 = 12 neutrons.
• Neutral, so 11 electrons.

Worked example: the ion Cl⁻ (chloride) from chlorine-35.

• Z = 17, so 17 protons; 35 − 17 = 18 neutrons.
• The 1− charge means one extra electron: 17 + 1 = 18 electrons.

A positive ion has lost electrons: Mg²⁺ has 12 protons and 10 electrons.

How the table is organised

• Periods are the rows (1–7). Moving left to right, Z goes up by one each step.
• Groups are the columns (1–18). Elements in a group have the same number of outer (valenceDescribes the outer electrons of an atom; main-group elements in the same group have the same number of them and behave alike.) electrons for the main-group elements, so they behave alike.
• Metals sit on the left and in the middle, nonmetals in the upper right, and metalloids (such as silicon) along the staircase between them.

Groups worth knowing by name:

Main-group elements gain or lose electrons to reach the same electron count as the nearest noble gas. That is why sodium forms Na⁺ (it loses 1 to match neon) and oxygen forms O²⁻ (it gains 2 to match neon).

Trends you can predict

• Atomic radius grows down a group (more electron shells) and shrinks across a period (more protons pull the electrons in).
• ElectronegativityHow strongly an atom pulls on shared electrons; it rises toward the upper right of the periodic table., how strongly an atom pulls on shared electrons, rises toward the upper right. Fluorine is the most electronegative element. You will use this in the next lesson to predict bond types.

Common mistakes

• Confusing atomic number and mass. The small whole number is Z; the decimal is the average mass.
• Changing protons to make an ion. Ions gain or lose electrons. Change the protons and you have a different element.
• Rounding the atomic mass and calling it the mass number of every atom. Chlorine atoms have mass number 35 or 37; none has 35.45.
• Expecting noble gases to form ions. Under ordinary conditions they almost never react.

Where this goes next

Chemistry is a gateway to lab, engineering and materials careers, but the platform teaches the knowledge, not the hands-on lab skill. Chemist roles typically require a bachelor's degree in chemistry (ACS-approved programmes are often preferred) with real laboratory coursework; medical laboratory scientists also need an accredited programme and certification. See the American Chemical Society and ASCP Board of Certification. Checked on 2026-10-23.

Quick recap

• Protons define the element (Z); protons + neutrons = mass number (A).
• Isotopes differ in neutrons; the table lists a weighted average mass.
• Ions form by gaining or losing electrons, never protons.
• Groups share valence electrons and behaviour; trends run across periods and down groups.

Try it

Open the Do tab and answer the quiz: you will read element data off the table and count particles in atoms and ions.