Atomic structure

1. Three particles, different jobs

An atom has a tiny nucleus surrounded by an electron cloud. The nucleus contains positively charged protons and electrically neutral neutrons. Negatively charged electrons occupy the much larger region outside the nucleus. Almost all the mass is concentrated in the nucleus, while the electrons account for most of the atom's volume. Charges are measured in coulombs, but introductory chemistry usually counts elementary charges as 1+, 0, and 1−.

The number of protons is the atom's identity. Every carbon atom has six protons; changing that number makes a different element. Atoms can gain or lose electrons without changing element identity. A neutron can change the isotope while leaving the element the same. This separation of roles prevents a common error: an ion is not a new element merely because its charge changed.

2. Atomic number and mass number

The atomic number, Z, equals the number of protons. An atom with no net charge has equal protons and electrons. The mass number, A, is a whole-number count of protons plus neutrons in one particular atom:

Z = protons     A = protons + neutrons     neutrons = A − Z

In isotope notation, A is written as a left superscript and Z may appear as a left subscript: ²³₁₁Na is sodium-23. The element symbol already communicates Z, so many tables omit the subscript. A mass number is not the decimal number printed in an ordinary periodic-table box. That decimal is usually an average atomic mass.

3. Isotopes and average atomic mass

Isotopes are atoms of one element with the same proton number but different neutron numbers. They generally have similar chemical behavior because their electron structures match when they are neutral, although their masses and some nuclear behavior differ. A natural sample can contain more than one isotope. The periodic-table average is a weighted average based on isotope masses and abundances, not a claim that every atom has a fractional mass number.

Use decimal abundance as a fraction in a weighted average:

average atomic mass = (isotope mass × fraction) + (isotope mass × fraction) + …

Do not round an isotope's mass number to the periodic-table average or call the average a mass number. The number 10.25 u represents a population average; no single isotope in this example has 10.25 nucleons.

4. Ions and electron counts

An ion has unequal numbers of protons and electrons. Its net charge is:

charge = protons − electrons

A positive ion, or cation, has lost electrons. A negative ion, or anion, has gained electrons. The charge tells how many more positive or negative elementary charges exist, not how many protons the atom has. For an ion with atomic number Z and charge q, electrons = Z − q when q is signed. Thus q = +2 means subtract two electrons, while q = −2 means add two.

Ion notation places the charge at the upper right. The mass number, when included, is at the upper left. A charge of 2+ is not the same as a superscript 2 in a formula; context and position matter.

5. A useful but limited shell model

Electrons occupy regions of allowed energy. For an introductory model of the first 20 elements, write shells from the nucleus outward and place up to 2 electrons in the first shell, up to 8 in the second, and up to 8 in the third before starting the fourth. This gives sodium as 2,8,1 and calcium as 2,8,8,2. The outer-shell electrons are called valence electrons in this simplified account and help explain why elements in a periodic-table group often react in related ways.

This shell count is a teaching model, not a literal set of circular tracks. A high-school extension uses orbitals and an electron configuration such as 1s² 2s² 2p⁶; the p sublevel contains three orbitals and can hold six electrons. Transition elements and many ions require more careful ordering than the 2,8,8 shortcut. Do not use a simple shell picture to claim that all atoms seek exactly eight electrons or that electrons are motionless dots.

For a neutral atom, total shell electrons must equal Z. For a cation, remove the stated number of electrons; for an anion, add them. At this level, write the resulting count and check its total against the charge.

6. Reading the periodic table

Elements are arranged by increasing atomic number. A period is a row; its number loosely indicates the highest occupied principal shell in the simple model. A group is a column. Group 1 elements have one valence electron in the main-group pattern, group 17 have seven, and group 18 have filled outer shells in the usual neutral atoms. Hydrogen is a special case and should not be treated as an ordinary metal simply because it appears above group 1.

Metals are generally on the left and center, nonmetals on the upper right, with metalloids near the zigzag boundary. These broad labels predict useful trends but have exceptions. The periodic table box may show symbol, atomic number, name, and average atomic mass. Always identify which number a question asks for before calculating.