What atoms are made of and how they form ions and bonds
TEAS 7 outline: Basic atomic structure
Also covers HESI A2 Chemistry (Atoms and the periodic table)
- Protons and neutrons sit in the nucleus; electrons move outside it.
- Atomic number = protons, and it names the element.
- Mass number = protons + neutrons; subtract atomic number to get neutrons.
- Isotopes differ in neutrons; ions differ in electrons.
- Ionic bonds transfer electrons; covalent bonds share them.
Key facts
- Protons (+1) and neutrons (0) sit in the nucleus and each have a mass of about 1 amu; electrons (−1) move outside the nucleus and have almost no mass.
- Atomic number (Z) = number of protons, which defines the element; in a neutral atom, electrons = protons.
- Mass number (A) = protons + neutrons, so neutrons = mass number − atomic number.
Show all 7 key factsShow fewer
- Isotopes of an element have the same number of protons but different numbers of neutrons, such as carbon-12 and carbon-14.
- An atom that loses electrons becomes a positive ion (cation); one that gains electrons becomes a negative ion (anion).
- Valence electrons are in the outermost shell; elements in the same group (column) have the same number of valence electrons and react similarly.
- Ionic bonds form when electrons are transferred and opposite ions attract (usually metal + nonmetal); covalent bonds form when atoms share electrons (usually nonmetals).
Quick check1 question · 2 more at the end
Which number identifies an atom as a particular element?
Correct answer: B.
The number of protons is the atomic number, and it defines the element: every atom with 6 protons is carbon, and every atom with 11 protons is sodium. Atoms of one element can have different numbers of neutrons (isotopes) or electrons (ions) and still be the same element. Protons plus neutrons is the mass number, which can differ between isotopes of the same element.
A Atoms of the same element can have different numbers of neutrons; those atoms are isotopes, not different elements.
B Correct: the number of protons is the atomic number, which defines the element.
C The number of electrons changes when an atom becomes an ion, but the element stays the same.
D Protons plus neutrons is the mass number, which varies among isotopes of one element, so it cannot identify the element by itself.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: An atom has a nucleus of positive protons and neutral neutrons, surrounded by negative electrons. The atomic number is the number of protons and identifies the element; the mass number is protons plus neutrons. Isotopes differ in neutrons, ions differ in electrons, and atoms bond by transferring electrons (ionic) or sharing them (covalent).
1 What is inside an atom?Every atom has a tiny, dense center called the nucleus, surrounded by a much larger region where electrons move. 1 min
Every atom has a tiny, dense center called the nucleus, surrounded by a much larger region where electrons move. The electrons take up almost all of an atom's volume, yet nearly all of its mass is packed into the nucleus.
| Particle | Charge | Approximate mass | Location |
|---|---|---|---|
| Proton | +1 | About 1 amu | Nucleus |
| Neutron | 0 | About 1 amu | Nucleus |
| Electron | −1 | About 0.0005 amu (almost nothing) | Outside the nucleus |
An amu (atomic mass unit) is the tiny unit used for atomic masses. Because electrons are so light, their mass is ignored when counting an atom's mass number.
2 How do atomic number and mass number differ?The atomic number (Z) is the number of protons. 1 min
The atomic number (Z) is the number of protons. It is the element's ID: every carbon atom has 6 protons, and any atom with 6 protons is carbon. In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.
The mass number (A) is protons plus neutrons. To find neutrons, subtract: neutrons = A − Z. Carbon-14 has a mass number of 14 and an atomic number of 6, so it has 14 − 6 = 8 neutrons.
3 What are isotopes?Isotopes are atoms of the same element with different numbers of neutrons. 1 min
Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 neutrons, carbon-13 has 7, and carbon-14 has 8. All three have 6 protons, so all are carbon. Hydrogen has three isotopes: ordinary hydrogen (no neutrons), deuterium (1 neutron) and tritium (2 neutrons).
Some isotopes are radioactive. Carbon-14, for example, slowly breaks down with a half-life of about 5,730 years, which lets scientists date ancient remains.
The atomic mass shown on the periodic table is a weighted average of an element's natural isotopes. That is why boron is listed as about 10.81 even though no single boron atom has that mass.
4 What are ions?An ion is an atom with a net charge because it has gained or lost electrons. 1 min
An ion is an atom with a net charge because it has gained or lost electrons. The protons never change, so the element stays the same.
- Losing electrons leaves more protons than electrons, making a positive ion, or cation. Sodium (11 protons) loses one electron to become Na⁺ with 10 electrons.
- Gaining electrons makes a negative ion, or anion. Chlorine gains one electron to become Cl⁻, and oxygen gains two to become O²⁻.
Metals tend to lose electrons and nonmetals tend to gain them. For main-group elements, the group number predicts the usual charge: group 1 forms 1+ ions, group 2 forms 2+, group 16 forms 2−, and group 17 forms 1−.
5 How are electrons arranged, and why does it matter?Electrons occupy shells (energy levels) and fill the shells closest to the nucleus first. 1 min
Electrons occupy shells (energy levels) and fill the shells closest to the nucleus first. The first shell holds up to 2 electrons and the second up to 8. The electrons in the outermost shell are valence electrons, and they decide how an atom bonds. By the octet rule, most atoms are most stable with 8 electrons in their outer shell.
The periodic table is arranged by increasing atomic number. Its horizontal rows are periods and its vertical columns are groups (families). Elements in the same group have the same number of valence electrons, so they behave alike. Group 1 (alkali metals) has 1 valence electron, and group 17 (halogens) has 7. Group 18 (noble gases) already has a full outer shell, which is why noble gases rarely bond.
6 How do atoms bond?Atoms bond to reach a stable outer shell, either by transferring or by sharing valence electrons. 1 min
Atoms bond to reach a stable outer shell, either by transferring or by sharing valence electrons.
| Ionic bond | Covalent bond | |
|---|---|---|
| What happens | Electrons transfer; opposite ions attract | Electrons are shared |
| Usual partners | Metal + nonmetal | Nonmetal + nonmetal |
| Example | Table salt, NaCl (Na⁺ and Cl⁻) | Water (H₂O), oxygen (O₂), methane (CH₄) |
| Typical properties | Solids with high melting points | Often gases, liquids or low-melting solids |
Covalent bonds can be single, double or triple; more shared pairs make a stronger connection. If electrons are shared unequally, the bond is polar, as in water, where oxygen pulls electrons harder than hydrogen. If they are shared equally, as in O₂ or CH₄, the bond is nonpolar.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Changing the number of electrons makes an ion, not a new element or an isotope; only changing the number of protons changes the element.
Watch out: Losing electrons makes an ion more positive, not more negative, because the protons now outnumber the electrons.
Watch out: The mass number counts protons and neutrons, not electrons.
Watch out: Groups are the vertical columns and periods are the horizontal rows, not the reverse.
Watch out: The atomic mass printed on the periodic table is a weighted average of the isotopes, not the mass number of any single atom.
How to remember it
Memory hook: "Protons pick the element, neutrons pick the isotope, electrons pick the charge." Change one particle and ask which of those three things changed.
Self-check:
- An atom has 15 protons and 16 neutrons. State its atomic number and mass number.
- Explain why Na⁺ is still sodium and why carbon-14 is still carbon.
- Predict whether magnesium (group 2) and chlorine (group 17) form an ionic or covalent bond, and why.