Describe the atom, determine atomic number, mass number and charge from particle counts, and model isotopes and ions

Learning goal: Describe the location, charge and relative mass of protons, neutrons and electrons; determine an element's identity, atomic number, mass number and charge from the numbers of its particles and the reverse; define isotope and give an example; draw a Bohr model and an electron dot diagram for elements in the first three periods; explain why valence electrons matter; and use the periodic table to predict the ionic charge of elements in groups 1, 2, 17 and 18.

Inside the atom. In middle school the atom was a tiny indestructible sphere. As instruments improved, the atom was found to have parts. Three particles matter here. Protons carry a positive charge of one unit and sit in the nucleus. Neutrons carry no charge, have almost the same mass as a proton, and also sit in the nucleus. Electrons carry a negative charge of one unit, move about in the space surrounding the nucleus, and have a mass roughly one two-thousandth of a proton's, so nearly all of an atom's mass is in its nucleus. The nucleus is extremely small compared with the atom as a whole; almost all of an atom is the empty space in which electrons move. An atom with equal numbers of protons and electrons has no net charge, because the positive and negative units cancel. The experiments that revealed this structure are studied in chemistry; here the model itself is the tool.

Atomic number, mass number and identity. The number of protons is the atomic number, Z, and it decides which element an atom is: every atom with 6 protons is carbon, every atom with 11 protons is sodium, with no exceptions. The periodic table lists elements in order of atomic number. The mass number, A, is the number of protons plus the number of neutrons, the total count of particles in the nucleus. So the number of neutrons is A − Z. Write an atom as the element name followed by its mass number, for example carbon-12 or sodium-23, or with the mass number above and the atomic number below the symbol. From a periodic table, sodium has Z = 11, so a sodium-23 atom has 11 protons, 23 − 11 = 12 neutrons and, if neutral, 11 electrons. The reverse works too: an atom with 8 protons, 8 neutrons and 8 electrons is oxygen (Z = 8), has mass number 16 and is neutral.

Isotopes. All atoms of an element have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons, and therefore different mass numbers, are isotopes of one another. Carbon-12 has 6 protons and 6 neutrons; carbon-13 has 6 protons and 7 neutrons; carbon-14 has 6 protons and 8 neutrons. All three are carbon and behave almost identically in chemical reactions, because chemical behavior depends mainly on the electrons, which the isotopes share; the small effects of their different masses, such as slightly different reaction speeds, are left for later courses. The isotopes differ in mass, and some isotopes, such as carbon-14, have unstable nuclei. Hydrogen-1, with a single proton and no neutron, and hydrogen-2, with one proton and one neutron, are another pair. The number listed under each element on most periodic tables is an average over the element's naturally occurring isotopes; how that average is calculated is left for chemistry. Do not confuse it with the mass number of any one atom, which is always a whole number.

The Bohr model and valence electrons. In the Bohr model, electrons occupy shells at increasing distances from the nucleus, and the shells fill from the inside out: 2 electrons in the first shell, then 8 in the second, then 8 in the third for the first eighteen elements. Sodium (11 electrons) is drawn as 2, 8, 1; chlorine (17 electrons) as 2, 8, 7; neon (10) as 2, 8. Today's understanding of electrons goes beyond this picture, but the model is still useful for one purpose: it shows the valence electrons, the electrons in the outermost occupied shell. Valence electrons are the ones involved when atoms bond or form ions, so an element's chemical behavior is largely set by how many it has. An electron dot diagram shows only these: the element symbol with one dot per valence electron around it, so Na has one dot, Mg two, Cl seven and Ne eight.

Ions: cations and anions. Atoms may gain or lose valence electrons. Because protons stay put, doing so gives the atom a net charge, and a charged atom is an ion. Losing electrons leaves more protons than electrons, a positive ion called a cation; gaining electrons produces a negative ion, an anion. Sodium, with one valence electron, loses it to form Na⁺ (11 protons, 10 electrons: charge +1). Chlorine, with seven, gains one to reach eight and forms Cl⁻ (17 protons, 18 electrons: charge −1). The pattern follows the group: group 1 elements form 1+ ions, group 2 elements form 2+ ions, group 17 elements form 1− ions, and group 18 elements, which already have a full outer shell, form no ions under ordinary conditions. Charge is worked out as protons minus electrons; an ion's identity is still fixed by its proton count, so Na⁺ is still sodium. A common error is to think that gaining an electron makes a positive ion because "it gained something"; charge counts sign, and an electron is negative.

Ions around you. Dissolved ions are everywhere in daily life. Hard water contains dissolved calcium and magnesium ions (Ca²⁺ and Mg²⁺) picked up from rock; these ions react with soap to form a scum and leave scale in kettles, so soap lathers poorly in hard water. Soft water has few of these ions and lathers easily. Many products work because of ions: fabric softeners and some hair conditioners carry positive ions that attach to negatively charged fiber or hair surfaces; road salt supplies Na⁺ and Cl⁻ that lower the freezing point of water on the pavement. Some dissolved ions in drinking water matter for health, such as fluoride in small amounts for teeth, while others such as lead ions are harmful at any level and are why water supplies are tested.

A worked model: from symbol to particle counts and back. Magnesium is element 12. A magnesium-24 atom has 12 protons, 24 − 12 = 12 neutrons and 12 electrons. Its Bohr model is 2, 8, 2, so it has two valence electrons and its dot diagram shows Mg with two dots. Losing both gives Mg²⁺: 12 protons, 12 neutrons, 10 electrons, charge 12 − 10 = +2, matching its position in group 2. A magnesium-26 atom has 14 neutrons and is an isotope of magnesium-24; it forms the same Mg²⁺ ion because it has the same electrons. Now reverse: an ion with 17 protons, 20 neutrons and 18 electrons is chlorine (Z = 17), mass number 37, charge 17 − 18 = −1, written chlorine-37 or ³⁷Cl⁻.

Investigation: test water samples for dissolved ions with a soap-lather test. Materials: distilled water, tap water, and a sample of distilled water in which about half a teaspoon of Epsom salt (magnesium sulfate) has been dissolved; three identical capped bottles or test tubes with stoppers; a dropper bottle of liquid castile soap or soap solution; a ruler; goggles. Steps: put 20 mL of one sample in each tube, add one drop of soap solution to each, cap and shake ten times, and measure the height of the lather after 30 seconds; keep adding one drop at a time, shaking each time, until the lather is at least 1 cm high and lasts 30 seconds; record the number of drops needed for each sample. Record: drops of soap needed and lather height for each water. A result that supports the claim: distilled water lathers with the fewest drops, the Epsom-salt water needs the most, and tap water falls somewhere between depending on the local supply; the sample with more dissolved magnesium ions needs more soap because those ions consume soap before lather can form. Safety: goggles, since soap solutions sting eyes; keep tubes capped while shaking. What the model cannot show: the test shows that ions that react with soap are present and gives a rough comparison, but it cannot identify which ion, cannot give a concentration and does not detect ions such as sodium or chloride that do not react with soap.

Worked example 1 — particle counts from the periodic table. Find protons, neutrons and electrons for a neutral atom of potassium-39. Potassium is element 19, so Z = 19 protons. Mass number 39, so neutrons = 39 − 19 = 20. The atom is neutral, so electrons = protons = 19. Its Bohr model is 2, 8, 8, 1, with one valence electron, so it is expected to form K⁺ by losing that electron: 19 protons, 20 neutrons, 18 electrons, charge +1.

Worked example 2 — identify an unknown from counts. A particle has 9 protons, 10 neutrons and 10 electrons. Nine protons means the element is fluorine, regardless of anything else. Mass number = 9 + 10 = 19, so it is fluorine-19. Charge = protons − electrons = 9 − 10 = −1, so it is the fluoride anion F⁻. This matches group 17, whose atoms have seven valence electrons and gain one to reach a full shell of eight.

Worked example 3 — isotopes or not? Atom P has 6 protons and 6 neutrons; atom Q has 6 protons and 8 neutrons; atom R has 7 protons and 7 neutrons. P and Q are isotopes of each other: same element (carbon, Z = 6), different mass numbers (12 and 14). R is not an isotope of either; with 7 protons it is nitrogen, a different element, even though its mass number 14 equals Q's. Isotopes share the proton count, not the mass number.

Worked example 4 — dot diagrams and ion charge. Calcium is in group 2 with Bohr model 2, 8, 8, 2, so its dot diagram shows Ca with two dots and it forms Ca²⁺ by losing both valence electrons. Bromine is in group 17 with seven valence electrons, so Br has seven dots and forms Br⁻ by gaining one. Argon in group 18 has eight dots, a full outer shell, and forms no ion under ordinary conditions. Writing the diagram for each element first makes the predicted charge visible: metals in groups 1 and 2 lose their one or two dots; halogens gain one dot to reach eight.

Representation task: a particle table and two Bohr models. Complete a table with columns element, atomic number, mass number, protons, neutrons, electrons, charge for these seven particles: carbon-12 atom, carbon-14 atom, sodium-23 atom, Na⁺ from sodium-23, Cl⁻ from chlorine-35, Mg²⁺ from magnesium-24, neon-20 atom. Then draw Bohr models of the sodium atom and the sodium ion side by side, and write one sentence explaining what changed and what did not.

Independent performance: model ion formation and explain hard water. Using counters or a drawing, model the formation of one cation from a group 1 or group 2 atom and one anion from a group 17 atom, showing valence electrons moving. Success criteria: each model shows the correct number of valence electrons before and after; the charge is calculated as protons minus electrons and matches the group's characteristic charge; the identity of the element is stated as unchanged; the model is labeled with what it does not show (the true size of the nucleus and the actual motion of electrons). Then write one paragraph, using the words cation, anion, dissolved and ion, explaining why hard water needs more soap than soft water and how the soap-lather test gives evidence for dissolved ions without identifying them.