8.1 Atomic Structure and the Periodic Table

8.1 Atomic Structure and the Periodic Table

Why This Matters for the PUPCET

Science makes up about 25% of the PUP College Entrance Test (PUPCET), and chemistry items almost always begin with the atom. Expect questions that ask you to name the charge or location of a subatomic particle, compute the number of neutrons from an atomic number and a mass number, read a periodic-table entry, or pick the correct chemical symbol. These are quick, predictable points — the kind a prepared examinee should never give away.

The Three Subatomic Particles

An atom is the smallest unit of an element that still keeps that element's identity. Every atom is built from three particles, and the PUPCET expects you to know all four columns of this table:

Three rules lock this in:

• The proton defines the element. Every atom with 6 protons is carbon; change the proton count and you change the element itself.
• The neutron only changes the atom's mass, never its identity. Its job is to help hold the nucleus together.
• The electron is what participates in chemical bonding. In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.

Atomic Number vs Mass Number

• Atomic number (Z) = number of protons. This is the element's ID number and fixes its position on the periodic table.
• Mass number (A) = protons + neutrons.

From these comes the most-used computation in this topic:

Number of neutrons = A − Z

Worked example. An atom of sodium has atomic number 11 and mass number 23. Protons = 11. Because the atom is neutral, electrons = 11. Neutrons = 23 − 11 = 12.

Common trap. The decimal printed under each element on the periodic table (for chlorine, 35.45) is the average atomic mass, not the mass number. It is a weighted average of all of that element's naturally occurring isotopes. You cannot subtract the atomic number from a decimal to count neutrons — always round to the given whole-number mass number first. PUPCET items love to test exactly this confusion.

Isotopes

Isotopes are atoms of the same element — same number of protons — that have different numbers of neutrons and therefore different mass numbers. Carbon-12, carbon-13, and carbon-14 are isotopes of carbon: each has 6 protons, but 6, 7, and 8 neutrons respectively. Hydrogen's isotopes even carry special names: protium (¹H, no neutrons), deuterium (²H, one neutron), and tritium (³H, two neutrons). Isotopes behave identically in chemical reactions because chemistry depends on electrons, and their electron counts are the same.

How the Periodic Table Is Organized

The periodic table arranges elements in order of increasing atomic number, and its layout encodes electron structure:

• Periods are the 7 horizontal rows. The period number tells you how many electron shells the atom has. Sodium, in Period 3, has electrons in three shells.
• Groups are the 18 vertical columns. Elements in the same group have the same number of valence electrons — the electrons in the outermost shell — so they react in similar ways.

Memorize these group names; they appear in exams constantly:

A bold staircase line divides the table: metals sit on the left and center (shiny, malleable, good conductors of heat and electricity), nonmetals sit on the right (dull, brittle when solid, poor conductors), and metalloids such as silicon and germanium sit along the staircase with properties of both.

The 20 Most-Tested Elements

Drill this table until you can reproduce it from memory:

Symbol traps: some symbols come from Latin names — Na (sodium, from natrium), K (potassium, from kalium), Fe (iron), Au (gold), Ag (silver), Hg (mercury), Pb (lead). Also watch capitalization: Co is the element cobalt, but CO is the compound carbon monoxide. One letter's case changes the entire answer.

Electron Arrangement for the First 20 Elements

For the PUPCET you only need the simple shell model. Shells fill in order: the first shell holds at most 2 electrons, the second at most 8, the third takes 8 more before the fourth begins filling. Examples: oxygen (8 electrons) is 2,6; chlorine (17) is 2,8,7; potassium (19) is 2,8,8,1. Atoms with 8 valence electrons (2 for the first shell) follow the octet rule and are stable — which is why the noble gases barely react, and why sodium (2,8,1) eagerly gives away one electron while chlorine (2,8,7) eagerly grabs one.

Final Exam Tips

• Count neutrons with the whole-number mass number, never the decimal average atomic mass.
• Same protons, different neutrons = isotope; different protons = different element.
• Period number = shells; group number (for the main groups) = valence electrons.