Chemistery

Two numbers sit next to almost every element symbol, and they trip up a lot of students: the atomic number and the mass number . They look similar, they're both whole numbers, and mixing them up quietly wrecks a lot of otherwise-correct answers. The short answer: the atomic number is the number of protons in an atom (it defines the element). The mass number is the total number of protons + neutrons in the nucleus. One identifies which element; the other tells you how heavy that particular atom is. Quick comparison at a glance Feature Atomic number (Z) Mass number (A) What it counts Protons only Protons + neutrons Symbol Z A What it tells you Which element it is The mass of that specific atom Same for every atom of an element? Yes — always No — can vary (isotopes) On the periodic table? Yes (the whole number for each element) Not directly (the table shows average atomic mass) In a neutral atom, also equals Number of electrons — Th...

Everything around you — this screen, the air you're breathing, the water in your glass, even you — is built from the same tiny piece: the atom . Get this one idea straight and the rest of chemistry stops feeling like a pile of rules and starts feeling like a story you can follow. The short answer: an atom is the smallest unit of an element that still behaves like that element. Every atom has a dense central nucleus made of protons and neutrons , with electrons moving in the space around it. Let's break down what an atom is actually made of, how the parts fit together, and how to picture one. What an atom actually is An atom is unimaginably small — a single hydrogen atom is about 0.1 nanometres across, so you could line up roughly ten million of them across the width of this letter "o". Yet each one is built from just three particles : Protons — positively charged, sitting in the nucleus. The number of protons is the atom's ID badge: it decides which el...

Why does a hand warmer get hot while an instant cold pack turns icy — using the same basic idea? It comes down to one of chemistry's most useful distinctions: whether a reaction releases energy or absorbs it. The short answer: exothermic reactions release energy (usually heat) to their surroundings, so things get warmer . Endothermic reactions absorb energy from their surroundings, so things get colder . An easy memory hook: exo = energy exits ; endo = energy goes in . Quick comparison at a glance Feature Exothermic Endothermic Energy flow Released to surroundings Absorbed from surroundings Surroundings feel… Hot Cold Products vs reactants Products have less energy Products have more energy Sign of ΔH (enthalpy) Negative (−) Positive (+) Examples Burning, respiration, hand warmers Photosynthesis, cold packs, thermal decomposition What is an exothermic reaction? An exothermic reaction gives out energy, most often as heat (sometimes light or sound). The prod...

Why does the periodic table list chlorine's mass as 35.5 when you can't have half a particle? The answer is isotopes — and once you understand them, a lot of small mysteries about atoms suddenly make sense. The short answer: isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons . Same element, different mass. A quick atom refresher Every atom is built from three particles: Protons — positive charge; the number of them defines which element you have (this is the atomic number). Neutrons — no charge; they add mass. Electrons — negative charge; they orbit the nucleus. The key fact: protons define the element . Change the protons and you have a different element entirely. What makes an isotope Isotopes come from changing the neutron count while keeping the protons the same. Because the protons don't change, it's still the same element — it just weighs a little more or less. We label isotopes by thei...

Melting ice, burning toast, dissolving sugar, rusting iron — all are "changes," but chemists split them into two big families. Knowing which is which is a classic exam skill, and the test for telling them apart is wonderfully simple. The short answer: a physical change alters how a substance looks or its state, but it's still the same substance underneath (melting ice is still water). A chemical change creates one or more brand-new substances (burning wood turns it into ash, smoke, and gas — you can't get the wood back). Quick comparison at a glance Feature Physical change Chemical change New substance formed? No Yes Chemical identity Stays the same Changes Easy to reverse? Usually yes Usually no What changes Shape, size, or state The actual substance Examples Melting, boiling, dissolving, cutting Burning, rusting, cooking, digesting What is a physical change? In a physical change , the molecules stay exactly the same — only their arrangement or form...

You've seen "pH balanced" on shampoo bottles and "pH 7" in science class — but what does the number actually mean? It's one of the most useful measurements in all of chemistry, and the idea behind it is refreshingly simple. The short answer: pH is a number from 0 to 14 that tells you how acidic or basic a solution is. It measures the concentration of hydrogen ions (H⁺) in the solution. A low pH means more acidic , 7 means neutral , and a high pH means more basic (alkaline). What pH actually measures pH is often described as the "power of hydrogen." It tracks how many hydrogen ions (H⁺) are floating around in a solution. The more H⁺ ions, the more acidic the solution — and the lower the pH number. If you've met logarithms, the exact definition is pH = −log[H⁺] . If you haven't, don't worry — the key consequence is the part below. The scale: 0 to 14 pH range Meaning Examples 0–6 Acidic Battery acid (~0), lemon juice (~2), vine...