Chemistry · Rung 3 of 24

Inside the atom

An atom has a nucleus with protons and neutrons, and electrons around it. The number of protons is the atomic number and it is the identity: change it and it is a different element. Change neutrons and you get an isotope, the same element with a different mass: carbon-12 and carbon-14 behave identically in chemistry and differ only in that one decays. Change electrons and you get an ion: lose electrons and it turns positive (cation), gain them and it turns negative (anion). Electrons do not orbit like planets; they sit in shells with room for 2, 8, 8, 18. Only the outermost ones, the valence electrons, do any chemistry. And when an electron drops from a high shell to a low one it releases a photon of an exact energy: that is why every element has its own barcode of colours.

Where this sits

Comes after
What everything is made of
Opens up
Bonds

Exercises from this rung

Generated by the app, from easiest to hardest

  1. An atom gains a neutron. What happens?

    • It becomes the next element
    • It stops existing
    • It becomes an ion
    • It stays the same element, but a different isotope

    Why An element's identity is its protons, not its neutrons. Adding a neutron changes the mass and sometimes the stability, but not the chemical behaviour: carbon-12 and carbon-14 undergo exactly the same reactions. Adding a proton would change the element, and adding or removing an electron would give an ion.

    easier

  2. An atom loses an electron. What does it become?

    • A cation, with a positive charge
    • A different element
    • An anion, with a negative charge
    • An isotope

    Why Electrons are negative, so losing one leaves the atom with more positive charge than negative: a cation. Gaining one would leave it negative: an anion. The mnemonic that works is that a cation is “pawsitive” and that anions go to the anode. Metals tend to form cations and nonmetals anions.

    in between

  3. A particle has 11 protons and 10 electrons. What is it?

    • A sodium ion, Na⁺
    • An isotope of sodium
    • A neon atom
    • A neon ion, Ne⁻

    Why Protons decide: eleven is sodium, always. With ten electrons it is one short of neutral, so it carries one positive charge: Na⁺. It has the same electron configuration as neon — which is why sodium sheds that electron so happily — but it is not neon, because the nucleus still holds eleven protons.

    harder