What makes an element more or less conductive than another one?
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An atomic structure that allows freer movement of electrons. If it takes less energy for an electron to move between atoms, it is more conductive.
Electricity is moving electrons around. If the electrons can move more freely there is more conductivity.
In the case of most metals, you have the metallic bond, where there’s a cloud of electrons floating around the metal (iron, silver, copper) atoms, making the electron movement way easier than in a non-metallic element.
Two things work together.
The first is the way atoms work. They consist of a nucleus and electrons. To be electrically neutral, the number of electrons is equal to the number of protons. Heavier elements have more protons, and thus more electrons. So oxygen has more electrons than hydrogen, and gold has more electrons that oxygen.
Those electrons are, through a complex process, forced into a series of discrete energy levels, maximum two to a level. You can visualize these levels as lying ever further from the nucleus. So heavier elements have more and more electrons far from the nucleus. The way the math works out is that these “orbitals” that are further from the nucleus are ever-closer to each other. This means the energy needed to move an electron from one orbital to another shrinks as you move further out from the nucleus. And since heavier atoms have more electrons, and those fill up the closer orbitals first, heavier atoms can more easily move their electrons around because they have more and more in those outer layers that are closely spaced.
The second part has to do with materials. They consist of atoms tied together in various ways by a number of different forces. They often, at least microscopically and under the right conditions, form into crystals. You can visualize a simple crystal as atoms at the corners of an infinite grid of cubes. Recall that heavier atoms have more electrons further from the nucleus, so in some cases the “outer layer” of one atom is really close to the outer layer of the next one. In that case, the energy needed to move the electron from one atom to another is not much more than it takes to move it from one level to another within a single atom. And in that case you have a conductor.
This is a toy model. In practice there are all sorts of additional factors: different atoms make different crystals, the energy level spacing is not a simple linear function, etc. When you consider all of these, what you end up with is that elements in the [periodic table](https://en.wikipedia.org/wiki/Periodic_table) in columns 10 to 12 are going to be more conductive than those in 3 or 4. The ones at either side of the table either don’t form a crystal normally (oxygen) or have electron structures that are highly charged (sodium). So copper is going to be more conductive than titanium, and both will be more conductive than gallium or calcium, and helium isn’t a conductor at all.
But it’s *never* that simple. You would also expect it to get more conductive as you go down the rows, because those atoms are heavier, and sure enough, silver is more conductive than copper, but then it breaks again, gold is *less* conductive than copper. And by the simple logic you might expect zinc to be more conductive than copper, but its the other way around.
So the real world its a bit of a mess, but generally speaking a good conductor is one that has lots of electrons in the right places, forms the right sort of crystal structure, and is easy to purify so you don’t have other atoms sucking up electrons in the conductor. And they end up conductive because the energy needed to move an electron from one atom to another in that overall structure is lower than other elements, which we call insulators.
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